llvm dev - Jan 2020 - Floating point semantic modes

Hi all,

I'm trying to put together a set of rules for how the various floating point
semantic modes should be handled in clang. A lot of this information will be
relevant to other front ends, but the details are necessarily bound to a front
end implementation so I'm framing the discussion here in terms of clang.
Other front ends can choose to follow clang or not. The existence of this set of
semantics is an LLVM property that applies to all front ends, but the front ends
will have to do something to initialize them.

I will eventually do something to convert this into an RST document and find a
home for it in the clang documentation, but I'd like to start by getting
input on whether everyone agrees with my judgment on how these things should
work and whether I've missed anything.

Here's what I've got.

=====================FP semantic modes
=====================except_behavior { ignore, strict, may_trap }
fenv_access { on, off }
rounding_mode { dynamic, tonearest, downward, upward, towardzero }
contract { on, off, fast }
denormal_fp_math { IEEE, PreserveSign, PositiveZero }
denormal_fp32_math { IEEE, PreserveSign, PositiveZero }
support_math_errno { on, off }
no_honor_nans { on, off }
no_honor_infinities { on, off }
no_signed_zeros { on, off }
allow_reciprocal { on, off }
allow_approximate_fns { on, off }
allow_reassociation { on, off }

---------------------
Dependencies
---------------------
rounding_mode must be "tonearest" if fenv_access is "off" --
It is the user's responsibility to make sure the dynamic rounding mode is
not left in another state.

if except_behavior is not "ignore", allow_reciprocal,
allow_approximate_fns and allow_reassociation must be "off".

=====================FP models
=====================-----------------------
precise (default)
-----------------------
except_behavior { ignore }
fenv_access { off }
rounding_mode { tonearest }
contract { on }
denormal_fp_math { IEEE }
denormal_fp32_math { IEEE }
support_math_errno { on }
no_honor_nans { off }
no_honor_infinities { off }
no_signed_zeros { off }
allow_reciprocal { off }
allow_approximate_fns { off }
allow_reassociation { off }

------------------
strict
------------------
except_behavior { strict }
fenv_access { on }
rounding_mode { dynamic }
contract { off }
denormal_fp_math { IEEE }
denormal_fp32_math { IEEE }
support_math_errno { on }
no_honor_nans { off }
no_honor_infinities { off }
no_signed_zeros { off }
allow_reciprocal { off }
allow_approximate_fns { off }
allow_reassociation { off }

------------------
fast
------------------
except_behavior { ignore }
fenv_access { off }
rounding_mode { tonearest }
contract { fast }
denormal_fp_math { PreserveSign }
denormal_fp32_math { PreserveSign }
support_math_errno { off }
no_honor_nans { on }
no_honor_infinities { on }
no_signed_zeros { on }
allow_reciprocal { on }
allow_approximate_fns { on }
allow_reassociation { on }

=====================Command-line options
=====================-ffp-model={precise|strict|fast}
  Sets all semantic modes as described above.

-ffast-math
  Equivalent to -ffp-model=fast. (I'm not sure that's currently true.)

-f[no-]math-errno
-ffp-contract={on|off|fast}
-f[no-]honor-infinities
-f[no-]honor-nans
-f[no-]associative-math
-f[no-]reciprocal-math
-f[no-]signed-zeros
-f[no-]trapping-math
-f[no-]rounding-math
-fdenormal-fp-math={ieee, preservesign, positivezero}
-fdenormal-fp-math-fp32={ieee, preservesign, positivezero}
-ffp-exception-behavior={ignore,maytrap,strict}
  Each of these has a 1-to-1 correspondance to an FP semantic mode.
  (I think several of these should set "except_behavior" to
"ignore".)


-f[no-]finite-math-only
  Controls no_honor_nans and no_honor_infinities.

-f[no-]unsafe-math-optimizations
  Turns no_signed_zeros, allow_reciprocal, allow_approximate_fns, and
allow_reassociation on or off.
  Also, sets except_behavior to "on" for -funsafe-math-optimizations.
  (Currently, -fno-]unsafe-math-optimizations clears except_behavior, but I
regard this as a bug.)

All command line options will override any previous values of all settings they
control with options taking effect in a left-to-right manner.

=====================pragmas
=====================STDC FENV_ACCESS {ON|OFF}
  Patch in progress. I think ON should force the following:

    except_behavior { strict }
    fenv_access { on }
    rounding_mode { dynamic }
    denormal_fp_math { IEEE }
    denormal_fp32_math { IEEE }
    no_signed_zeros { off }
    allow_reciprocal { off }
    allow_approximate_fns { off }
    allow_reassociation { off }

  And OFF should set fenv_access to off, except_behavior to ignore, and
rounding_mode to tonearest. Other modes should be reset to their command line
defined settings.

  I don't think this pragma should have any effect on contract,
support_math_errno, no_honor_nans, or no_honor_infinities.

STDC FP_CONTRACT {ON|OFF|DEFAULT}
  This pragma controls the contract FP semantic mode. No other FP semantic modes
are effected.

float_control ({precise|except}, {on|off}[, push])
float_control (pop)
  Patch in progress. These are tricky.
  I think they should have the following effects:

float_control (precise, on[, push])
  contract { on }
  denormal_fp_math { IEEE }
  denormal_fp32_math { IEEE }
  no_signed_zeros { off }
  allow_reciprocal { off }
  allow_approximate_fns { off }
  allow_reassociation { off }

float_control (precise, off[, push])
  contract { fast }
  denormal_fp_math { preservesign }
  denormal_fp32_math { preservesign }
  no_signed_zeros { on }
  allow_reciprocal { on }
  allow_approximate_fns { on }
  allow_reassociation { on }

Note, this is less than what the -ffp-model=precise control does. Should this
override support_math_errno, no_honor_nans, or no_honor_infinities?

float_control (except, on[, push])
  except_behavior { strict }

float_control (except, off[, push])
  except_behavior { ignore }

The MSVC documentation says you can only use the float_control pragma to turn
exception semantics on when precise semantics are enabled. For us, this would
mean:
  denormal_fp_math { IEEE }
  denormal_fp32_math { IEEE }
  no_signed_zeros { off }
  allow_reciprocal { off }
  allow_approximate_fns { off }
  allow_reassociation { off }

The MSVC documentation also says you can't use the float_control pragma to
turn excpetion semantics off when precise semantics are enabled, and you
can't use the float_control pragma to turn precise off when fenv_access is
on.

I believe we should follow the MSVC restrictions.

========================Code-visible identifiers
========================__FAST_MATH__

This symbol will only be defined if and only if all of the following are set
(before pragmas are applied):
  except_behavior { ignore }
  fenv_access { off }
  rounding_mode { tonearest }
  contract { fast }
  denormal_fp_math { PreserveSign }
  denormal_fp32_math { PreserveSign }
  support_math_errno { off }
  no_honor_nans { on }
  no_honor_infinities { on }
  no_signed_zeros { on }
  allow_reciprocal { on }
  allow_approximate_fns { on }
  allow_reassociation { on }

__FINITE_MATH_ONLY__

This symbol will only be defined if and only if all of the following are set
(before pragmas are applied):
  no_honor_nans { on }
  no_honor_infinities { on }

FLT_ROUNDS

Should be set to -1 (indeterminable) if rounding_mode() is dynamic or 1
(tonearest) if rounding_mode is tonearest. There are values for other rounding
modes, but clang offers no way to set those rounding modes.

FLT_EVAL_METHOD

Should be set to -1 if any of allow_reciprocal, allow_approximate_fns, or
allow_reassociation is set. Should any other flags also make this -1? Otherwise,
the setting is target-defined.

math_errhandling

The MATH_ERRNO bit will be set or cleared based on the setting of
support_math_errno. Should MATH_ERREXCEPT be set or cleared based on
except_behavior?


Thanks in advance for any opinions and suggestions.

-Andy
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> On Jan 27, 2020, at 18:24, Kaylor, Andrew via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> denormal_fp_math { IEEE, PreserveSign, PositiveZero }
> denormal_fp32_math { IEEE, PreserveSign, PositiveZero }
As far as this part, ping on https://reviews.llvm.org/D69978
<https://reviews.llvm.org/D69978>
The input mode and output modes are really different settings

-Matt
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On Mon, Jan 27, 2020, 15:24 Kaylor, Andrew via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> rounding_mode { dynamic, tonearest, downward, upward, towardzero }
>
I don't know if LLVM supports it, but RISC-V as well as a few other
architectures also support to-nearest-ties-to-max-magnitude.

Jacob Lifshay
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> I don't know if LLVM supports it, but RISC-V as well as a few other
architectures also support to-nearest-ties-to-max-magnitude.
Yeah, that’s come up a few times in reviews. We should start factoring it into
our plans, but from a clang point of view, I’m not sure how you’d get into that
mode.

We don’t have it in the list of possible rounding modes for the constrained
intrinsics either. We should fix that.

-Andy

From: Jacob Lifshay <programmerjake at gmail.com>
Sent: Monday, January 27, 2020 3:53 PM
To: Kaylor, Andrew <andrew.kaylor at intel.com>
Cc: Clang Dev <cfe-dev at lists.llvm.org>; LLVM Developers Mailing List
<llvm-dev at lists.llvm.org>; Ristow, Warren <warren.ristow at
sony.com>; Ulrich Weigand (Ulrich.Weigand at de.ibm.com) <Ulrich.Weigand
at de.ibm.com>; Cameron McInally <cameron.mcinally at nyu.edu>; Kevin
Neal <Kevin.Neal at sas.com>; Blower, Melanie I <melanie.blower at
intel.com>; sepavloff at gmail.com; hfinkel at anl.gov; Sanjay Patel
<spatel at rotateright.com>; Wang, Pengfei <pengfei.wang at
intel.com>
Subject: Re: [llvm-dev] Floating point semantic modes

On Mon, Jan 27, 2020, 15:24 Kaylor, Andrew via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
rounding_mode { dynamic, tonearest, downward, upward, towardzero }
I don't know if LLVM supports it, but RISC-V as well as a few other
architectures also support to-nearest-ties-to-max-magnitude.

Jacob Lifshay
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Hi,
> I'm trying to put together a set of rules for how the various floating
point semantic modes should be handled in clang.
Great to see this.  Thanks!

Regarding the aspect:
> ========================> Code-visible identifiers
> ========================> __FAST_MATH__
>
> This symbol will only be defined if and only if all of the following are
set (before pragmas are applied):
>   except_behavior { ignore }
>   fenv_access { off }
>   rounding_mode { tonearest }
>   contract { fast }
>   denormal_fp_math { PreserveSign }
>   denormal_fp32_math { PreserveSign }
>   support_math_errno { off }
>   no_honor_nans { on }
>   no_honor_infinities { on }
>   no_signed_zeros { on }
>   allow_reciprocal { on }
>   allow_approximate_fns { on }
>   allow_reassociation { on }
This makes logical sense to me.  That said, it's different than what GCC
does.  GCC defines __FAST_MATH__ irrespective of the settings of:

  contract (-ffp-contract={on|off|fast})
  allow_reassociation (-f[no-]associative-math)
  allow_reciprocal (-f[no-]reciprocal-math)
  rounding_mode (-f[no-]rounding-math)

(See https://reviews.llvm.org/D72675#1829810.)

I had previously thought we should be compatible with GCC (unless we find that
GCC's behavior is viewed by them as a bug).  But after reading this, the
simple rule of defining __FAST_MATH__ when all of the collective flags that
-ffast-math defines are set appropriately, makes more sense to me.  In any case,
I thought we should overtly note the difference here, in case anyone objects.

Thanks,
-Warren Ristow


From: Kaylor, Andrew <andrew.kaylor at intel.com>
Sent: Monday, January 27, 2020 3:24 PM
To: cfe-dev at lists.llvm.org; LLVM Developers Mailing List <llvm-dev at
lists.llvm.org>; Ristow, Warren <warren.ristow at sony.com>; Ulrich
Weigand (Ulrich.Weigand at de.ibm.com) <Ulrich.Weigand at de.ibm.com>;
Cameron McInally <cameron.mcinally at nyu.edu>; Kevin Neal <Kevin.Neal
at sas.com>; Blower, Melanie I <melanie.blower at intel.com>; sepavloff
at gmail.com; hfinkel at anl.gov; Sanjay Patel <spatel at
rotateright.com>; Wang, Pengfei <pengfei.wang at intel.com>
Subject: Floating point semantic modes

Hi all,

I'm trying to put together a set of rules for how the various floating point
semantic modes should be handled in clang. A lot of this information will be
relevant to other front ends, but the details are necessarily bound to a front
end implementation so I'm framing the discussion here in terms of clang.
Other front ends can choose to follow clang or not. The existence of this set of
semantics is an LLVM property that applies to all front ends, but the front ends
will have to do something to initialize them.

I will eventually do something to convert this into an RST document and find a
home for it in the clang documentation, but I'd like to start by getting
input on whether everyone agrees with my judgment on how these things should
work and whether I've missed anything.

Here's what I've got.

=====================FP semantic modes
=====================except_behavior { ignore, strict, may_trap }
fenv_access { on, off }
rounding_mode { dynamic, tonearest, downward, upward, towardzero }
contract { on, off, fast }
denormal_fp_math { IEEE, PreserveSign, PositiveZero }
denormal_fp32_math { IEEE, PreserveSign, PositiveZero }
support_math_errno { on, off }
no_honor_nans { on, off }
no_honor_infinities { on, off }
no_signed_zeros { on, off }
allow_reciprocal { on, off }
allow_approximate_fns { on, off }
allow_reassociation { on, off }

---------------------
Dependencies
---------------------
rounding_mode must be "tonearest" if fenv_access is "off" --
It is the user's responsibility to make sure the dynamic rounding mode is
not left in another state.

if except_behavior is not "ignore", allow_reciprocal,
allow_approximate_fns and allow_reassociation must be "off".

=====================FP models
=====================-----------------------
precise (default)
-----------------------
except_behavior { ignore }
fenv_access { off }
rounding_mode { tonearest }
contract { on }
denormal_fp_math { IEEE }
denormal_fp32_math { IEEE }
support_math_errno { on }
no_honor_nans { off }
no_honor_infinities { off }
no_signed_zeros { off }
allow_reciprocal { off }
allow_approximate_fns { off }
allow_reassociation { off }

------------------
strict
------------------
except_behavior { strict }
fenv_access { on }
rounding_mode { dynamic }
contract { off }
denormal_fp_math { IEEE }
denormal_fp32_math { IEEE }
support_math_errno { on }
no_honor_nans { off }
no_honor_infinities { off }
no_signed_zeros { off }
allow_reciprocal { off }
allow_approximate_fns { off }
allow_reassociation { off }

------------------
fast
------------------
except_behavior { ignore }
fenv_access { off }
rounding_mode { tonearest }
contract { fast }
denormal_fp_math { PreserveSign }
denormal_fp32_math { PreserveSign }
support_math_errno { off }
no_honor_nans { on }
no_honor_infinities { on }
no_signed_zeros { on }
allow_reciprocal { on }
allow_approximate_fns { on }
allow_reassociation { on }

=====================Command-line options
=====================-ffp-model={precise|strict|fast}
  Sets all semantic modes as described above.

-ffast-math
  Equivalent to -ffp-model=fast. (I'm not sure that's currently true.)

-f[no-]math-errno
-ffp-contract={on|off|fast}
-f[no-]honor-infinities
-f[no-]honor-nans
-f[no-]associative-math
-f[no-]reciprocal-math
-f[no-]signed-zeros
-f[no-]trapping-math
-f[no-]rounding-math
-fdenormal-fp-math={ieee, preservesign, positivezero}
-fdenormal-fp-math-fp32={ieee, preservesign, positivezero}
-ffp-exception-behavior={ignore,maytrap,strict}
  Each of these has a 1-to-1 correspondance to an FP semantic mode.
  (I think several of these should set "except_behavior" to
"ignore".)


-f[no-]finite-math-only
  Controls no_honor_nans and no_honor_infinities.

-f[no-]unsafe-math-optimizations
  Turns no_signed_zeros, allow_reciprocal, allow_approximate_fns, and
allow_reassociation on or off.
  Also, sets except_behavior to "on" for -funsafe-math-optimizations.
  (Currently, -fno-]unsafe-math-optimizations clears except_behavior, but I
regard this as a bug.)

All command line options will override any previous values of all settings they
control with options taking effect in a left-to-right manner.

=====================pragmas
=====================STDC FENV_ACCESS {ON|OFF}
  Patch in progress. I think ON should force the following:

    except_behavior { strict }
    fenv_access { on }
    rounding_mode { dynamic }
    denormal_fp_math { IEEE }
    denormal_fp32_math { IEEE }
    no_signed_zeros { off }
    allow_reciprocal { off }
    allow_approximate_fns { off }
    allow_reassociation { off }

  And OFF should set fenv_access to off, except_behavior to ignore, and
rounding_mode to tonearest. Other modes should be reset to their command line
defined settings.

  I don't think this pragma should have any effect on contract,
support_math_errno, no_honor_nans, or no_honor_infinities.

STDC FP_CONTRACT {ON|OFF|DEFAULT}
  This pragma controls the contract FP semantic mode. No other FP semantic modes
are effected.

float_control ({precise|except}, {on|off}[, push])
float_control (pop)
  Patch in progress. These are tricky.
  I think they should have the following effects:

float_control (precise, on[, push])
  contract { on }
  denormal_fp_math { IEEE }
  denormal_fp32_math { IEEE }
  no_signed_zeros { off }
  allow_reciprocal { off }
  allow_approximate_fns { off }
  allow_reassociation { off }

float_control (precise, off[, push])
  contract { fast }
  denormal_fp_math { preservesign }
  denormal_fp32_math { preservesign }
  no_signed_zeros { on }
  allow_reciprocal { on }
  allow_approximate_fns { on }
  allow_reassociation { on }

Note, this is less than what the -ffp-model=precise control does. Should this
override support_math_errno, no_honor_nans, or no_honor_infinities?

float_control (except, on[, push])
  except_behavior { strict }

float_control (except, off[, push])
  except_behavior { ignore }

The MSVC documentation says you can only use the float_control pragma to turn
exception semantics on when precise semantics are enabled. For us, this would
mean:
  denormal_fp_math { IEEE }
  denormal_fp32_math { IEEE }
  no_signed_zeros { off }
  allow_reciprocal { off }
  allow_approximate_fns { off }
  allow_reassociation { off }

The MSVC documentation also says you can't use the float_control pragma to
turn excpetion semantics off when precise semantics are enabled, and you
can't use the float_control pragma to turn precise off when fenv_access is
on.

I believe we should follow the MSVC restrictions.

========================Code-visible identifiers
========================__FAST_MATH__

This symbol will only be defined if and only if all of the following are set
(before pragmas are applied):
  except_behavior { ignore }
  fenv_access { off }
  rounding_mode { tonearest }
  contract { fast }
  denormal_fp_math { PreserveSign }
  denormal_fp32_math { PreserveSign }
  support_math_errno { off }
  no_honor_nans { on }
  no_honor_infinities { on }
  no_signed_zeros { on }
  allow_reciprocal { on }
  allow_approximate_fns { on }
  allow_reassociation { on }

__FINITE_MATH_ONLY__

This symbol will only be defined if and only if all of the following are set
(before pragmas are applied):
  no_honor_nans { on }
  no_honor_infinities { on }

FLT_ROUNDS

Should be set to -1 (indeterminable) if rounding_mode() is dynamic or 1
(tonearest) if rounding_mode is tonearest. There are values for other rounding
modes, but clang offers no way to set those rounding modes.

FLT_EVAL_METHOD

Should be set to -1 if any of allow_reciprocal, allow_approximate_fns, or
allow_reassociation is set. Should any other flags also make this -1? Otherwise,
the setting is target-defined.

math_errhandling

The MATH_ERRNO bit will be set or cleared based on the setting of
support_math_errno. Should MATH_ERREXCEPT be set or cleared based on
except_behavior?


Thanks in advance for any opinions and suggestions.

-Andy
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Nice database, definitely worth to be placed into documentation.

FLT_ROUNDS
>
> Should be set to -1 (indeterminable) if rounding_mode() is dynamic or 1
> (tonearest) if rounding_mode is tonearest. There are values for other
> rounding modes, but clang offers no way to set those rounding modes.

I would remove this symbol from this database. According to the C11
standard (5.2.4.2.2p8):

Evaluation of FLT_ROUNDS correctly reflects any execution-time change of
rounding mode through

the function fesetround in <fenv.h>.

So on a target which is capable of reading rounding mode it must report
actual rounding mode and never -1. It is actually a wrapper over fegetround.


Thanks,
--Serge


On Tue, Jan 28, 2020 at 6:24 AM Kaylor, Andrew <andrew.kaylor at
intel.com>
wrote:
> Hi all,
>
>
>
> I’m trying to put together a set of rules for how the various floating
> point semantic modes should be handled in clang. A lot of this information
> will be relevant to other front ends, but the details are necessarily bound
> to a front end implementation so I’m framing the discussion here in terms
> of clang. Other front ends can choose to follow clang or not. The existence
> of this set of semantics is an LLVM property that applies to all front
> ends, but the front ends will have to do something to initialize them.
>
>
>
> I will eventually do something to convert this into an RST document and
> find a home for it in the clang documentation, but I’d like to start by
> getting input on whether everyone agrees with my judgment on how these
> things should work and whether I’ve missed anything.
>
>
>
> Here’s what I’ve got.
>
>
>
> =====================>
> FP semantic modes
>
> =====================>
> except_behavior { ignore, strict, may_trap }
>
> fenv_access { on, off }
>
> rounding_mode { dynamic, tonearest, downward, upward, towardzero }
>
> contract { on, off, fast }
>
> denormal_fp_math { IEEE, PreserveSign, PositiveZero }
>
> denormal_fp32_math { IEEE, PreserveSign, PositiveZero }
>
> support_math_errno { on, off }
>
> no_honor_nans { on, off }
>
> no_honor_infinities { on, off }
>
> no_signed_zeros { on, off }
>
> allow_reciprocal { on, off }
>
> allow_approximate_fns { on, off }
>
> allow_reassociation { on, off }
>
>
>
> ---------------------
>
> Dependencies
>
> ---------------------
>
> rounding_mode must be "tonearest" if fenv_access is
"off" -- It is the
> user's responsibility to make sure the dynamic rounding mode is not
left in
> another state.
>
>
>
> if except_behavior is not "ignore", allow_reciprocal,
> allow_approximate_fns and allow_reassociation must be "off".
>
>
>
> =====================>
> FP models
>
> =====================>
> -----------------------
>
> precise (default)
>
> -----------------------
>
> except_behavior { ignore }
>
> fenv_access { off }
>
> rounding_mode { tonearest }
>
> contract { on }
>
> denormal_fp_math { IEEE }
>
> denormal_fp32_math { IEEE }
>
> support_math_errno { on }
>
> no_honor_nans { off }
>
> no_honor_infinities { off }
>
> no_signed_zeros { off }
>
> allow_reciprocal { off }
>
> allow_approximate_fns { off }
>
> allow_reassociation { off }
>
>
>
> ------------------
>
> strict
>
> ------------------
>
> except_behavior { strict }
>
> fenv_access { on }
>
> rounding_mode { dynamic }
>
> contract { off }
>
> denormal_fp_math { IEEE }
>
> denormal_fp32_math { IEEE }
>
> support_math_errno { on }
>
> no_honor_nans { off }
>
> no_honor_infinities { off }
>
> no_signed_zeros { off }
>
> allow_reciprocal { off }
>
> allow_approximate_fns { off }
>
> allow_reassociation { off }
>
>
>
> ------------------
>
> fast
>
> ------------------
>
> except_behavior { ignore }
>
> fenv_access { off }
>
> rounding_mode { tonearest }
>
> contract { fast }
>
> denormal_fp_math { PreserveSign }
>
> denormal_fp32_math { PreserveSign }
>
> support_math_errno { off }
>
> no_honor_nans { on }
>
> no_honor_infinities { on }
>
> no_signed_zeros { on }
>
> allow_reciprocal { on }
>
> allow_approximate_fns { on }
>
> allow_reassociation { on }
>
>
>
> =====================>
> Command-line options
>
> =====================>
> -ffp-model={precise|strict|fast}
>
>   Sets all semantic modes as described above.
>
>
>
> -ffast-math
>
>   Equivalent to -ffp-model=fast. (I'm not sure that's currently
true.)
>
>
>
> -f[no-]math-errno
>
> -ffp-contract={on|off|fast}
>
> -f[no-]honor-infinities
>
> -f[no-]honor-nans
>
> -f[no-]associative-math
>
> -f[no-]reciprocal-math
>
> -f[no-]signed-zeros
>
> -f[no-]trapping-math
>
> -f[no-]rounding-math
>
> -fdenormal-fp-math={ieee, preservesign, positivezero}
>
> -fdenormal-fp-math-fp32={ieee, preservesign, positivezero}
>
> -ffp-exception-behavior={ignore,maytrap,strict}
>
>   Each of these has a 1-to-1 correspondance to an FP semantic mode.
>
>   (I think several of these should set "except_behavior" to
"ignore".)
>
>
>
>
>
> -f[no-]finite-math-only
>
>   Controls no_honor_nans and no_honor_infinities.
>
>
>
> -f[no-]unsafe-math-optimizations
>
>   Turns no_signed_zeros, allow_reciprocal, allow_approximate_fns, and
> allow_reassociation on or off.
>
>   Also, sets except_behavior to "on" for
-funsafe-math-optimizations.
>
>   (Currently, -fno-]unsafe-math-optimizations clears except_behavior, but
> I regard this as a bug.)
>
>
>
> All command line options will override any previous values of all settings
> they control with options taking effect in a left-to-right manner.
>
>
>
> =====================>
> pragmas
>
> =====================>
> STDC FENV_ACCESS {ON|OFF}
>
>   Patch in progress. I think ON should force the following:
>
>
>
>     except_behavior { strict }
>
>     fenv_access { on }
>
>     rounding_mode { dynamic }
>
>     denormal_fp_math { IEEE }
>
>     denormal_fp32_math { IEEE }
>
>     no_signed_zeros { off }
>
>     allow_reciprocal { off }
>
>     allow_approximate_fns { off }
>
>     allow_reassociation { off }
>
>
>
>   And OFF should set fenv_access to off, except_behavior to ignore, and
> rounding_mode to tonearest. Other modes should be reset to their command
> line defined settings.
>
>
>
>   I don't think this pragma should have any effect on contract,
> support_math_errno, no_honor_nans, or no_honor_infinities.
>
>
>
> STDC FP_CONTRACT {ON|OFF|DEFAULT}
>
>   This pragma controls the contract FP semantic mode. No other FP semantic
> modes are effected.
>
>
>
> float_control ({precise|except}, {on|off}[, push])
>
> float_control (pop)
>
>   Patch in progress. These are tricky.
>
>   I think they should have the following effects:
>
>
>
> float_control (precise, on[, push])
>
>   contract { on }
>
>   denormal_fp_math { IEEE }
>
>   denormal_fp32_math { IEEE }
>
>   no_signed_zeros { off }
>
>   allow_reciprocal { off }
>
>   allow_approximate_fns { off }
>
>   allow_reassociation { off }
>
>
>
> float_control (precise, off[, push])
>
>   contract { fast }
>
>   denormal_fp_math { preservesign }
>
>   denormal_fp32_math { preservesign }
>
>   no_signed_zeros { on }
>
>   allow_reciprocal { on }
>
>   allow_approximate_fns { on }
>
>   allow_reassociation { on }
>
>
>
> Note, this is less than what the -ffp-model=precise control does. Should
> this override support_math_errno, no_honor_nans, or no_honor_infinities?
>
>
>
> float_control (except, on[, push])
>
>   except_behavior { strict }
>
>
>
> float_control (except, off[, push])
>
>   except_behavior { ignore }
>
>
>
> The MSVC documentation says you can only use the float_control pragma to
> turn exception semantics on when precise semantics are enabled. For us,
> this would mean:
>
>   denormal_fp_math { IEEE }
>
>   denormal_fp32_math { IEEE }
>
>   no_signed_zeros { off }
>
>   allow_reciprocal { off }
>
>   allow_approximate_fns { off }
>
>   allow_reassociation { off }
>
>
>
> The MSVC documentation also says you can't use the float_control pragma
to
> turn excpetion semantics off when precise semantics are enabled, and you
> can't use the float_control pragma to turn precise off when fenv_access
is
> on.
>
>
>
> I believe we should follow the MSVC restrictions.
>
>
>
> ========================>
> Code-visible identifiers
>
> ========================>
> __FAST_MATH__
>
>
>
> This symbol will only be defined if and only if all of the following are
> set (before pragmas are applied):
>
>   except_behavior { ignore }
>
>   fenv_access { off }
>
>   rounding_mode { tonearest }
>
>   contract { fast }
>
>   denormal_fp_math { PreserveSign }
>
>   denormal_fp32_math { PreserveSign }
>
>   support_math_errno { off }
>
>   no_honor_nans { on }
>
>   no_honor_infinities { on }
>
>   no_signed_zeros { on }
>
>   allow_reciprocal { on }
>
>   allow_approximate_fns { on }
>
>   allow_reassociation { on }
>
>
>
> __FINITE_MATH_ONLY__
>
>
>
> This symbol will only be defined if and only if all of the following are
> set (before pragmas are applied):
>
>   no_honor_nans { on }
>
>   no_honor_infinities { on }
>
>
>
> FLT_ROUNDS
>
>
>
> Should be set to -1 (indeterminable) if rounding_mode() is dynamic or 1
> (tonearest) if rounding_mode is tonearest. There are values for other
> rounding modes, but clang offers no way to set those rounding modes.
>
>
>
> FLT_EVAL_METHOD
>
>
>
> Should be set to -1 if any of allow_reciprocal, allow_approximate_fns, or
> allow_reassociation is set. Should any other flags also make this -1?
> Otherwise, the setting is target-defined.
>
>
>
> math_errhandling
>
>
>
> The MATH_ERRNO bit will be set or cleared based on the setting of
> support_math_errno. Should MATH_ERREXCEPT be set or cleared based on
> except_behavior?
>
>
>
>
>
> Thanks in advance for any opinions and suggestions.
>
>
>
> -Andy
>
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* Kaylor, Andrew via llvm-dev <llvm-dev at lists.llvm.org> [2020-01-27
23:24:10 +0000]:
> Hi all,
> 
> I'm trying to put together a set of rules for how the various floating
point semantic modes should be handled in clang. A lot of this information will
be relevant to other front ends, but the details are necessarily bound to a
front end implementation so I'm framing the discussion here in terms of
clang. Other front ends can choose to follow clang or not. The existence of this
set of semantics is an LLVM property that applies to all front ends, but the
front ends will have to do something to initialize them.
> 
> I will eventually do something to convert this into an RST document and
find a home for it in the clang documentation, but I'd like to start by
getting input on whether everyone agrees with my judgment on how these things
should work and whether I've missed anything.
> 
> Here's what I've got.
> 
i'm not an llvm/clang dev, i hope this mail wont bounce.
> =====================> FP semantic modes
> =====================> except_behavior { ignore, strict, may_trap }
> fenv_access { on, off }
> rounding_mode { dynamic, tonearest, downward, upward, towardzero }
> contract { on, off, fast }
> denormal_fp_math { IEEE, PreserveSign, PositiveZero }
> denormal_fp32_math { IEEE, PreserveSign, PositiveZero }
> support_math_errno { on, off }
note that math errno handling can be

1) errno is set,
2) errno may be set and
3) errno is guaranteed to be untouched

iso c math_errhandling can select between 1 and 2,
(user can or cannot rely on errno) but for optimizing
math calls as side-effect-free pure functions, 3 is
needed.

-f(no-)math-errno selects between 1 and 3.
with 3, moving math calls across errno checks or
calls that set errno can break semantics depending
on how libm is implemented (e.g. glibc will set
errno independently of how you compiled your code).
> no_honor_nans { on, off }
ideally there would be a way to support snan too.
(e.g. isnan(x) cannot be turned into x!=x then)
> no_honor_infinities { on, off }
> no_signed_zeros { on, off }
> allow_reciprocal { on, off }
> allow_approximate_fns { on, off }
> allow_reassociation { on, off }
excess precision handling is missing from this list
which matters for x87 and m68k fpu support and may
matter for _Float16 implementations that fall back
to _Float32 arithmetic.

the granularity of these knobs is also interesting
(expression, code block, function or translation unit),
iso c pragmas work on code block level.
> =====================> FP models
> =====================> -----------------------
> precise (default)
> -----------------------
> except_behavior { ignore }
> fenv_access { off }
> rounding_mode { tonearest }
> contract { on }
> denormal_fp_math { IEEE }
> denormal_fp32_math { IEEE }
> support_math_errno { on }
> no_honor_nans { off }
> no_honor_infinities { off }
> no_signed_zeros { off }
> allow_reciprocal { off }
> allow_approximate_fns { off }
> allow_reassociation { off }
> 
> ------------------
> strict
> ------------------
> except_behavior { strict }
> fenv_access { on }
> rounding_mode { dynamic }
> contract { off }
> denormal_fp_math { IEEE }
> denormal_fp32_math { IEEE }
> support_math_errno { on }
> no_honor_nans { off }
> no_honor_infinities { off }
> no_signed_zeros { off }
> allow_reciprocal { off }
> allow_approximate_fns { off }
> allow_reassociation { off }
> 
> ------------------
> fast
> ------------------
> except_behavior { ignore }
> fenv_access { off }
> rounding_mode { tonearest }
> contract { fast }
> denormal_fp_math { PreserveSign }
> denormal_fp32_math { PreserveSign }
> support_math_errno { off }
> no_honor_nans { on }
> no_honor_infinities { on }
> no_signed_zeros { on }
> allow_reciprocal { on }
> allow_approximate_fns { on }
> allow_reassociation { on }
> 
> =====================> Command-line options
> =====================> -ffp-model={precise|strict|fast}
>   Sets all semantic modes as described above.
> 
> -ffast-math
>   Equivalent to -ffp-model=fast. (I'm not sure that's currently
true.)
> 
> -f[no-]math-errno
> -ffp-contract={on|off|fast}
> -f[no-]honor-infinities
> -f[no-]honor-nans
> -f[no-]associative-math
> -f[no-]reciprocal-math
> -f[no-]signed-zeros
> -f[no-]trapping-math
> -f[no-]rounding-math
> -fdenormal-fp-math={ieee, preservesign, positivezero}
> -fdenormal-fp-math-fp32={ieee, preservesign, positivezero}
> -ffp-exception-behavior={ignore,maytrap,strict}
>   Each of these has a 1-to-1 correspondance to an FP semantic mode.
>   (I think several of these should set "except_behavior" to
"ignore".)
-ftrapping-math vs -ffp-exception-behaviour=maytrap
is unclear.

(-ftrapping-math is weird in gcc, it does not handle
all fp exception cases, not sure what clang plans to
do with that)
> 
> -f[no-]finite-math-only
>   Controls no_honor_nans and no_honor_infinities.
> 
> -f[no-]unsafe-math-optimizations
>   Turns no_signed_zeros, allow_reciprocal, allow_approximate_fns, and
allow_reassociation on or off.
>   Also, sets except_behavior to "on" for
-funsafe-math-optimizations.
>   (Currently, -fno-]unsafe-math-optimizations clears except_behavior, but I
regard this as a bug.)
> 
> All command line options will override any previous values of all settings
they control with options taking effect in a left-to-right manner.
> 
> =====================> pragmas
> =====================> STDC FENV_ACCESS {ON|OFF}
>   Patch in progress. I think ON should force the following:
> 
>     except_behavior { strict }
>     fenv_access { on }
>     rounding_mode { dynamic }
>     denormal_fp_math { IEEE }
>     denormal_fp32_math { IEEE }
>     no_signed_zeros { off }
>     allow_reciprocal { off }
>     allow_approximate_fns { off }
>     allow_reassociation { off }
> 
>   And OFF should set fenv_access to off, except_behavior to ignore, and
rounding_mode to tonearest. Other modes should be reset to their command line
defined settings.
> 
>   I don't think this pragma should have any effect on contract,
support_math_errno, no_honor_nans, or no_honor_infinities.
> 
> STDC FP_CONTRACT {ON|OFF|DEFAULT}
>   This pragma controls the contract FP semantic mode. No other FP semantic
modes are effected.
> 
> float_control ({precise|except}, {on|off}[, push])
> float_control (pop)
>   Patch in progress. These are tricky.
>   I think they should have the following effects:
> 
> float_control (precise, on[, push])
>   contract { on }
>   denormal_fp_math { IEEE }
>   denormal_fp32_math { IEEE }
>   no_signed_zeros { off }
>   allow_reciprocal { off }
>   allow_approximate_fns { off }
>   allow_reassociation { off }
> 
> float_control (precise, off[, push])
>   contract { fast }
>   denormal_fp_math { preservesign }
>   denormal_fp32_math { preservesign }
>   no_signed_zeros { on }
>   allow_reciprocal { on }
>   allow_approximate_fns { on }
>   allow_reassociation { on }
> 
> Note, this is less than what the -ffp-model=precise control does. Should
this override support_math_errno, no_honor_nans, or no_honor_infinities?
> 
> float_control (except, on[, push])
>   except_behavior { strict }
> 
> float_control (except, off[, push])
>   except_behavior { ignore }
> 
> The MSVC documentation says you can only use the float_control pragma to
turn exception semantics on when precise semantics are enabled. For us, this
would mean:
>   denormal_fp_math { IEEE }
>   denormal_fp32_math { IEEE }
>   no_signed_zeros { off }
>   allow_reciprocal { off }
>   allow_approximate_fns { off }
>   allow_reassociation { off }
> 
> The MSVC documentation also says you can't use the float_control pragma
to turn excpetion semantics off when precise semantics are enabled, and you
can't use the float_control pragma to turn precise off when fenv_access is
on.
> 
> I believe we should follow the MSVC restrictions.
> 
> ========================> Code-visible identifiers
> ========================> __FAST_MATH__
> 
> This symbol will only be defined if and only if all of the following are
set (before pragmas are applied):
>   except_behavior { ignore }
>   fenv_access { off }
>   rounding_mode { tonearest }
>   contract { fast }
>   denormal_fp_math { PreserveSign }
>   denormal_fp32_math { PreserveSign }
>   support_math_errno { off }
>   no_honor_nans { on }
>   no_honor_infinities { on }
>   no_signed_zeros { on }
>   allow_reciprocal { on }
>   allow_approximate_fns { on }
>   allow_reassociation { on }
> 
> __FINITE_MATH_ONLY__
> 
> This symbol will only be defined if and only if all of the following are
set (before pragmas are applied):
>   no_honor_nans { on }
>   no_honor_infinities { on }
> 
> FLT_ROUNDS
> 
> Should be set to -1 (indeterminable) if rounding_mode() is dynamic or 1
(tonearest) if rounding_mode is tonearest. There are values for other rounding
modes, but clang offers no way to set those rounding modes.
> 
> FLT_EVAL_METHOD
> 
> Should be set to -1 if any of allow_reciprocal, allow_approximate_fns, or
allow_reassociation is set. Should any other flags also make this -1? Otherwise,
the setting is target-defined.
> 
> math_errhandling
> 
> The MATH_ERRNO bit will be set or cleared based on the setting of
support_math_errno. Should MATH_ERREXCEPT be set or cleared based on
except_behavior?

FLT_ROUNDS, FLT_EVAL_METHOD and math_errhandling

are controlled by the c runtime, so a compiler has no business
changing them, the compiler can define its own __FLT_ROUNDS,
etc macros and the libc may or may not use those, but e.g.
in case of FLT_ROUNDS it makes no sense for the compiler to
try to do anything: the mode changes at runtime, the libc macro
will expand to a function call that determines the current
rounding mode. (same problem arises if you can change the
other modes on a per function or code block granularity.)

and i don't think it's a good idea to change FLT_EVAL_METHOD
with non-precise arithmetic modes, because it is used to decide
if excess range and precision is available, but arithmetic
changes don't affect that. (e.g. float_t is still same as float).
> 
> 
> Thanks in advance for any opinions and suggestions.
> 
> -Andy
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

About ftrapping-math:
I think we should eliminate ftrapping-math, a boolean option, because it
overlaps with ffp-exception-behavior, a 3 valued option.  Or we can keep it in
the clang driver for compatibility, but it should be rewritten by clang driver
into ffp-exception-behavior=ignore and ffp-exception-behavior=strict.  There are
various fields in llvm and/or clang that maintain Boolean TrappingMath, those
should be removed/rewritten.

About ffp-contract:
Currently the clang driver, at default setting, doesn't specify a value for
ffp-contract, nothing is passed through to llvm in LangOpts or CodegenOpts,
digging around in llvm I find that the default setting in llvm is
"Standard"==on just like you said.  Shall we change clang to always
pass through the option values rather than relying on llvm default?  To me it
seems more secure to have all settings exactly specified.
    static cl::opt<llvm::FPOpFusion::FPOpFusionMode> FuseFPOps(
    "fp-contract", cl::desc("Enable aggressive formation of fused
FP ops"),
    cl::init(FPOpFusion::Standard),
    cl::values(
        clEnumValN(FPOpFusion::Fast, "fast", "Fuse FP ops
whenever profitable"),
        clEnumValN(FPOpFusion::Standard, "on", "Only fuse
'blessed' FP ops."),
        clEnumValN(FPOpFusion::Strict, "off",
                   "Only fuse FP ops when the result won't be
affected.")));


About math-errno:
There are comments in the clang code that describe the settings for math-errno
as toolchain dependent. Here's the initialization:
  // -fmath-errno is the default on some platforms, e.g. BSD-derived OSes.
  bool MathErrno = TC.IsMathErrnoDefault();
  Here's another comment,
      // Turning *off* -ffast-math restores the toolchain default.
      MathErrno = TC.IsMathErrnoDefault();
> -----Original Message-----
> From: Szabolcs Nagy <nsz at port70.net>
> Sent: Tuesday, January 28, 2020 8:09 AM
> To: Kaylor, Andrew <andrew.kaylor at intel.com>
> Cc: cfe-dev at lists.llvm.org; LLVM Developers Mailing List <llvm-
> dev at lists.llvm.org>; Ristow, Warren <warren.ristow at
sony.com>; Ulrich
> Weigand (Ulrich.Weigand at de.ibm.com) <Ulrich.Weigand at
de.ibm.com>;
> Cameron McInally <cameron.mcinally at nyu.edu>; Kevin Neal
> <Kevin.Neal at sas.com>; Blower, Melanie I <melanie.blower at
intel.com>;
> sepavloff at gmail.com; hfinkel at anl.gov; Sanjay Patel
> <spatel at rotateright.com>; Wang, Pengfei <pengfei.wang at
intel.com>
> Subject: Re: [llvm-dev] Floating point semantic modes
> 
> * Kaylor, Andrew via llvm-dev <llvm-dev at lists.llvm.org>
[2020-01-27 23:24:10
> +0000]:
> > Hi all,
> >
> > I'm trying to put together a set of rules for how the various
floating point
> semantic modes should be handled in clang. A lot of this information will
be
> relevant to other front ends, but the details are necessarily bound to a
front end
> implementation so I'm framing the discussion here in terms of clang.
Other front
> ends can choose to follow clang or not. The existence of this set of
semantics is
> an LLVM property that applies to all front ends, but the front ends will
have to
> do something to initialize them.
> >
> > I will eventually do something to convert this into an RST document
and find a
> home for it in the clang documentation, but I'd like to start by
getting input on
> whether everyone agrees with my judgment on how these things should work
> and whether I've missed anything.
> >
> > Here's what I've got.
> >
> 
> i'm not an llvm/clang dev, i hope this mail wont bounce.
> 
> > =====================> > FP semantic modes
> > =====================> > except_behavior { ignore, strict,
may_trap } fenv_access { on, off }
> > rounding_mode { dynamic, tonearest, downward, upward, towardzero }
> > contract { on, off, fast } denormal_fp_math { IEEE, PreserveSign,
> > PositiveZero } denormal_fp32_math { IEEE, PreserveSign, PositiveZero }
> > support_math_errno { on, off }
> 
> note that math errno handling can be
> 
> 1) errno is set,
> 2) errno may be set and
> 3) errno is guaranteed to be untouched
> 
> iso c math_errhandling can select between 1 and 2, (user can or cannot rely
on
> errno) but for optimizing math calls as side-effect-free pure functions, 3
is
> needed.
> 
> -f(no-)math-errno selects between 1 and 3.
> with 3, moving math calls across errno checks or calls that set errno can
break
> semantics depending on how libm is implemented (e.g. glibc will set errno
> independently of how you compiled your code).
> 
> > no_honor_nans { on, off }
> 
> ideally there would be a way to support snan too.
> (e.g. isnan(x) cannot be turned into x!=x then)
> 
> > no_honor_infinities { on, off }
> > no_signed_zeros { on, off }
> > allow_reciprocal { on, off }
> > allow_approximate_fns { on, off }
> > allow_reassociation { on, off }
> 
> excess precision handling is missing from this list which matters for x87
and
> m68k fpu support and may matter for _Float16 implementations that fall back
> to _Float32 arithmetic.
> 
> the granularity of these knobs is also interesting (expression, code block,
> function or translation unit), iso c pragmas work on code block level.
> 
> > =====================> > FP models
> > =====================> > -----------------------
> > precise (default)
> > -----------------------
> > except_behavior { ignore }
> > fenv_access { off }
> > rounding_mode { tonearest }
> > contract { on }
> > denormal_fp_math { IEEE }
> > denormal_fp32_math { IEEE }
> > support_math_errno { on }
> > no_honor_nans { off }
> > no_honor_infinities { off }
> > no_signed_zeros { off }
> > allow_reciprocal { off }
> > allow_approximate_fns { off }
> > allow_reassociation { off }
> >
> > ------------------
> > strict
> > ------------------
> > except_behavior { strict }
> > fenv_access { on }
> > rounding_mode { dynamic }
> > contract { off }
> > denormal_fp_math { IEEE }
> > denormal_fp32_math { IEEE }
> > support_math_errno { on }
> > no_honor_nans { off }
> > no_honor_infinities { off }
> > no_signed_zeros { off }
> > allow_reciprocal { off }
> > allow_approximate_fns { off }
> > allow_reassociation { off }
> >
> > ------------------
> > fast
> > ------------------
> > except_behavior { ignore }
> > fenv_access { off }
> > rounding_mode { tonearest }
> > contract { fast }
> > denormal_fp_math { PreserveSign }
> > denormal_fp32_math { PreserveSign }
> > support_math_errno { off }
> > no_honor_nans { on }
> > no_honor_infinities { on }
> > no_signed_zeros { on }
> > allow_reciprocal { on }
> > allow_approximate_fns { on }
> > allow_reassociation { on }
> >
> > =====================> > Command-line options
> > =====================> > -ffp-model={precise|strict|fast}
> >   Sets all semantic modes as described above.
> >
> > -ffast-math
> >   Equivalent to -ffp-model=fast. (I'm not sure that's
currently true.)
> >
> > -f[no-]math-errno
> > -ffp-contract={on|off|fast}
> > -f[no-]honor-infinities
> > -f[no-]honor-nans
> > -f[no-]associative-math
> > -f[no-]reciprocal-math
> > -f[no-]signed-zeros
> > -f[no-]trapping-math
> > -f[no-]rounding-math
> > -fdenormal-fp-math={ieee, preservesign, positivezero}
> > -fdenormal-fp-math-fp32={ieee, preservesign, positivezero}
> > -ffp-exception-behavior={ignore,maytrap,strict}
> >   Each of these has a 1-to-1 correspondance to an FP semantic mode.
> >   (I think several of these should set "except_behavior" to
"ignore".)
> 
> -ftrapping-math vs -ffp-exception-behaviour=maytrap is unclear.
> 
> (-ftrapping-math is weird in gcc, it does not handle all fp exception
cases, not
> sure what clang plans to do with that)
> 
> >
> > -f[no-]finite-math-only
> >   Controls no_honor_nans and no_honor_infinities.
> >
> > -f[no-]unsafe-math-optimizations
> >   Turns no_signed_zeros, allow_reciprocal, allow_approximate_fns, and
> allow_reassociation on or off.
> >   Also, sets except_behavior to "on" for
-funsafe-math-optimizations.
> >   (Currently, -fno-]unsafe-math-optimizations clears except_behavior,
> > but I regard this as a bug.)
> >
> > All command line options will override any previous values of all
settings they
> control with options taking effect in a left-to-right manner.
> >
> > =====================> > pragmas
> > =====================> > STDC FENV_ACCESS {ON|OFF}
> >   Patch in progress. I think ON should force the following:
> >
> >     except_behavior { strict }
> >     fenv_access { on }
> >     rounding_mode { dynamic }
> >     denormal_fp_math { IEEE }
> >     denormal_fp32_math { IEEE }
> >     no_signed_zeros { off }
> >     allow_reciprocal { off }
> >     allow_approximate_fns { off }
> >     allow_reassociation { off }
> >
> >   And OFF should set fenv_access to off, except_behavior to ignore,
and
> rounding_mode to tonearest. Other modes should be reset to their command
> line defined settings.
> >
> >   I don't think this pragma should have any effect on contract,
> support_math_errno, no_honor_nans, or no_honor_infinities.
> >
> > STDC FP_CONTRACT {ON|OFF|DEFAULT}
> >   This pragma controls the contract FP semantic mode. No other FP
semantic
> modes are effected.
> >
> > float_control ({precise|except}, {on|off}[, push]) float_control (pop)
> >   Patch in progress. These are tricky.
> >   I think they should have the following effects:
> >
> > float_control (precise, on[, push])
> >   contract { on }
> >   denormal_fp_math { IEEE }
> >   denormal_fp32_math { IEEE }
> >   no_signed_zeros { off }
> >   allow_reciprocal { off }
> >   allow_approximate_fns { off }
> >   allow_reassociation { off }
> >
> > float_control (precise, off[, push])
> >   contract { fast }
> >   denormal_fp_math { preservesign }
> >   denormal_fp32_math { preservesign }
> >   no_signed_zeros { on }
> >   allow_reciprocal { on }
> >   allow_approximate_fns { on }
> >   allow_reassociation { on }
> >
> > Note, this is less than what the -ffp-model=precise control does.
Should this
> override support_math_errno, no_honor_nans, or no_honor_infinities?
> >
> > float_control (except, on[, push])
> >   except_behavior { strict }
> >
> > float_control (except, off[, push])
> >   except_behavior { ignore }
> >
> > The MSVC documentation says you can only use the float_control pragma
to
> turn exception semantics on when precise semantics are enabled. For us,
this
> would mean:
> >   denormal_fp_math { IEEE }
> >   denormal_fp32_math { IEEE }
> >   no_signed_zeros { off }
> >   allow_reciprocal { off }
> >   allow_approximate_fns { off }
> >   allow_reassociation { off }
> >
> > The MSVC documentation also says you can't use the float_control
pragma to
> turn excpetion semantics off when precise semantics are enabled, and you
can't
> use the float_control pragma to turn precise off when fenv_access is on.
> >
> > I believe we should follow the MSVC restrictions.
> >
> > ========================> > Code-visible identifiers
> > ========================> > __FAST_MATH__
> >
> > This symbol will only be defined if and only if all of the following
are set
> (before pragmas are applied):
> >   except_behavior { ignore }
> >   fenv_access { off }
> >   rounding_mode { tonearest }
> >   contract { fast }
> >   denormal_fp_math { PreserveSign }
> >   denormal_fp32_math { PreserveSign }
> >   support_math_errno { off }
> >   no_honor_nans { on }
> >   no_honor_infinities { on }
> >   no_signed_zeros { on }
> >   allow_reciprocal { on }
> >   allow_approximate_fns { on }
> >   allow_reassociation { on }
> >
> > __FINITE_MATH_ONLY__
> >
> > This symbol will only be defined if and only if all of the following
are set
> (before pragmas are applied):
> >   no_honor_nans { on }
> >   no_honor_infinities { on }
> >
> > FLT_ROUNDS
> >
> > Should be set to -1 (indeterminable) if rounding_mode() is dynamic or
1
> (tonearest) if rounding_mode is tonearest. There are values for other
rounding
> modes, but clang offers no way to set those rounding modes.
> >
> > FLT_EVAL_METHOD
> >
> > Should be set to -1 if any of allow_reciprocal, allow_approximate_fns,
or
> allow_reassociation is set. Should any other flags also make this -1?
Otherwise,
> the setting is target-defined.
> >
> > math_errhandling
> >
> > The MATH_ERRNO bit will be set or cleared based on the setting of
> support_math_errno. Should MATH_ERREXCEPT be set or cleared based on
> except_behavior?
> 
> 
> FLT_ROUNDS, FLT_EVAL_METHOD and math_errhandling
> 
> are controlled by the c runtime, so a compiler has no business changing
them,
> the compiler can define its own __FLT_ROUNDS, etc macros and the libc may
or
> may not use those, but e.g.
> in case of FLT_ROUNDS it makes no sense for the compiler to try to do
anything:
> the mode changes at runtime, the libc macro will expand to a function call
that
> determines the current rounding mode. (same problem arises if you can
change
> the other modes on a per function or code block granularity.)
> 
> and i don't think it's a good idea to change FLT_EVAL_METHOD with
non-precise
> arithmetic modes, because it is used to decide if excess range and
precision is
> available, but arithmetic changes don't affect that. (e.g. float_t is
still same as
> float).
> 
> >
> >
> > Thanks in advance for any opinions and suggestions.
> >
> > -Andy
> 
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

On Mon, Jan 27, 2020 at 6:24 PM Kaylor, Andrew <andrew.kaylor at
intel.com> wrote:
...
> =====================>
> FP models
>
> =====================>
> -----------------------
>
> precise (default)
>
> -----------------------
>
> except_behavior { ignore }
>
> fenv_access { off }
>
> rounding_mode { tonearest }
>
> contract { on }
>
> denormal_fp_math { IEEE }
>
> denormal_fp32_math { IEEE }
>
> support_math_errno { on }
>
> no_honor_nans { off }
>
> no_honor_infinities { off }
>
> no_signed_zeros { off }
>
> allow_reciprocal { off }
>
> allow_approximate_fns { off }
>
> allow_reassociation { off }
>
>
>
> ------------------
>
> strict
>
> ------------------
>
> except_behavior { strict }
>
> fenv_access { on }
>
> rounding_mode { dynamic }
>
> contract { off }
>
> denormal_fp_math { IEEE }
>
> denormal_fp32_math { IEEE }
>
> support_math_errno { on }
>
> no_honor_nans { off }
>
> no_honor_infinities { off }
>
> no_signed_zeros { off }
>
> allow_reciprocal { off }
>
> allow_approximate_fns { off }
>
> allow_reassociation { off }
>
>
>
> ------------------
>
> fast
>
> ------------------
>
> except_behavior { ignore }
>
> fenv_access { off }
>
> rounding_mode { tonearest }
>
> contract { fast }
>
> denormal_fp_math { PreserveSign }
>
> denormal_fp32_math { PreserveSign }
>
> support_math_errno { off }
>
> no_honor_nans { on }
>
> no_honor_infinities { on }
>
> no_signed_zeros { on }
>
> allow_reciprocal { on }
>
> allow_approximate_fns { on }
>
> allow_reassociation { on }
I misunderstood the purpose of -ffp-model=fast. I assumed that was a
*trap-safe mode* where we care more about performance than strict
IEEE-754 conformance. I.e., I would like a mode where *hard*
trap-unsafe optimizations (e.g. hoisting) are disabled, but *soft*
trap-unsafe optimizations (e.g. reassociation, optimized math libs)
are performed. The thinking is that Invalid, DivisionByZero, and
Overflow are the most important traps, but I'm willing to give up some
edge cases around Overflow.

I suppose that having orthogonal FMFs is a good start, but that would
require updating Reassociate et al. to handle the constrained
intrinsics. From my off-line discussion with Andy, it doesn't sound
like that was a consideration.

Would anyone else like to see a heavily optimized (risky) trap-safe
mode like this?

>
> STDC FENV_ACCESS {ON|OFF}
>   Patch in progress. I think ON should force the following:
>
>     except_behavior { strict }
>     fenv_access { on }
>     rounding_mode { dynamic }
>     denormal_fp_math { IEEE }
>     denormal_fp32_math { IEEE }
>     no_signed_zeros { off }
>     allow_reciprocal { off }
>     allow_approximate_fns { off }
>     allow_reassociation { off }

The pragma `STDC FENV_ACCESS` notifies compiler about access to FP
environment, so `except_behavior`, `fenv_access` and `rounding_mode` should
be set according to this list. But other properties should not be set by
this pragma unless they are required by some of these. `allow_reassociation
{ off }` look like required for ` except_behavior { strict }`. What
prevents us from using `no_signed_zeros { on }` together with the pragma
`STDC FENV_ACCESS`?

Thanks,
--Serge


On Tue, Jan 28, 2020 at 6:24 AM Kaylor, Andrew <andrew.kaylor at
intel.com>
wrote:
> Hi all,
>
>
>
> I’m trying to put together a set of rules for how the various floating
> point semantic modes should be handled in clang. A lot of this information
> will be relevant to other front ends, but the details are necessarily bound
> to a front end implementation so I’m framing the discussion here in terms
> of clang. Other front ends can choose to follow clang or not. The existence
> of this set of semantics is an LLVM property that applies to all front
> ends, but the front ends will have to do something to initialize them.
>
>
>
> I will eventually do something to convert this into an RST document and
> find a home for it in the clang documentation, but I’d like to start by
> getting input on whether everyone agrees with my judgment on how these
> things should work and whether I’ve missed anything.
>
>
>
> Here’s what I’ve got.
>
>
>
> =====================>
> FP semantic modes
>
> =====================>
> except_behavior { ignore, strict, may_trap }
>
> fenv_access { on, off }
>
> rounding_mode { dynamic, tonearest, downward, upward, towardzero }
>
> contract { on, off, fast }
>
> denormal_fp_math { IEEE, PreserveSign, PositiveZero }
>
> denormal_fp32_math { IEEE, PreserveSign, PositiveZero }
>
> support_math_errno { on, off }
>
> no_honor_nans { on, off }
>
> no_honor_infinities { on, off }
>
> no_signed_zeros { on, off }
>
> allow_reciprocal { on, off }
>
> allow_approximate_fns { on, off }
>
> allow_reassociation { on, off }
>
>
>
> ---------------------
>
> Dependencies
>
> ---------------------
>
> rounding_mode must be "tonearest" if fenv_access is
"off" -- It is the
> user's responsibility to make sure the dynamic rounding mode is not
left in
> another state.
>
>
>
> if except_behavior is not "ignore", allow_reciprocal,
> allow_approximate_fns and allow_reassociation must be "off".
>
>
>
> =====================>
> FP models
>
> =====================>
> -----------------------
>
> precise (default)
>
> -----------------------
>
> except_behavior { ignore }
>
> fenv_access { off }
>
> rounding_mode { tonearest }
>
> contract { on }
>
> denormal_fp_math { IEEE }
>
> denormal_fp32_math { IEEE }
>
> support_math_errno { on }
>
> no_honor_nans { off }
>
> no_honor_infinities { off }
>
> no_signed_zeros { off }
>
> allow_reciprocal { off }
>
> allow_approximate_fns { off }
>
> allow_reassociation { off }
>
>
>
> ------------------
>
> strict
>
> ------------------
>
> except_behavior { strict }
>
> fenv_access { on }
>
> rounding_mode { dynamic }
>
> contract { off }
>
> denormal_fp_math { IEEE }
>
> denormal_fp32_math { IEEE }
>
> support_math_errno { on }
>
> no_honor_nans { off }
>
> no_honor_infinities { off }
>
> no_signed_zeros { off }
>
> allow_reciprocal { off }
>
> allow_approximate_fns { off }
>
> allow_reassociation { off }
>
>
>
> ------------------
>
> fast
>
> ------------------
>
> except_behavior { ignore }
>
> fenv_access { off }
>
> rounding_mode { tonearest }
>
> contract { fast }
>
> denormal_fp_math { PreserveSign }
>
> denormal_fp32_math { PreserveSign }
>
> support_math_errno { off }
>
> no_honor_nans { on }
>
> no_honor_infinities { on }
>
> no_signed_zeros { on }
>
> allow_reciprocal { on }
>
> allow_approximate_fns { on }
>
> allow_reassociation { on }
>
>
>
> =====================>
> Command-line options
>
> =====================>
> -ffp-model={precise|strict|fast}
>
>   Sets all semantic modes as described above.
>
>
>
> -ffast-math
>
>   Equivalent to -ffp-model=fast. (I'm not sure that's currently
true.)
>
>
>
> -f[no-]math-errno
>
> -ffp-contract={on|off|fast}
>
> -f[no-]honor-infinities
>
> -f[no-]honor-nans
>
> -f[no-]associative-math
>
> -f[no-]reciprocal-math
>
> -f[no-]signed-zeros
>
> -f[no-]trapping-math
>
> -f[no-]rounding-math
>
> -fdenormal-fp-math={ieee, preservesign, positivezero}
>
> -fdenormal-fp-math-fp32={ieee, preservesign, positivezero}
>
> -ffp-exception-behavior={ignore,maytrap,strict}
>
>   Each of these has a 1-to-1 correspondance to an FP semantic mode.
>
>   (I think several of these should set "except_behavior" to
"ignore".)
>
>
>
>
>
> -f[no-]finite-math-only
>
>   Controls no_honor_nans and no_honor_infinities.
>
>
>
> -f[no-]unsafe-math-optimizations
>
>   Turns no_signed_zeros, allow_reciprocal, allow_approximate_fns, and
> allow_reassociation on or off.
>
>   Also, sets except_behavior to "on" for
-funsafe-math-optimizations.
>
>   (Currently, -fno-]unsafe-math-optimizations clears except_behavior, but
> I regard this as a bug.)
>
>
>
> All command line options will override any previous values of all settings
> they control with options taking effect in a left-to-right manner.
>
>
>
> =====================>
> pragmas
>
> =====================>
> STDC FENV_ACCESS {ON|OFF}
>
>   Patch in progress. I think ON should force the following:
>
>
>
>     except_behavior { strict }
>
>     fenv_access { on }
>
>     rounding_mode { dynamic }
>
>     denormal_fp_math { IEEE }
>
>     denormal_fp32_math { IEEE }
>
>     no_signed_zeros { off }
>
>     allow_reciprocal { off }
>
>     allow_approximate_fns { off }
>
>     allow_reassociation { off }
>
>
>
>   And OFF should set fenv_access to off, except_behavior to ignore, and
> rounding_mode to tonearest. Other modes should be reset to their command
> line defined settings.
>
>
>
>   I don't think this pragma should have any effect on contract,
> support_math_errno, no_honor_nans, or no_honor_infinities.
>
>
>
> STDC FP_CONTRACT {ON|OFF|DEFAULT}
>
>   This pragma controls the contract FP semantic mode. No other FP semantic
> modes are effected.
>
>
>
> float_control ({precise|except}, {on|off}[, push])
>
> float_control (pop)
>
>   Patch in progress. These are tricky.
>
>   I think they should have the following effects:
>
>
>
> float_control (precise, on[, push])
>
>   contract { on }
>
>   denormal_fp_math { IEEE }
>
>   denormal_fp32_math { IEEE }
>
>   no_signed_zeros { off }
>
>   allow_reciprocal { off }
>
>   allow_approximate_fns { off }
>
>   allow_reassociation { off }
>
>
>
> float_control (precise, off[, push])
>
>   contract { fast }
>
>   denormal_fp_math { preservesign }
>
>   denormal_fp32_math { preservesign }
>
>   no_signed_zeros { on }
>
>   allow_reciprocal { on }
>
>   allow_approximate_fns { on }
>
>   allow_reassociation { on }
>
>
>
> Note, this is less than what the -ffp-model=precise control does. Should
> this override support_math_errno, no_honor_nans, or no_honor_infinities?
>
>
>
> float_control (except, on[, push])
>
>   except_behavior { strict }
>
>
>
> float_control (except, off[, push])
>
>   except_behavior { ignore }
>
>
>
> The MSVC documentation says you can only use the float_control pragma to
> turn exception semantics on when precise semantics are enabled. For us,
> this would mean:
>
>   denormal_fp_math { IEEE }
>
>   denormal_fp32_math { IEEE }
>
>   no_signed_zeros { off }
>
>   allow_reciprocal { off }
>
>   allow_approximate_fns { off }
>
>   allow_reassociation { off }
>
>
>
> The MSVC documentation also says you can't use the float_control pragma
to
> turn excpetion semantics off when precise semantics are enabled, and you
> can't use the float_control pragma to turn precise off when fenv_access
is
> on.
>
>
>
> I believe we should follow the MSVC restrictions.
>
>
>
> ========================>
> Code-visible identifiers
>
> ========================>
> __FAST_MATH__
>
>
>
> This symbol will only be defined if and only if all of the following are
> set (before pragmas are applied):
>
>   except_behavior { ignore }
>
>   fenv_access { off }
>
>   rounding_mode { tonearest }
>
>   contract { fast }
>
>   denormal_fp_math { PreserveSign }
>
>   denormal_fp32_math { PreserveSign }
>
>   support_math_errno { off }
>
>   no_honor_nans { on }
>
>   no_honor_infinities { on }
>
>   no_signed_zeros { on }
>
>   allow_reciprocal { on }
>
>   allow_approximate_fns { on }
>
>   allow_reassociation { on }
>
>
>
> __FINITE_MATH_ONLY__
>
>
>
> This symbol will only be defined if and only if all of the following are
> set (before pragmas are applied):
>
>   no_honor_nans { on }
>
>   no_honor_infinities { on }
>
>
>
> FLT_ROUNDS
>
>
>
> Should be set to -1 (indeterminable) if rounding_mode() is dynamic or 1
> (tonearest) if rounding_mode is tonearest. There are values for other
> rounding modes, but clang offers no way to set those rounding modes.
>
>
>
> FLT_EVAL_METHOD
>
>
>
> Should be set to -1 if any of allow_reciprocal, allow_approximate_fns, or
> allow_reassociation is set. Should any other flags also make this -1?
> Otherwise, the setting is target-defined.
>
>
>
> math_errhandling
>
>
>
> The MATH_ERRNO bit will be set or cleared based on the setting of
> support_math_errno. Should MATH_ERREXCEPT be set or cleared based on
> except_behavior?
>
>
>
>
>
> Thanks in advance for any opinions and suggestions.
>
>
>
> -Andy
>
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Yes, you’re probably right about this. I was originally thinking of FENV_ACCESS
as a fully strict mode of operation, but what you’re suggesting aligns with what
Cameron suggested and even some of my own reasoning on other points. So, let me
amend my previous proposal to say:

STDC FENV_ACCESS {ON|OFF}
  Patch in progress. I think ON should force the following:
    except_behavior { strict }
    fenv_access { on }
    rounding_mode { dynamic }
  Other modes should be unchanged.

Thanks,
Andy

From: Serge Pavlov <sepavloff at gmail.com>
Sent: Wednesday, January 29, 2020 9:34 AM
To: Kaylor, Andrew <andrew.kaylor at intel.com>
Cc: cfe-dev at lists.llvm.org; LLVM Developers Mailing List <llvm-dev at
lists.llvm.org>; Ristow, Warren <warren.ristow at sony.com>; Ulrich
Weigand (Ulrich.Weigand at de.ibm.com) <Ulrich.Weigand at de.ibm.com>;
Cameron McInally <cameron.mcinally at nyu.edu>; Kevin Neal <Kevin.Neal
at sas.com>; Blower, Melanie I <melanie.blower at intel.com>; hfinkel
at anl.gov; Sanjay Patel <spatel at rotateright.com>; Wang, Pengfei
<pengfei.wang at intel.com>
Subject: Re: Floating point semantic modes

STDC FENV_ACCESS {ON|OFF}
  Patch in progress. I think ON should force the following:

    except_behavior { strict }
    fenv_access { on }
    rounding_mode { dynamic }
    denormal_fp_math { IEEE }
    denormal_fp32_math { IEEE }
    no_signed_zeros { off }
    allow_reciprocal { off }
    allow_approximate_fns { off }
    allow_reassociation { off }

The pragma `STDC FENV_ACCESS` notifies compiler about access to FP environment,
so `except_behavior`, `fenv_access` and `rounding_mode` should be set according
to this list. But other properties should not be set by this pragma unless they
are required by some of these. `allow_reassociation { off }` look like required
for ` except_behavior { strict }`. What prevents us from using `no_signed_zeros
{ on }` together with the pragma  `STDC FENV_ACCESS`?

Thanks,
--Serge


On Tue, Jan 28, 2020 at 6:24 AM Kaylor, Andrew <andrew.kaylor at
intel.com<mailto:andrew.kaylor at intel.com>> wrote:
Hi all,

I’m trying to put together a set of rules for how the various floating point
semantic modes should be handled in clang. A lot of this information will be
relevant to other front ends, but the details are necessarily bound to a front
end implementation so I’m framing the discussion here in terms of clang. Other
front ends can choose to follow clang or not. The existence of this set of
semantics is an LLVM property that applies to all front ends, but the front ends
will have to do something to initialize them.

I will eventually do something to convert this into an RST document and find a
home for it in the clang documentation, but I’d like to start by getting input
on whether everyone agrees with my judgment on how these things should work and
whether I’ve missed anything.

Here’s what I’ve got.

=====================FP semantic modes
=====================except_behavior { ignore, strict, may_trap }
fenv_access { on, off }
rounding_mode { dynamic, tonearest, downward, upward, towardzero }
contract { on, off, fast }
denormal_fp_math { IEEE, PreserveSign, PositiveZero }
denormal_fp32_math { IEEE, PreserveSign, PositiveZero }
support_math_errno { on, off }
no_honor_nans { on, off }
no_honor_infinities { on, off }
no_signed_zeros { on, off }
allow_reciprocal { on, off }
allow_approximate_fns { on, off }
allow_reassociation { on, off }

---------------------
Dependencies
---------------------
rounding_mode must be "tonearest" if fenv_access is "off" --
It is the user's responsibility to make sure the dynamic rounding mode is
not left in another state.

if except_behavior is not "ignore", allow_reciprocal,
allow_approximate_fns and allow_reassociation must be "off".

=====================FP models
=====================-----------------------
precise (default)
-----------------------
except_behavior { ignore }
fenv_access { off }
rounding_mode { tonearest }
contract { on }
denormal_fp_math { IEEE }
denormal_fp32_math { IEEE }
support_math_errno { on }
no_honor_nans { off }
no_honor_infinities { off }
no_signed_zeros { off }
allow_reciprocal { off }
allow_approximate_fns { off }
allow_reassociation { off }

------------------
strict
------------------
except_behavior { strict }
fenv_access { on }
rounding_mode { dynamic }
contract { off }
denormal_fp_math { IEEE }
denormal_fp32_math { IEEE }
support_math_errno { on }
no_honor_nans { off }
no_honor_infinities { off }
no_signed_zeros { off }
allow_reciprocal { off }
allow_approximate_fns { off }
allow_reassociation { off }

------------------
fast
------------------
except_behavior { ignore }
fenv_access { off }
rounding_mode { tonearest }
contract { fast }
denormal_fp_math { PreserveSign }
denormal_fp32_math { PreserveSign }
support_math_errno { off }
no_honor_nans { on }
no_honor_infinities { on }
no_signed_zeros { on }
allow_reciprocal { on }
allow_approximate_fns { on }
allow_reassociation { on }

=====================Command-line options
=====================-ffp-model={precise|strict|fast}
  Sets all semantic modes as described above.

-ffast-math
  Equivalent to -ffp-model=fast. (I'm not sure that's currently true.)

-f[no-]math-errno
-ffp-contract={on|off|fast}
-f[no-]honor-infinities
-f[no-]honor-nans
-f[no-]associative-math
-f[no-]reciprocal-math
-f[no-]signed-zeros
-f[no-]trapping-math
-f[no-]rounding-math
-fdenormal-fp-math={ieee, preservesign, positivezero}
-fdenormal-fp-math-fp32={ieee, preservesign, positivezero}
-ffp-exception-behavior={ignore,maytrap,strict}
  Each of these has a 1-to-1 correspondance to an FP semantic mode.
  (I think several of these should set "except_behavior" to
"ignore".)


-f[no-]finite-math-only
  Controls no_honor_nans and no_honor_infinities.

-f[no-]unsafe-math-optimizations
  Turns no_signed_zeros, allow_reciprocal, allow_approximate_fns, and
allow_reassociation on or off.
  Also, sets except_behavior to "on" for -funsafe-math-optimizations.
  (Currently, -fno-]unsafe-math-optimizations clears except_behavior, but I
regard this as a bug.)

All command line options will override any previous values of all settings they
control with options taking effect in a left-to-right manner.

=====================pragmas
=====================STDC FENV_ACCESS {ON|OFF}
  Patch in progress. I think ON should force the following:

    except_behavior { strict }
    fenv_access { on }
    rounding_mode { dynamic }
    denormal_fp_math { IEEE }
    denormal_fp32_math { IEEE }
    no_signed_zeros { off }
    allow_reciprocal { off }
    allow_approximate_fns { off }
    allow_reassociation { off }

  And OFF should set fenv_access to off, except_behavior to ignore, and
rounding_mode to tonearest. Other modes should be reset to their command line
defined settings.

  I don't think this pragma should have any effect on contract,
support_math_errno, no_honor_nans, or no_honor_infinities.

STDC FP_CONTRACT {ON|OFF|DEFAULT}
  This pragma controls the contract FP semantic mode. No other FP semantic modes
are effected.

float_control ({precise|except}, {on|off}[, push])
float_control (pop)
  Patch in progress. These are tricky.
  I think they should have the following effects:

float_control (precise, on[, push])
  contract { on }
  denormal_fp_math { IEEE }
  denormal_fp32_math { IEEE }
  no_signed_zeros { off }
  allow_reciprocal { off }
  allow_approximate_fns { off }
  allow_reassociation { off }

float_control (precise, off[, push])
  contract { fast }
  denormal_fp_math { preservesign }
  denormal_fp32_math { preservesign }
  no_signed_zeros { on }
  allow_reciprocal { on }
  allow_approximate_fns { on }
  allow_reassociation { on }

Note, this is less than what the -ffp-model=precise control does. Should this
override support_math_errno, no_honor_nans, or no_honor_infinities?

float_control (except, on[, push])
  except_behavior { strict }

float_control (except, off[, push])
  except_behavior { ignore }

The MSVC documentation says you can only use the float_control pragma to turn
exception semantics on when precise semantics are enabled. For us, this would
mean:
  denormal_fp_math { IEEE }
  denormal_fp32_math { IEEE }
  no_signed_zeros { off }
  allow_reciprocal { off }
  allow_approximate_fns { off }
  allow_reassociation { off }

The MSVC documentation also says you can't use the float_control pragma to
turn excpetion semantics off when precise semantics are enabled, and you
can't use the float_control pragma to turn precise off when fenv_access is
on.

I believe we should follow the MSVC restrictions.

========================Code-visible identifiers
========================__FAST_MATH__

This symbol will only be defined if and only if all of the following are set
(before pragmas are applied):
  except_behavior { ignore }
  fenv_access { off }
  rounding_mode { tonearest }
  contract { fast }
  denormal_fp_math { PreserveSign }
  denormal_fp32_math { PreserveSign }
  support_math_errno { off }
  no_honor_nans { on }
  no_honor_infinities { on }
  no_signed_zeros { on }
  allow_reciprocal { on }
  allow_approximate_fns { on }
  allow_reassociation { on }

__FINITE_MATH_ONLY__

This symbol will only be defined if and only if all of the following are set
(before pragmas are applied):
  no_honor_nans { on }
  no_honor_infinities { on }

FLT_ROUNDS

Should be set to -1 (indeterminable) if rounding_mode() is dynamic or 1
(tonearest) if rounding_mode is tonearest. There are values for other rounding
modes, but clang offers no way to set those rounding modes.

FLT_EVAL_METHOD

Should be set to -1 if any of allow_reciprocal, allow_approximate_fns, or
allow_reassociation is set. Should any other flags also make this -1? Otherwise,
the setting is target-defined.

math_errhandling

The MATH_ERRNO bit will be set or cleared based on the setting of
support_math_errno. Should MATH_ERREXCEPT be set or cleared based on
except_behavior?


Thanks in advance for any opinions and suggestions.

-Andy
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> ... math errno ...
I wouldn't recommend to anyone that they should rely on math errno (because
I don't trust libraries to correctly support it). My goal here was to
incorporate our existing support for it into the rest of what I'm trying to
document.

My understanding is that for clang this primarily controls whether or not we
feel free to substitute intrinsics for recognized math library calls. I
don't know if we have any code in the optimizer that introduces access to or
modification of errno in the user's program. The library calls that test
should always act as barriers to one another.

> ideally there would be a way to support snan too. (e.g. isnan(x) cannot be
turned into x!=x then)
The except_behavior mode is supposed to handle this. The LLVM support for
constrained intrinsics is considering all manner of FP exceptions that could be
raised, including the distinction between QNaN and SNaN. The default LLVM IR
definition does not support this distinction.

We seem to have an issue with isnan() in clang though. If you call isnan() you
get a call to __isnan() which should be fine (assuming the library does the
right thing), but we're translating __builtin_isnan() to x!=x. That's
not what we should be doing if except_behavior isn't "ignore".

> excess precision handling is missing from this list which matters for x87
and m68k fpu support and may matter for _Float16 implementations that fall back
to _Float32 arithmetic.
Yeah, we don't currently have any support for controlling that, at least in
the x87 case. I think our current strategy is nothing more than setting
FLT_EVAL_METHOD to reflect that we might not be using source precision for
intermediate results. This is something we should consider adding.

> the granularity of these knobs is also interesting (expression, code block,
function or translation unit), iso c pragmas work on code block level.
I'd have to defer to someone more familiar with the front end to say how
that is handled.

> -ftrapping-math vs -ffp-exception-behaviour=maytrap is unclear.
The "maytrap" setting is supposed to prevent the optimizer from
introducing spurious exceptions (e.g. speculative execution) while still
allowing it to optimize away potential exceptions (e.g. dce operations whose
results are never used).

> FLT_ROUNDS, FLT_EVAL_METHOD and math_errhandling are controlled by the c
runtime, so a compiler has no business changing them, the compiler can define
its own __FLT_ROUNDS, etc macros and the libc may or may not use those, but e.g.
in case of FLT_ROUNDS it makes no sense for the compiler to try to do anything:
the mode changes at runtime, the libc macro will expand to a function call that
determines the current rounding mode. (same problem arises if you can change the
other modes on a per function or code block granularity.)
I don't think I understand what you're saying here. FLT_ROUNDS, in
particular, I thought was supposed to be implemented without reference to the
runtime library. In clang we're mapping this to an intrinsic that gets a
target-specific inline expansion. For example: https://godbolt.org/z/uQD-AF

And FLT_EVAL_METHOD I take to be an indicator of how the compiler is handling
intermediate results.



-----Original Message-----
From: Szabolcs Nagy <nsz at port70.net> 
Sent: Tuesday, January 28, 2020 5:09 AM
To: Kaylor, Andrew <andrew.kaylor at intel.com>
Cc: cfe-dev at lists.llvm.org; LLVM Developers Mailing List <llvm-dev at
lists.llvm.org>; Ristow, Warren <warren.ristow at sony.com>; Ulrich
Weigand (Ulrich.Weigand at de.ibm.com) <Ulrich.Weigand at de.ibm.com>;
Cameron McInally <cameron.mcinally at nyu.edu>; Kevin Neal <Kevin.Neal
at sas.com>; Blower, Melanie I <melanie.blower at intel.com>; sepavloff
at gmail.com; hfinkel at anl.gov; Sanjay Patel <spatel at
rotateright.com>; Wang, Pengfei <pengfei.wang at intel.com>
Subject: Re: [llvm-dev] Floating point semantic modes

* Kaylor, Andrew via llvm-dev <llvm-dev at lists.llvm.org> [2020-01-27
23:24:10 +0000]:
> Hi all,
> 
> I'm trying to put together a set of rules for how the various floating
point semantic modes should be handled in clang. A lot of this information will
be relevant to other front ends, but the details are necessarily bound to a
front end implementation so I'm framing the discussion here in terms of
clang. Other front ends can choose to follow clang or not. The existence of this
set of semantics is an LLVM property that applies to all front ends, but the
front ends will have to do something to initialize them.
> 
> I will eventually do something to convert this into an RST document and
find a home for it in the clang documentation, but I'd like to start by
getting input on whether everyone agrees with my judgment on how these things
should work and whether I've missed anything.
> 
> Here's what I've got.
> 
i'm not an llvm/clang dev, i hope this mail wont bounce.
> =====================> FP semantic modes
> =====================> except_behavior { ignore, strict, may_trap }
fenv_access { on, off }
> rounding_mode { dynamic, tonearest, downward, upward, towardzero } 
> contract { on, off, fast } denormal_fp_math { IEEE, PreserveSign, 
> PositiveZero } denormal_fp32_math { IEEE, PreserveSign, PositiveZero } 
> support_math_errno { on, off }
note that math errno handling can be

1) errno is set,
2) errno may be set and
3) errno is guaranteed to be untouched

iso c math_errhandling can select between 1 and 2, (user can or cannot rely on
errno) but for optimizing math calls as side-effect-free pure functions, 3 is
needed.

-f(no-)math-errno selects between 1 and 3.
with 3, moving math calls across errno checks or calls that set errno can break
semantics depending on how libm is implemented (e.g. glibc will set errno
independently of how you compiled your code).
> no_honor_nans { on, off }
ideally there would be a way to support snan too.
(e.g. isnan(x) cannot be turned into x!=x then)
> no_honor_infinities { on, off }
> no_signed_zeros { on, off }
> allow_reciprocal { on, off }
> allow_approximate_fns { on, off }
> allow_reassociation { on, off }
excess precision handling is missing from this list which matters for x87 and
m68k fpu support and may matter for _Float16 implementations that fall back to
_Float32 arithmetic.

the granularity of these knobs is also interesting (expression, code block,
function or translation unit), iso c pragmas work on code block level.
> =====================> FP models
> =====================> -----------------------
> precise (default)
> -----------------------
> except_behavior { ignore }
> fenv_access { off }
> rounding_mode { tonearest }
> contract { on }
> denormal_fp_math { IEEE }
> denormal_fp32_math { IEEE }
> support_math_errno { on }
> no_honor_nans { off }
> no_honor_infinities { off }
> no_signed_zeros { off }
> allow_reciprocal { off }
> allow_approximate_fns { off }
> allow_reassociation { off }
> 
> ------------------
> strict
> ------------------
> except_behavior { strict }
> fenv_access { on }
> rounding_mode { dynamic }
> contract { off }
> denormal_fp_math { IEEE }
> denormal_fp32_math { IEEE }
> support_math_errno { on }
> no_honor_nans { off }
> no_honor_infinities { off }
> no_signed_zeros { off }
> allow_reciprocal { off }
> allow_approximate_fns { off }
> allow_reassociation { off }
> 
> ------------------
> fast
> ------------------
> except_behavior { ignore }
> fenv_access { off }
> rounding_mode { tonearest }
> contract { fast }
> denormal_fp_math { PreserveSign }
> denormal_fp32_math { PreserveSign }
> support_math_errno { off }
> no_honor_nans { on }
> no_honor_infinities { on }
> no_signed_zeros { on }
> allow_reciprocal { on }
> allow_approximate_fns { on }
> allow_reassociation { on }
> 
> =====================> Command-line options
> =====================> -ffp-model={precise|strict|fast}
>   Sets all semantic modes as described above.
> 
> -ffast-math
>   Equivalent to -ffp-model=fast. (I'm not sure that's currently
true.)
> 
> -f[no-]math-errno
> -ffp-contract={on|off|fast}
> -f[no-]honor-infinities
> -f[no-]honor-nans
> -f[no-]associative-math
> -f[no-]reciprocal-math
> -f[no-]signed-zeros
> -f[no-]trapping-math
> -f[no-]rounding-math
> -fdenormal-fp-math={ieee, preservesign, positivezero} 
> -fdenormal-fp-math-fp32={ieee, preservesign, positivezero} 
> -ffp-exception-behavior={ignore,maytrap,strict}
>   Each of these has a 1-to-1 correspondance to an FP semantic mode.
>   (I think several of these should set "except_behavior" to
"ignore".)
-ftrapping-math vs -ffp-exception-behaviour=maytrap is unclear.

(-ftrapping-math is weird in gcc, it does not handle all fp exception cases, not
sure what clang plans to do with that)
> 
> -f[no-]finite-math-only
>   Controls no_honor_nans and no_honor_infinities.
> 
> -f[no-]unsafe-math-optimizations
>   Turns no_signed_zeros, allow_reciprocal, allow_approximate_fns, and
allow_reassociation on or off.
>   Also, sets except_behavior to "on" for
-funsafe-math-optimizations.
>   (Currently, -fno-]unsafe-math-optimizations clears except_behavior, 
> but I regard this as a bug.)
> 
> All command line options will override any previous values of all settings
they control with options taking effect in a left-to-right manner.
> 
> =====================> pragmas
> =====================> STDC FENV_ACCESS {ON|OFF}
>   Patch in progress. I think ON should force the following:
> 
>     except_behavior { strict }
>     fenv_access { on }
>     rounding_mode { dynamic }
>     denormal_fp_math { IEEE }
>     denormal_fp32_math { IEEE }
>     no_signed_zeros { off }
>     allow_reciprocal { off }
>     allow_approximate_fns { off }
>     allow_reassociation { off }
> 
>   And OFF should set fenv_access to off, except_behavior to ignore, and
rounding_mode to tonearest. Other modes should be reset to their command line
defined settings.
> 
>   I don't think this pragma should have any effect on contract,
support_math_errno, no_honor_nans, or no_honor_infinities.
> 
> STDC FP_CONTRACT {ON|OFF|DEFAULT}
>   This pragma controls the contract FP semantic mode. No other FP semantic
modes are effected.
> 
> float_control ({precise|except}, {on|off}[, push]) float_control (pop)
>   Patch in progress. These are tricky.
>   I think they should have the following effects:
> 
> float_control (precise, on[, push])
>   contract { on }
>   denormal_fp_math { IEEE }
>   denormal_fp32_math { IEEE }
>   no_signed_zeros { off }
>   allow_reciprocal { off }
>   allow_approximate_fns { off }
>   allow_reassociation { off }
> 
> float_control (precise, off[, push])
>   contract { fast }
>   denormal_fp_math { preservesign }
>   denormal_fp32_math { preservesign }
>   no_signed_zeros { on }
>   allow_reciprocal { on }
>   allow_approximate_fns { on }
>   allow_reassociation { on }
> 
> Note, this is less than what the -ffp-model=precise control does. Should
this override support_math_errno, no_honor_nans, or no_honor_infinities?
> 
> float_control (except, on[, push])
>   except_behavior { strict }
> 
> float_control (except, off[, push])
>   except_behavior { ignore }
> 
> The MSVC documentation says you can only use the float_control pragma to
turn exception semantics on when precise semantics are enabled. For us, this
would mean:
>   denormal_fp_math { IEEE }
>   denormal_fp32_math { IEEE }
>   no_signed_zeros { off }
>   allow_reciprocal { off }
>   allow_approximate_fns { off }
>   allow_reassociation { off }
> 
> The MSVC documentation also says you can't use the float_control pragma
to turn excpetion semantics off when precise semantics are enabled, and you
can't use the float_control pragma to turn precise off when fenv_access is
on.
> 
> I believe we should follow the MSVC restrictions.
> 
> ========================> Code-visible identifiers
> ========================> __FAST_MATH__
> 
> This symbol will only be defined if and only if all of the following are
set (before pragmas are applied):
>   except_behavior { ignore }
>   fenv_access { off }
>   rounding_mode { tonearest }
>   contract { fast }
>   denormal_fp_math { PreserveSign }
>   denormal_fp32_math { PreserveSign }
>   support_math_errno { off }
>   no_honor_nans { on }
>   no_honor_infinities { on }
>   no_signed_zeros { on }
>   allow_reciprocal { on }
>   allow_approximate_fns { on }
>   allow_reassociation { on }
> 
> __FINITE_MATH_ONLY__
> 
> This symbol will only be defined if and only if all of the following are
set (before pragmas are applied):
>   no_honor_nans { on }
>   no_honor_infinities { on }
> 
> FLT_ROUNDS
> 
> Should be set to -1 (indeterminable) if rounding_mode() is dynamic or 1
(tonearest) if rounding_mode is tonearest. There are values for other rounding
modes, but clang offers no way to set those rounding modes.
> 
> FLT_EVAL_METHOD
> 
> Should be set to -1 if any of allow_reciprocal, allow_approximate_fns, or
allow_reassociation is set. Should any other flags also make this -1? Otherwise,
the setting is target-defined.
> 
> math_errhandling
> 
> The MATH_ERRNO bit will be set or cleared based on the setting of
support_math_errno. Should MATH_ERREXCEPT be set or cleared based on
except_behavior?

FLT_ROUNDS, FLT_EVAL_METHOD and math_errhandling

are controlled by the c runtime, so a compiler has no business changing them,
the compiler can define its own __FLT_ROUNDS, etc macros and the libc may or may
not use those, but e.g.
in case of FLT_ROUNDS it makes no sense for the compiler to try to do anything:
the mode changes at runtime, the libc macro will expand to a function call that
determines the current rounding mode. (same problem arises if you can change the
other modes on a per function or code block granularity.)

and i don't think it's a good idea to change FLT_EVAL_METHOD with
non-precise arithmetic modes, because it is used to decide if excess range and
precision is available, but arithmetic changes don't affect that. (e.g.
float_t is still same as float).
> 
> 
> Thanks in advance for any opinions and suggestions.
> 
> -Andy
> _______________________________________________
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