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Finkel, Hal J. via llvm-dev

2019-Apr-17 17:31 UTC

[llvm-dev] [FP] Constant folding math library functions

On 4/16/19 4:18 PM, Amara Emerson wrote:


On Apr 16, 2019, at 1:47 PM, Kaylor, Andrew via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Thanks, Hal.

I hear what you are saying about the accuracy. The problem, from my perspective,
is trying to explain to users what they are going to get. The constant folding
may be as accurate as the lib call would have been, but it isn’t necessarily
value safe. I’ve been operating on the assumption that LLVM’s FP optimizations
are value safe unless fast math flags are used. For the most part that appears
to be true. This case breaks my assumption.

I realize that any call to a library function puts claims of value safety on
shaky ground, but the standard I’m going for is that you’ll get the same bitwise
results compiling at -O0 as you will at -O2 (for instance).

That said, I agree that the difference between constant folding a library call
and substituting an approximate calculation is significant. Most users would
probably prefer to have this optimization enabled by default. It just leads to a
kind of murky answer to the question of whether or not we’re value safe by
default for the users who do care about that.
I guess what I’m saying is that I do like the idea of a separate flag for this,
though as I recall we’re running out of bits for fast math flags. I’m also not
sure whether it should be on by default. If we want to permit this
transformation by default, then it shouldn’t be a fast math flag. Probably an
attribute on the call site is better? And in that case it feels like we’d be
circling back toward “nobuiltin” but can the front end identify which call sites
would need that?
Could this not be a function attribute if it’s intended to be consistent across
entire functions/programs?

I agree with Hal that afn doesn’t sound like the right approach, and in terms of
how the compiler actually treats these calls (I’m thinking about more than
constant folding here) then it seems to be that this is the same as
-fno-builtin. For example, can the optimizer assume some properties about the
result value of a call if it knowns some (partial) information about the
argument (e.g. sign)? If we prevent constant folding then that also precludes
this kind of optimization. Perhaps an umbrella flag like -fno-builtin-math-lib
that would turn on -fno-builtin for all of the libm functions?


I certainly think that the umbrella flag is a good approach. The tricky part is
implementing it so that Clang does not need to have a list of the relevant
functions that LLVM's optimizer knows about (thus, I don't think that we
can implement this just by having Clang add metadata to a predefined list of
math functions).

 -Hal


Amara

-Andy

From: Finkel, Hal J. <hfinkel at anl.gov<mailto:hfinkel at anl.gov>>
Sent: Tuesday, April 16, 2019 1:01 PM
To: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>>; Kaylor, Andrew <andrew.kaylor at
intel.com<mailto:andrew.kaylor at intel.com>>
Subject: Re: [FP] Constant folding math library functions

Hi, Andy,

This is somewhat tricky. 'afn' is for approximate functions, to
"allow substitution of approximate calculations for functions", but in
this case, the answers aren't any more approximate than the original
function calls. Different, but likely no less accurate. This has long caused
these kinds of subtle differences when cross compiling, etc. but it's not
clear what the best thing to do actually is. Users often want the constant
folding, and I've certainly seen code where the performance depends
critically on it, and yet, the compiler will likely never be able to exactly
replicate the behavior of whatever libm implementation is used at runtime. Maybe
having a dedicated flag to disable just this behavior, aside from suggesting
that users use -fno-builtin=..., would be useful for users who depend on the
compiler not folding these kinds of expressions in ways that might differ from
their runtime libm behavior?

 -Hal

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces
at lists.llvm.org>> on behalf of Kaylor, Andrew via llvm-dev <llvm-dev
at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Sent: Tuesday, April 16, 2019 2:23 PM
To: llvm-dev
Subject: [llvm-dev] [FP] Constant folding math library functions

Hi everyone,

I noticed today that LLVM’s constant folding of math library functions can lead
to minor differences in results. A colleague sent me the following test case
which demonstrates the issue:

#include <stdio.h>
#include <math.h>

typedef union {
  double d;
  unsigned long long i;
} my_dbl;

int main(void) {
  my_dbl res, x;
  x.i = 0x3feeb39556255de2ull;
  res.d = tanh(x.d);
  printf("tanh(%f) = %f = %016LX\n", x.d, res.d, res.i);
  return 0;
}

Compiling with “clang -O2 -g0 -emit-llvm” I get this:

define dso_local i32 @main() local_unnamed_addr #0 {
  %1 = tail call double @tanh(double 0x3FEEB39556255DE2) #2
  %2 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([24 x i8],
[24 x i8]* @.str, i64 0, i64 0),
                                                             double
0x3FEEB39556255DE2, double 0x3FE7CF009CE7F169,
                                                             i64
4604876745549017449)
  ret i32 0
}

We’re still calling ‘tanh’ but all the values passed to printf are constant
folded. The constant folding is based on a call to tanh made by the compiler.
The problem with this is that if I am linking my program against a different
version of the math library than was used by the compiler I may get a different
result.

I can prevent this constant folding with either the ‘nobuiltin’ or ‘strictfp’
attribute. However, it seems to me like this optimization should really be
checking the ‘afn’ fast math flag.

Opinions?

Thanks,
Andy

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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Scott Manley via llvm-dev

2019-Apr-17 19:35 UTC

head link

[llvm-dev] [FP] Constant folding math library functions

Hi,

I don't mean to hijack this thread but after reading through some
discussion here and poking around in the constant folder myself just now,
is my understanding correct in that LLVM only uses host routines for
constant folding? Has there been discussion in the past about using MPFR or
something like it instead? (I'm aware that doing so wouldn't solve the
original problem in the general case -- I'm asking out of curiosity).

What happens currently when cross compiling and endianness does not match
between host/target? What about folding types that are unsupported on the
host (such as __float128)? Does LLVM reject the fold in those cases?

Regards,

Scott

On Wed, Apr 17, 2019 at 10:31 AM Finkel, Hal J. via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> On 4/16/19 4:18 PM, Amara Emerson wrote:
>
>
>
> On Apr 16, 2019, at 1:47 PM, Kaylor, Andrew via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Thanks, Hal.
>
> I hear what you are saying about the accuracy. The problem, from my
> perspective, is trying to explain to users what they are going to get. The
> constant folding may be as accurate as the lib call would have been, but it
> isn’t necessarily value safe. I’ve been operating on the assumption that
> LLVM’s FP optimizations are value safe unless fast math flags are used. For
> the most part that appears to be true. This case breaks my assumption.
>
> I realize that any call to a library function puts claims of value safety
> on shaky ground, but the standard I’m going for is that you’ll get the same
> bitwise results compiling at -O0 as you will at -O2 (for instance).
>
> That said, I agree that the difference between constant folding a library
> call and substituting an approximate calculation is significant. Most users
> would probably prefer to have this optimization enabled by default. It just
> leads to a kind of murky answer to the question of whether or not we’re
> value safe by default for the users who do care about that.
> I guess what I’m saying is that I do like the idea of a separate flag for
> this, though as I recall we’re running out of bits for fast math flags. I’m
> also not sure whether it should be on by default. If we want to permit this
> transformation by default, then it shouldn’t be a fast math flag. Probably
> an attribute on the call site is better? And in that case it feels like
> we’d be circling back toward “nobuiltin” but can the front end identify
> which call sites would need that?
>
> Could this not be a function attribute if it’s intended to be consistent
> across entire functions/programs?
>
> I agree with Hal that afn doesn’t sound like the right approach, and in
> terms of how the compiler actually treats these calls (I’m thinking about
> more than constant folding here) then it seems to be that this is the same
> as -fno-builtin. For example, can the optimizer assume some properties
> about the result value of a call if it knowns some (partial) information
> about the argument (e.g. sign)? If we prevent constant folding then that
> also precludes this kind of optimization. Perhaps an umbrella flag like
> -fno-builtin-math-lib that would turn on -fno-builtin for all of the libm
> functions?
>
>
> I certainly think that the umbrella flag is a good approach. The tricky
> part is implementing it so that Clang does not need to have a list of the
> relevant functions that LLVM's optimizer knows about (thus, I don't
think
> that we can implement this just by having Clang add metadata to a
> predefined list of math functions).
>
>  -Hal
>
>
>
> Amara
>
>
> -Andy
>
> *From:* Finkel, Hal J. <hfinkel at anl.gov>
> *Sent:* Tuesday, April 16, 2019 1:01 PM
> *To:* llvm-dev <llvm-dev at lists.llvm.org>; Kaylor, Andrew <
> andrew.kaylor at intel.com>
> *Subject:* Re: [FP] Constant folding math library functions
>
> Hi, Andy,
>
> This is somewhat tricky. 'afn' is for approximate functions, to
"allow
> substitution of approximate calculations for functions", but in this
case,
> the answers aren't any more approximate than the original function
calls.
> Different, but likely no less accurate. This has long caused these kinds of
> subtle differences when cross compiling, etc. but it's not clear what
the
> best thing to do actually is. Users often want the constant folding, and
> I've certainly seen code where the performance depends critically on
it,
> and yet, the compiler will likely never be able to exactly replicate the
> behavior of whatever libm implementation is used at runtime. Maybe having a
> dedicated flag to disable just this behavior, aside from suggesting that
> users use -fno-builtin=..., would be useful for users who depend on the
> compiler not folding these kinds of expressions in ways that might differ
> from their runtime libm behavior?
>
>  -Hal
>
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> ------------------------------
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
Kaylor,
> Andrew via llvm-dev <llvm-dev at lists.llvm.org>
> *Sent:* Tuesday, April 16, 2019 2:23 PM
> *To:* llvm-dev
> *Subject:* [llvm-dev] [FP] Constant folding math library functions
>
> Hi everyone,
>
> I noticed today that LLVM’s constant folding of math library functions can
> lead to minor differences in results. A colleague sent me the following
> test case which demonstrates the issue:
>
> #include <stdio.h>
> #include <math.h>
>
> typedef union {
>   double d;
>   unsigned long long i;
> } my_dbl;
>
> int main(void) {
>   my_dbl res, x;
>   x.i = 0x3feeb39556255de2ull;
>   res.d = tanh(x.d);
>   printf("tanh(%f) = %f = %016LX\n", x.d, res.d, res.i);
>   return 0;
> }
>
> Compiling with “clang -O2 -g0 -emit-llvm” I get this:
>
> define dso_local i32 @main() local_unnamed_addr #0 {
>   %1 = tail call double @tanh(double 0x3FEEB39556255DE2) #2
>   %2 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([24 x
> i8], [24 x i8]* @.str, i64 0, i64 0),
>                                                              double
> 0x3FEEB39556255DE2, double 0x3FE7CF009CE7F169,
>                                                              i64
> 4604876745549017449)
>   ret i32 0
> }
>
> We’re still calling ‘tanh’ but all the values passed to printf are
> constant folded. The constant folding is based on a call to tanh made by
> the compiler. The problem with this is that if I am linking my program
> against a different version of the math library than was used by the
> compiler I may get a different result.
>
> I can prevent this constant folding with either the ‘nobuiltin’ or
> ‘strictfp’ attribute. However, it seems to me like this optimization should
> really be checking the ‘afn’ fast math flag.
>
> Opinions?
>
> Thanks,
> Andy
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Finkel, Hal J. via llvm-dev

2019-Apr-17 20:08 UTC

head link

[llvm-dev] [FP] Constant folding math library functions

Scott,

Yes, LLVM uses host routines for constant-folding math functions (e.g., sin,
cos). For arithmetic operations, LLVM has its own builtin implementations
(called APFloat). I know people have talked about having our own implementations
of trigonometric functions, etc., but I don't believe that anyone has viewed
this as a high-priority item. We wouldn't use MPFR as a core dependency for
licensing reasons, but in general, we shy away from any external dependencies
that would affect the output of the compiler. In any case, as you say, this
doesn't solve the problem of matching the libm at runtime.

This can cause issues for cross compiling for the same reason it can cause
issues when the runtime libm doesn't match the one used by LLVM itself. The
values are properly encoded for the target regardless, however, so there's
no endianness concern. We only ever fold the float and double variants of these
calls, so we don't need to worry about host support for 128-bit float
formats (see lib/Analysis/ConstantFolding.cpp).

 -Hal

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

________________________________
From: Scott Manley <rscottmanley at gmail.com>
Sent: Wednesday, April 17, 2019 2:35 PM
To: Finkel, Hal J.
Cc: Amara Emerson; Kaylor, Andrew; llvm-dev
Subject: Re: [llvm-dev] [FP] Constant folding math library functions

Hi,

I don't mean to hijack this thread but after reading through some discussion
here and poking around in the constant folder myself just now, is my
understanding correct in that LLVM only uses host routines for constant folding?
Has there been discussion in the past about using MPFR or something like it
instead? (I'm aware that doing so wouldn't solve the original problem in
the general case -- I'm asking out of curiosity).

What happens currently when cross compiling and endianness does not match
between host/target? What about folding types that are unsupported on the host
(such as __float128)? Does LLVM reject the fold in those cases?

Regards,

Scott

On Wed, Apr 17, 2019 at 10:31 AM Finkel, Hal J. via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:


On 4/16/19 4:18 PM, Amara Emerson wrote:


On Apr 16, 2019, at 1:47 PM, Kaylor, Andrew via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Thanks, Hal.

I hear what you are saying about the accuracy. The problem, from my perspective,
is trying to explain to users what they are going to get. The constant folding
may be as accurate as the lib call would have been, but it isn’t necessarily
value safe. I’ve been operating on the assumption that LLVM’s FP optimizations
are value safe unless fast math flags are used. For the most part that appears
to be true. This case breaks my assumption.

I realize that any call to a library function puts claims of value safety on
shaky ground, but the standard I’m going for is that you’ll get the same bitwise
results compiling at -O0 as you will at -O2 (for instance).

That said, I agree that the difference between constant folding a library call
and substituting an approximate calculation is significant. Most users would
probably prefer to have this optimization enabled by default. It just leads to a
kind of murky answer to the question of whether or not we’re value safe by
default for the users who do care about that.
I guess what I’m saying is that I do like the idea of a separate flag for this,
though as I recall we’re running out of bits for fast math flags. I’m also not
sure whether it should be on by default. If we want to permit this
transformation by default, then it shouldn’t be a fast math flag. Probably an
attribute on the call site is better? And in that case it feels like we’d be
circling back toward “nobuiltin” but can the front end identify which call sites
would need that?
Could this not be a function attribute if it’s intended to be consistent across
entire functions/programs?

I agree with Hal that afn doesn’t sound like the right approach, and in terms of
how the compiler actually treats these calls (I’m thinking about more than
constant folding here) then it seems to be that this is the same as
-fno-builtin. For example, can the optimizer assume some properties about the
result value of a call if it knowns some (partial) information about the
argument (e.g. sign)? If we prevent constant folding then that also precludes
this kind of optimization. Perhaps an umbrella flag like -fno-builtin-math-lib
that would turn on -fno-builtin for all of the libm functions?


I certainly think that the umbrella flag is a good approach. The tricky part is
implementing it so that Clang does not need to have a list of the relevant
functions that LLVM's optimizer knows about (thus, I don't think that we
can implement this just by having Clang add metadata to a predefined list of
math functions).

 -Hal


Amara

-Andy

From: Finkel, Hal J. <hfinkel at anl.gov<mailto:hfinkel at anl.gov>>
Sent: Tuesday, April 16, 2019 1:01 PM
To: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>>; Kaylor, Andrew <andrew.kaylor at
intel.com<mailto:andrew.kaylor at intel.com>>
Subject: Re: [FP] Constant folding math library functions

Hi, Andy,

This is somewhat tricky. 'afn' is for approximate functions, to
"allow substitution of approximate calculations for functions", but in
this case, the answers aren't any more approximate than the original
function calls. Different, but likely no less accurate. This has long caused
these kinds of subtle differences when cross compiling, etc. but it's not
clear what the best thing to do actually is. Users often want the constant
folding, and I've certainly seen code where the performance depends
critically on it, and yet, the compiler will likely never be able to exactly
replicate the behavior of whatever libm implementation is used at runtime. Maybe
having a dedicated flag to disable just this behavior, aside from suggesting
that users use -fno-builtin=..., would be useful for users who depend on the
compiler not folding these kinds of expressions in ways that might differ from
their runtime libm behavior?

 -Hal

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces
at lists.llvm.org>> on behalf of Kaylor, Andrew via llvm-dev <llvm-dev
at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Sent: Tuesday, April 16, 2019 2:23 PM
To: llvm-dev
Subject: [llvm-dev] [FP] Constant folding math library functions

Hi everyone,

I noticed today that LLVM’s constant folding of math library functions can lead
to minor differences in results. A colleague sent me the following test case
which demonstrates the issue:

#include <stdio.h>
#include <math.h>

typedef union {
  double d;
  unsigned long long i;
} my_dbl;

int main(void) {
  my_dbl res, x;
  x.i = 0x3feeb39556255de2ull;
  res.d = tanh(x.d);
  printf("tanh(%f) = %f = %016LX\n", x.d, res.d, res.i);
  return 0;
}

Compiling with “clang -O2 -g0 -emit-llvm” I get this:

define dso_local i32 @main() local_unnamed_addr #0 {
  %1 = tail call double @tanh(double 0x3FEEB39556255DE2) #2
  %2 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([24 x i8],
[24 x i8]* @.str, i64 0, i64 0),
                                                             double
0x3FEEB39556255DE2, double 0x3FE7CF009CE7F169,
                                                             i64
4604876745549017449)
  ret i32 0
}

We’re still calling ‘tanh’ but all the values passed to printf are constant
folded. The constant folding is based on a call to tanh made by the compiler.
The problem with this is that if I am linking my program against a different
version of the math library than was used by the compiler I may get a different
result.

I can prevent this constant folding with either the ‘nobuiltin’ or ‘strictfp’
attribute. However, it seems to me like this optimization should really be
checking the ‘afn’ fast math flag.

Opinions?

Thanks,
Andy

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Joerg Sonnenberger via llvm-dev

2019-Apr-17 21:04 UTC

head link

[llvm-dev] [FP] Constant folding math library functions

On Wed, Apr 17, 2019 at 12:35:16PM -0700, Scott Manley via llvm-dev
wrote:> I don't mean to hijack this thread but after reading through some
> discussion here and poking around in the constant folder myself just now,
> is my understanding correct in that LLVM only uses host routines for
> constant folding? Has there been discussion in the past about using MPFR or
> something like it instead? (I'm aware that doing so wouldn't solve
the
> original problem in the general case -- I'm asking out of curiosity).
Using MPFR doesn't solve anything, in fact it will often be worse as it
won't agree with even the native libm.

Joerg

llvm dev - Apr 2019 - [FP] Constant folding math library functions

[llvm-dev] [FP] Constant folding math library functions

[llvm-dev] [FP] Constant folding math library functions

[llvm-dev] [FP] Constant folding math library functions

[llvm-dev] [FP] Constant folding math library functions