[llvm-dev] [RFC] FP Environment and Rounding mode handling in LLVM

Chandler Carruth via llvm-dev
Fri, 05 Feb 2016 22:36:54 -0000

On Fri, Feb 5, 2016 at 2:10 PM Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Chandler,
>
> This scheme has significant advantages over what was being pursued, but
> one question (or two)...
>
> Under the proposed system, how would you represent the necessary
> dependency edges between the fp intrinsics and function calls? How is the
> state 'returned' to the caller? [I was thinking that our new
operand
> bundles could help for the inputs, but the outputs? Plus what about the
> live-in state?]
>
> This is important because any external subroutine call could (potentially)
> change the rounding mode or any other part of the floating-point
> environment.
>

So, one thing that was missing in my original email and that talking with
Steve Canon offline clarified was that we need a way to directly query the
current modes for systems where those can be set externally.

My suggestion was to have an intrinsic that "loads" this state. This
could
then be used to load whatever the current state is, and pass that to the
floating point intrinsics proposed in order to pick up whatever the
"current" state happens to be on systems where this is truly a
background
stateful thing, while still allowing us to model operation-specific state
for other systems. Naturally, there should be a complimenting "store"
of
the state as well.

Then, for code which really needs this degree of faithful FP environment
handling, you would expect the #pragma to be present enabling that mode.
While that pragma is in place, all floating point operations would be
lowered using these intrinsics, and external function calls could be
guarded by storing and reloading this state at the IR level. This would
make the IR substantially more verbose when the pragma is enabled, but that
seems like an acceptable tradeoff given that we expect this code to be rare
(see my preconditions section). And naturally, on any system that actually
manages FP environment in a state "register" or whatever, we'd
want to do
some work to try to optimize away state changes. Much like we have
attributes that can be inferred about access to memory, we could infer
attributes on functions about whether they change the FP environment state,
and if not, propagate across the function call boundaries.

But even though this would be some amount of work to optimize, the nice
thing (IMO) is that it would be localized. We would have specific code that
dealt with optimizing the FP environment concerns, while the rest of LLVM
could remain oblivious and rely on simple common constructs to provide
conservatively correct behavior.

What do you think?
-Chandler


>
> Thanks again,
> Hal
>
> ----- Original Message -----
> > From: "Chandler Carruth" <chandlerc at gmail.com>
> > To: "Mehdi Amini" <mehdi.amini at apple.com>,
"llvm-dev" <
> llvm-dev at lists.llvm.org>
> > Cc: "Steve (Numerics) Canon" <scanon at apple.com>,
"Sergey Dmitrouk" <
> sdmitrouk at accesssoftek.com>, "David Majnemer"
> > <david.majnemer at gmail.com>, "Hal Finkel"
<hfinkel at anl.gov>
> > Sent: Thursday, February 4, 2016 8:05:38 PM
> > Subject: Re: [RFC] FP Environment and Rounding mode handling in LLVM
> >
> >
> > First, thanks Mehdi for putting something on llvm-dev and getting
> > wider awareness of this.
> >
> >
> > I am actually really interested in finding a way for LLVM to support
> > the interesting functionality we are missing from fenv-like
> > interfaces. Things like rounding modes, exceptions, etc. However, I
> > think the current design is going to be a really high burden for the
> > entire optimizer and I think there is a simpler model that we might
> > pursue instead.
> >
> >
> > I'll start off with some underlying principles that I'm
operating
> > from:
> > a) Most code in the world will be very happy with the default
> > floating point environment, doesn't need to carefully model
floating
> > point exceptions, etc. Essentially, I think that LLVM's behavior
> > today is probably right for most code. Now, the code which needs
> > support for the other features of floating point isn't bad or
> > unimportant! But it is relatively speaking rare, and so I think it
> > is reasonable to optimize the *representation* model for the common
> > case provided we don't lose support for functionality.
> >
> >
> > a) When outside the default floating point environment's rules,
there
> > are few if any optimizations that we realistically expect from LLVM.
> > Certainly, any changes to the LLVM optimizer which impact code
> > outside the default needs to be done *much* more carefully to avoid
> > introducing subtle bugs.
> >
> >
> > OK, based on that, consider the following model:
> > We provide intrinsics that mirror the instructions 'fadd',
'fsub',
> > 'fmul', 'fdiv', and 'frem' (so 5 total). From
here on out, I'll
> > exclusively use 'fadd' as my examples. The intrinsics would
look
> > like:
> >
> > declare {f32, i1} @llvm.fadd.with.environment.f32(f32 %lhs, f32 %rhs,
> > i8 %rounding_mode, i8 %exception_behavior)
> >
> >
> > Then we define specific values to be used for the IEEE rounding
> > modes. And we define values to control exception behavior. I'm not
> > an expert on floating point exceptions in particular (my platforms
> > don't use them) but I'm imagining three states
"ignore", "return",
> > and "trap". I've used a single 'i1', but I'm
assuming it would need
> > to be several i1s or an iN in order to model the set of FP
> > exceptions. I'm using i1 here just to simplify the explanation, I
> > think it generalizes and I'll let the experts suggest the exact
> > formulation.
> >
> >
> > If the default rounding mode is provided to these intrinsics and the
> > "ignore" exception behavior is provided, they behave exactly
as the
> > existing instructions do, and instcombine should canonicalize to the
> > existing instructions.
> >
> >
> > The semantics of non-default rounding modes are to perform the
> > operation with that rounding mode.
> >
> >
> > If "return" is provided for the exception behavior, then the
i1
> > component of the result is true if an FP exception occured and false
> > otherwise. If "ignore" is provided then any FP exceptions
are
> > ignored and the i1 is always false. If "trap" is provided
then the
> > i1 is always false, but the call to the intrinsic might trap. We
> > could either define a trap as precisely the same as a call to
> > @llvm.trap(), or we could introduce an @llvm.fp.trap() and define it
> > as a call to that.
> >
> >
> > The frontend would then be responsible for lowering floating point
> > arithmetic using these intrinsics. This may be somewhat challenging
> > because in the frontend behavior is controlled dynamically in some
> > languages. In those situations, we can either allow these intrinsics
> > to accept non-constant arguments for %rounding_mode and
> > %exception_behavior so that frontends can emit code that just
> > dynamically computes them, or we could follow the same model that
> > atomics use, and if the frontend cannot trivially compute a
> > constant, it can emit a switch over the possible states with a
> > specific intrinsic call in each case. I don't have strong opinions
> > about which would be best, I think either could be made to work.
> >
> >
> > If we go with constant arguments being required, we could use
> > "metadata arguments" which aren't actually metadata but
just encoded
> > arguments for intrinsics.
> >
> >
> > When emitting constants and trying to respect floating point
> > environment settings, frontends will have to emit runtime calls
> > instead of actual constants. But this seems actually good because
> > that is what we'll need anyways -- we aren't able to with full
> > generality emulate all the environment options if I understand
> > things correctly (and let me know if I've misunderstood).
> >
> >
> >
> >
> > The two really big reasons why I like this model much more than
> > extending flags are:
> >
> >
> > 1) This avoids implicit state. The implicit state of the floating
> > point environment makes things like code motion extremely hard to
> > reason about. I think we will just get it wrong too often to make
> > this a good approach. By modeling all of this as actual SSA values I
> > think there is a much better chance we'll get this stuff right.
For
> > example by or-ing all the i1s for floating point exceptions and
> > testing the result to implement fetestexcept. Then the backend can
> > spill the state when necessary and reload it when needed even if
> > other floating point math is introduced. I admit that first class
> > aggregate returns aren't a beautiful way to encapsulate this, but
> > they are an *effective* way that we know how to work with in the
> > LLVM IR. If we ever come up with a better multi-def model, we can
> > always switch these and all the other intrinsics which need this to
> > that model.
> >
> >
> > 2) Every pass will conservatively correctly model the operations.
> > This is most significant when modeling trapping on exceptions. We
> > need every pass to realize that control flow might not proceed past
> > such operations. We already have this logic for calls, and it seems
> > a really nice fit for allowing most of the optimizer to be unaware
> > of these constructs while respecting them and preserving behavior in
> > the face of them.
> >
> >
> >
> >
> > I suspect that there are things this model doesn't handle that
I've
> > not thought of (as this is outside the are of FP that I'm deeply
> > familiar with), but I really think this model would be easier to
> > reason about and would be much less invasive within the IR and
> > optimizer. I wonder if folks think this could work and would be up
> > for moving their efforts in this direction?
> >
> >
> > -Chandler
> >
> >
> > On Wed, Feb 3, 2016 at 3:04 PM Mehdi Amini < mehdi.amini at
apple.com >
> > wrote:
> >
> >
> > Hi everyone,
> >
> > Sergey (CC’ed) worked on a series of patches to add support for
> > floating-point environment and floating-point rounding modes in
> > LLVM.
> > This started *in 2014* and the patches after multiple rounds of
> > review in the last months (involving amongst other Steve Canon, Hal
> > Finkel, David Majnemer, and myself) are getting very close (IMO) to
> > be in a state where we can land them.
> >
> > This is the thread that started this development: “ [LLVMdev] More
> > careful treatment of floating point exceptions"
> > http://marc.info/?l=llvm-dev&m=141113983302113&w=2
> > And this is the thread where most of the discussion on the design
> > occurred: "[PATCH] Flag to enable IEEE-754 friendly FP
> > optimizations”
> > http://marc.info/?l=llvm-commits&m=141235814915999&w=2
> >
> > Since Chandler raised some concerns on IRC today, so I figured I
> > should send a heads-up on this topic to allow any one to comment on
> > the current plan.
> >
> > We plan on adding two new FP env flags to the existing FMF (fast-math
> > flags). Without these flags set, the optimizer has to assume that
> > the FP env can be observed, or the rounding mode can be changed. For
> > clang, these flags would be set unless a command line option would
> > require to preserve the FP env.
> >
> > Here is the list of patches:
> >
> > [FPEnv Core 01/14] Add flags and command-line switches for FPEnv:
> > http://reviews.llvm.org/D14066
> > [FPEnv Core 02/14] Add FPEnv access flags to fast-math flags:
> > http://reviews.llvm.org/D14067
> > [FPEnv Core 03/14] Make SelectionDAG aware of FPEnv flags:
> > http://reviews.llvm.org/D14068
> > [FPEnv Core 04/14] Skip constant folding to preserve FPEnv:
> > http://reviews.llvm.org/D14069
> > [FPEnv Core 05/14] Teach IR builder and folders about new flags:
> > http://reviews.llvm.org/D14070
> > [FPEnv Core 06/14] Do not fold constants on reading in IR
> > asm/bitcode: http://reviews.llvm.org/D14071
> > [FPEnv Core 07/14] Prevent undesired folding by InstSimplify:
> > http://reviews.llvm.org/D14072
> > [FPEnv Core 08/14] Do not simplify expressions with FPEnv access:
> > http://reviews.llvm.org/D14073
> > [FPEnv Core 09/14] Make Strict flag available for more clients:
> > http://reviews.llvm.org/D14074
> > [FPEnv Core 10/14] Use Strict in IRBuilder:
> > http://reviews.llvm.org/D14075
> > [FPEnv Core 11/14] Don't convert fpops to constexprs in SCCP:
> > http://reviews.llvm.org/D14076
> > [FPEnv Core 13/14] Don't hoist FP-ops with side-effects in LICM:
> > http://reviews.llvm.org/D14078
> > [FPEnv Core 14/14] Introduce F*_W_CHAIN instrs to prevent reordering:
> > http://reviews.llvm.org/D14079
> >
> >
> > —
> > Mehdi
> >
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> _______________________________________________
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