[cfe-dev] what does -ffp-contract=fast allow?

Hal Finkel via cfe-dev cfe-dev at lists.llvm.org
Fri Nov 18 10:19:19 PST 2016 

----- Original Message -----

> From: "Sanjay Patel" <spatel at rotateright.com>
> To: "Hal J. Finkel" <hfinkel at anl.gov>
> Cc: "Mehdi Amini" <mehdi.amini at apple.com>, "llvm-dev"
> <llvm-dev at lists.llvm.org>, "cfe-dev" <cfe-dev at lists.llvm.org>,
> "andrew kaylor" <andrew.kaylor at intel.com>, "Nicolai Hähnle"
> <nhaehnle at gmail.com>, "Warren Ristow" <warren.ristow at sony.com>
> Sent: Friday, November 18, 2016 10:37:08 AM
> Subject: Re: what does -ffp-contract=fast allow?

> fp-contract is confusing, so let me try to summarize that and the
> underlying implementation:

> 1. -ffp-contract=on means honor the compiler's default FP_CONTRACT
> setting or any FP_CONTRACT pragmas in the source. Currently, clang
> defaults to "OFF". The shouting is not an accident; this is not the
> same as the flag's "off" setting. This is described nicely here:
> https://reviews.llvm.org/D24481

> If we set "on" in the invocation *and* we set "ON" in the source,
> clang will generate @llvm.fmuladd intrinsics for expressions like
> x*y+z. If you split that into 2 lines in C with a temp variable
> assignment, it's no longer a single expression, so no FMA for you.
> The @llvm.fmuladd intrinsic is our way of preserving the C source
> information through the optimizer. If we don't end up producing an
> FMA instruction for the target in this case, it's a bug.

This is not correct. 

First, the behavior of -ffp-contract=on/off should just set the default state of the pragma. Once we finish fixing up the test suite to allow us to actually flip the default, this will actually be the case (the review description referenced above is not clear on the desired end state in this regard). Hopefully, this work will be done soon. 

Second, it is specifically *not* a bug if @llvm.fmuladd does not become an FMA on the target. It only represents an allowable place to form an FMA. The LangRef specifically states, "Fusion is not guaranteed, even if the target platform supports it." The @llvm.fma intrinsic should become an FMA if the target supports it. 

> 2. -ffp-contract=fast means override the compiler's default "OFF"
> setting and override source pragmas to generate FMA when possible,
> even across C expressions. The "fast" naming is unfortunate because
> this does *not* enable most fast-math. Ie, as everyone in this
> thread agrees so far, we are not allowed to do the reassociation in
> the example. It's not strict math though because of that trailing
> clause that let's us generate FMA across expressions.

> Here's where it gets more complicated and possibly buggy. Clang does
> not generate llvm.fmuladd intrinsics with this setting. In this
> mode, clang generates individual fmul and fadd instructions and
> relies on the backend to fuse those back together.
This is definitely not a bug. The problem with the C rules for contraction, which only allow fusion within a C-language statement, don't allow fusion opportunities that appear only after function inlining (or, obviously, across statements in any other sense). This is a real problem, especially in C++ code, where there are a lot of small inline functions in abstraction layers that users expect the compiler to see through before deciding on fusion. Even within a function, the fusions allowed by the C rules are not necessarily performance-optimal. 

> More background here:
> https://llvm.org/bugs/show_bug.cgi?id=17211

> I don't know if it's possible, but if we're in this mode and some IR
> transform pass managed to move/kill an fmul or fadd that was
> destined to be part of an FMA, I think that would be a bug.
No, this also would not be a bug (although could be bad for performance on some architectures). 

> This mode is also completely broken with LTO because we're using a
> TargetOption to communicate the FMA mode to the backend; there is no
> instruction-level or function-level attribute/metadata for FMA-ness:
> https://llvm.org/bugs/show_bug.cgi?id=25721

Interesting; we should at least have a function-attribute for this that Clang uses. 

Thanks again, 
Hal 

> To tie this back to the earlier thread about changes to IR FMF, the
> possibility of adding FMA bits to FMF (as well as storing all FMF in
> metadata) was discussed here:
> https://llvm.org/bugs/show_bug.cgi?id=13118

> 3. The backend needs a thread of its own. We have at least these
> mechanisms to handle FMA codegen:

> a. TargetOptions for LessPreciseFPMADOption, UnsafeFPMath,
> NoInfsFPMath, NoNaNsFPMath, AllowFPOpFusion (Fast, Standard, Strict)

> b. SDNodeFlags for UnsafeAlgebra, NoNaNs, NoInfs, NoSignedZeros (but
> nothing for FMA since IR FMF has nothing for FMA)

> c. SelectionDAGTargetInfo::generateFMAsInMachineCombiner()

> d. TargetLoweringBase::isFMAFasterThanFMulAndFAdd()

> e. TargetLoweringBase::enableAggressiveFMAFusion()

> f. ISD::FMA (no intermediate rounding step) and ISD::FMAD (has
> intermediate rounding) nodes

> On Thu, Nov 17, 2016 at 6:03 PM, Finkel, Hal J. < hfinkel at anl.gov >
> wrote:

> > Sent from my Verizon Wireless 4G LTE DROID
> 
> > On Nov 17, 2016 5:53 PM, Mehdi Amini < mehdi.amini at apple.com >
> > wrote:
> 
> > >
> 
> > >
> 
> > >> On Nov 17, 2016, at 4:33 PM, Hal Finkel < hfinkel at anl.gov >
> > >> wrote:
> 
> > >>
> 
> > >>
> 
> > >> ________________________________
> 
> > >>>
> 
> > >>> From: "Warren Ristow" < warren.ristow at sony.com >
> 
> > >>> To: "Sanjay Patel" < spatel at rotateright.com >, "cfe-dev" <
> > >>> cfe-dev at lists.llvm.org >, "llvm-dev" < llvm-dev at lists.llvm.org
> > >>> >
> 
> > >>> Cc: "Nicolai Hähnle" < nhaehnle at gmail.com >, "Hal Finkel" <
> > >>> hfinkel at anl.gov >, "Mehdi Amini" < mehdi.amini at apple.com >,
> > >>> "andrew kaylor" < andrew.kaylor at intel.com >
> 
> > >>> Sent: Thursday, November 17, 2016 5:58:58 PM
> 
> > >>> Subject: RE: what does -ffp-contract=fast allow?
> 
> > >>>
> 
> > >>> > Is this a bug? We transformed the original expression into:
> 
> > >>> > x * y + x
> 
> > >>>
> 
> > >>> I’d say yes, it’s a bug.
> 
> > >>>
> 
> > >>>
> 
> > >>>
> 
> > >>> Unless ‑ffast‑math is used (or some appropriate subset that
> > >>> gives
> > >>> us leeway, like ‑fno‑honor‑infinities or ‑fno‑honor‑nans, or
> > >>> somesuch), the re-association isn’t allowed, and that blocks
> > >>> the
> > >>> madd contraction.
> 
> > >>
> 
> > >> I agree. FP contraction alone only allows us to do x*y+z ->
> > >> fma(x,y,z).
> 
> > >
> 
> > >
> 
> > > I agree too, but the more difficult question is "which flags are
> > > needed here?”
> 
> > > Would FPContract + no-inf be enough? If not why and how to
> > > document
> > > it?
> 

> > I think that the relevant question is: Is the contracted form more
> > precise for all inputs (or the same precision as the original)? If
> > so, then this should be allowed with just fp-contract+no-inf.
> > Otherwise, more is required.
> 

> > -Hal
> 

> > >
> 
> > >
> 
> > > —
> 
> > > Mehdi
> 
> > >
> 
> > >
> 
> > >
> 
> > >>>
> 
> > >>>
> 
> > >>> From: Sanjay Patel [mailto: spatel at rotateright.com ]
> 
> > >>> Sent: Thursday, November 17, 2016 3:22 PM
> 
> > >>> To: cfe-dev < cfe-dev at lists.llvm.org >; llvm-dev <
> > >>> llvm-dev at lists.llvm.org >
> 
> > >>> Cc: Nicolai Hähnle < nhaehnle at gmail.com >; Hal Finkel <
> > >>> hfinkel at anl.gov >; Mehdi Amini < mehdi.amini at apple.com >;
> > >>> Ristow, Warren < warren.ristow at sony.com >;
> > >>> andrew.kaylor at intel.com
> 
> > >>> Subject: what does -ffp-contract=fast allow?
> 
> > >>>
> 
> > >>>
> 
> > >>>
> 
> > >>> This is just paraphrasing from D26602, so credit to Nicolai for
> > >>> first raising the issue there.
> 
> > >>>
> 
> > >>> float foo(float x, float y) {
> 
> > >>> return x * (y + 1);
> 
> > >>> }
> 
> > >>>
> 
> > >>> $ ./clang -O2 xy1.c -S -o - -target aarch64 -ffp-contract=fast
> > >>> |
> > >>> grep fm
> 
> > >>> fmadd s0, s1, s0, s0
> 
> > >>>
> 
> > >>> Is this a bug? We transformed the original expression into:
> 
> > >>> x * y + x
> 
> > >>>
> 
> > >>> When x=INF and y=0, the code returns INF if we don't
> > >>> reassociate.
> > >>> With reassociation to FMA, it returns NAN because 0 * INF =
> > >>> NAN.
> 
> > >>>
> 
> > >>> 1. I used aarch64 as the example target, but this is not
> > >>> target-dependent (as long as the target has FMA).
> 
> > >>>
> 
> > >>> 2. This is *not* -ffast-math...or is it? The C standard only
> > >>> shows on/off settings for the associated FP_CONTRACT pragma.
> 
> > >>>
> 
> > >>> 3. AFAIK, clang has no documentation for -ffp-contract:
> 
> > >>> http://clang.llvm.org/docs/UsersManual.html
> 
> > >>>
> 
> > >>> 4. GCC says:
> 
> > >>> https://gcc.gnu.org/onlinedocs/gcc-6.2.0/gcc/Optimize-Options.html#Optimize-Options
> 
> > >>> "-ffp-contract=fast enables floating-point expression
> > >>> contraction
> > >>> such as forming of fused multiply-add operations if the target
> > >>> has native support for them."
> 
> > >>>
> 
> > >>> 5. The LLVM backend (where this reassociation currently
> > >>> happens)
> > >>> shows:
> 
> > >>> FPOpFusion::Fast - Enable fusion of FP ops wherever it's
> > >>> profitable.
> 
> > >>
> 
> > >>
> 
> > >>
> 
> > >>
> 
> > >> --
> 
> > >> Hal Finkel
> 
> > >> Lead, Compiler Technology and Programming Languages
> 
> > >> Leadership Computing Facility
> 
> > >> Argonne National Laboratory
> 
> > >
> 
> > >
> 

-- 

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