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

Mehdi Amini via llvm-dev llvm-dev at lists.llvm.org
Thu Nov 17 16:53:43 PST 2016

> 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?


> 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 <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 <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

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