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

Hal Finkel via llvm-dev
Fri, 18 Nov 2016 00:33:04 -0000

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

> 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).

-Hal

> -Warren

> 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
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