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

[Ristow, Warren via cfe-dev]

> 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.

-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.
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