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

[Sanjay Patel via cfe-dev]

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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