[llvm-dev] Fusing contract fadd/fsub with normal fmul

[Yichao Yu via llvm-dev]

Hi,

On LLVM 5.0 (current trunk), fadd/fsub and fmul that are both marked
with `contract` or `fast` can be merged to a fma instruction by the
backend.

I'm wondering about the exact semantic of this new flag as well as
`fast` and in particular, would it be valid to do this when only the
`fadd`/`fsub` (and not the `fmul`) is marked with `contract` or at
least `fast`. The reasoning is that doing this will have a similar
effect as if the `fadd`/`fsub` is performed not to IEEE spec so a
single flag on this instruction should be enough for the
transformation.

The particular case I'm interested in is vectorized loop with
reduction like in pseudo C code `s += a[i] * b[i]`. Our front end will
recognize this and mark the `+` as `fast` to enable vectorization.
It'll be great if this can enable the reduction to be done with `fma`
instructions.

Yichao Yu