[llvm] r371518 - [InstCombine] Use SimplifyFMulInst to simplify multiply in fma.

[Florian Hahn via llvm-commits]

On Tue, Sep 10, 2019 at 8:39 PM Philip Reames <listmail at philipreames.com> wrote:
>
> Wait, is this correct?  I thought that FMA had different rounding
> behavior than fadd/fmul.  Is it legal to take the simplified mul and
> simple feed it as an operand to a fadd?
>

Yes, I think if SimplifyFMulInst would return a new `simplified` fmul
instruction, than the transform as it is would introduce additional
rounding error. For example, a few lines above, we have the following
transform: fma fabs(x), fabs(x), z -> fma x, x, z. Here it would be
wrong to replace that with fadd z, fmul x, x.

But SimplifyFMul won't create any new instructions as far as I am
aware, it will only return an existing value if it could simplify the
fmul to it. In all those cases, we skip the multiply as part of the
FMA and there is no extra rounding error introduced. And I think
SimplifyFMulInst should only return a value if we can use it to
replace the FMul.

Does this make sense?

Cheers,
Florian