[LLVMdev] Question about FMA formation

[Lang Hames]

A little background:

The fmuladd intrinsic was introduced to support the FP_CONTRACT pragma in
C. llvm.fmuladd.* is generated by clang when it sees an expression of the
form  'a * b + c' within a single source statement.

If you want to opportunistically form FMA target instructions my
inclination would be to skip llvm.fmuladd.* and just form them from a*b+c
expressions at isel time. I don't see any fundamental problem with forming
llvm.fmuladd.* to model FMA formation opportunities in an IR pass though.

- Lang.


On Wed, Dec 12, 2012 at 4:11 PM, Michael Ilseman <milseman at apple.com> wrote:

>
> On Dec 12, 2012, at 3:40 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
>
> Hi, Dear All:
>
>   I'm going implement FMA formation. On some architectures, "FMA a, b, c"
> is more precise than
> "a * b + c".  I'm wondering if FMA could be less precise. In the former
> case, can we enable FMA
> formation despite restrictive FP mode?
>
>
> I believe that a pass to form fmuladd[1] intrinsic calls would be very
> useful! The fmuladd intrinsic is defined such that its formation should be
> isolated from worries about strictness. It simply means "a * b + c" and
> leaves the decision of whether or not to fuse up to the code generator. Of
> course, one probably would only run your pass if they wanted the code
> generator to fuse it, but the pass itself should be valid.
>
> Someone please correct me if I misunderstand this intrinsic.
>
> [1] http://llvm.org/docs/LangRef.html#llvm-fmuladd-intrinsic
>
> Thanks
> Shuxin
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