[llvm-commits] [PATCH] Add llvm.fmuladd intrinsic.

[Lang Hames]

Hi All,

I want to improve Clang/LLVM's support for the fp_contract pragma.
(Background: when the fp_contract pragma is ON, multiplications and
additions within expressions can be fused to FMAs. This is a performance
win on targets that have a fast fma instruction.)

As a step towards implementing this functionality, this patch adds a new
family of intrinsics, llvm.fmuladd.*, that represent mul+add expressions
that can be fused. The frontend will emit calls to these intrinsics when it
sees a mul+add in a single source expression and fp_contract is ON (I'll
mail a patch for this functionality to cfe-commits in a minute). During
isel, the legalizer will check the Subtarget::isFMACheap() method to decide
whether to lower these intrinsics to llvm.fma.* intrinsic calls, or a
regular unfused mul and add expression.

I haven't taught the optimizers to reason about these expressions yet -
that's coming next. For now I just wanted to see if anyone had any
objections to the proposed scheme. If not I'll go ahead and commit.

Cheers,
Lang.
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