[PATCH] D99675: RFC [llvm][clang] Create new intrinsic llvm.arith.fence to control FP optimization at expression level

[Eli Friedman via Phabricator via cfe-commits]

efriedma added a comment.

> The expression “llvm.arith.fence(a * b) + c” means that “a * b” must happen before “+ c” and FMA guarantees that, but to prevent later optimizations from unpacking the FMA the correct transformation needs to be:
>
> llvm.arith.fence(a * b) + c  →  llvm.arith.fence(FMA(a, b, c))

Does this actually block later transforms from unpacking the FMA?  Maybe if the FMA isn't marked "fast"...

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How is llvm.arith.fence() different from using "freeze" on a floating-point value?  The goal isn't really the same, sure, but the effects seem similar at first glance.


Repository:
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https://reviews.llvm.org/D99675