[PATCH] D72820: [FPEnv] Add pragma FP_CONTRACT support under strict FP.

[Andy Kaylor via Phabricator via llvm-commits]

andrew.w.kaylor added inline comments.


================
Comment at: clang/lib/CodeGen/CGExprScalar.cpp:3436
+    }
+  }
+
----------------
cameron.mcinally wrote:
> cameron.mcinally wrote:
> > cameron.mcinally wrote:
> > > I don't think it's safe to fuse a FMUL and FADD if the intermediate rounding isn't exactly the same as those individual operations. FMULADD doesn't guarantee that, does it?
> > To be clear, we could miss very-edge-case overflow/underflow exceptions.
> Ah, but I see C/C++ FP_CONTRACT allows the exceptions to be optimized away. Sorry for the noise.
We've talked about this before but I don't think we ever documented a decision as to whether we want to allow constrained intrinsics and fast math flags to be combined. This patch moves that decision into clang's decision to generate this intrinsic or not.

I think it definitely makes sense in the case of fp contraction, because even if a user cares about value safety they might want FMA, which is theorectically more accurate than the separate values even though it produces a different value. This is consistent with gcc (which produces FMA under "-ffp-contract=fast -fno-fast-math") and icc (which produced FMA under "-fp-model strict -fma").

For the record, I also think it makes sense to use nnan, ninf, and nsz with constrained intrinsics.


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  https://reviews.llvm.org/D72820/new/

https://reviews.llvm.org/D72820