SLPVectorizer does not respect isFPVectorizationPotentiallyUnsafe()

[nikic]

SLPVectorizer currently incorrectly vectorizes FP instructions using ARM NEON (https://llvm.godbolt.org/z/c9866Yv13), which has non-IEEE compliant denormal flushing (#16648).

There is a isFPVectorizationPotentiallyUnsafe() TTI hook for this, which is checked by LoopVectorize, but not SLPVectorizer.

(Technically at fault here is the ARM backend for using NEON instructions for vector floating point ops, but LLVM currently handles this by preventing their implicit formation.)

[alexey-bataev]

I cannot find where LoopVectorizer uses isFPVectorizationPotentiallyUnsafe. Could you point exact line of code? Searching through the LLVM code base provides only declarations of the function, but I cannot find a single reference

[nikic]

Sure, the call is here:

llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

Line 9878 in 84ed3c2

TTI->isFPVectorizationPotentiallyUnsafe()) {

See also the related code in

llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp

Lines 1001 to 1004 in 84ed3c2

	} else if (I.getType()->isFloatingPointTy() && (CI || I.isBinaryOp()) &&
	!I.isFast()) {
	LLVM_DEBUG(dbgs() << "LV: Found FP op with unsafe algebra.\n");
	Hints->setPotentiallyUnsafe();