`@simd` enables unexpected fastmath optimizations

[mikmoore]

@simd results in @fastmath being applied. This is undocumented behavior (except for re-association of reduction variables). The behavior needs to be documented or the @fastmath needs to not occur.

Reduced example derived from an effort to write a faster norm(x, 2):

# Julia v1.8.0

function test(x)
	scale = exp2(768)
	s1 = 0.0
	s2 = 0.0
	@simd for z in x
		s1 += abs2(z)
		s2 += abs2(z*scale)
	end
	s = s2==Inf ? s1 : s2
	return s,s1,s2
end

test(1:3)
# (Inf, 14.0, Inf)

Inspecting the @code_llvm shows many @fastmath operations and the @code_native shows unauthorized fma instructions. The result should be s == s1 == 14.0, but instead we get s == s2 == Inf because the s2==Inf check is replaced by false by the "noinf" property inherited from @fastmath.

[giordano]

I don't think "@simd invokes @fastmath" is an accurate description because as far as I know @simd never invokes the @fastmath macro, but the code it generates enables LLVM fast-math flags under certain conditions, I presume around

julia/src/llvm-simdloop.cpp

Lines 67 to 131 in c6ed7d7

	/// If Phi is part of a reduction cycle of FAdd, FSub, FMul or FDiv,
	/// mark the ops as permitting reassociation/commuting.
	/// As of LLVM 4.0, FDiv is not handled by the loop vectorizer
	static void enableUnsafeAlgebraIfReduction(PHINode *Phi, Loop *L) JL_NOTSAFEPOINT
	{
	typedef SmallVector<Instruction*, 8> chainVector;
	chainVector chain;
	Instruction *J;
	unsigned opcode = 0;
	for (Instruction *I = Phi; ; I=J) {
	J = NULL;
	// Find the user of instruction I that is within loop L.
	for (User *UI : I->users()) { /*}*/
	Instruction *U = cast<Instruction>(UI);
	if (L->contains(U)) {
	if (J) {
	LLVM_DEBUG(dbgs() << "LSL: not a reduction var because op has two internal uses: " << *I << "\n");
	return;
	}
	J = U;
	}
	}
	if (!J) {
	LLVM_DEBUG(dbgs() << "LSL: chain prematurely terminated at " << *I << "\n");
	return;
	}
	if (J == Phi) {
	// Found the entire chain.
	break;
	}
	if (opcode) {
	// Check that arithmetic op matches prior arithmetic ops in the chain.
	if (getReduceOpcode(J, I) != opcode) {
	LLVM_DEBUG(dbgs() << "LSL: chain broke at " << *J << " because of wrong opcode\n");
	return;
	}
	}
	else {
	// First arithmetic op in the chain.
	opcode = getReduceOpcode(J, I);
	if (!opcode) {
	LLVM_DEBUG(dbgs() << "LSL: first arithmetic op in chain is uninteresting" << *J << "\n");
	return;
	}
	}
	chain.push_back(J);
	}
	switch (opcode) {
	case Instruction::FAdd:
	++AddChains;
	break;
	case Instruction::FMul:
	++MulChains;
	break;
	}
	++ReductionChains;
	int length = 0;
	for (chainVector::const_iterator K=chain.begin(); K!=chain.end(); ++K) {
	LLVM_DEBUG(dbgs() << "LSL: marking " << **K << "\n");
	(*K)->setFast(true);
	++length;
	}
	ReductionChainLength += length;
	MaxChainLength.updateMax(length);
	}

probably

julia/src/llvm-simdloop.cpp

Line 126 in c6ed7d7

(*K)->setFast(true);

in particular

[vchuravy]

help?> @simd
  @simd

    •  Floating-point operations on reduction variables can be reordered,
       possibly causing different results than without @simd.

There is an argument to only enable reassoc/contract.