[InstCombine] Wrong negation of vector selection with a poison value

[bongjunj]

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp

Lines 327 to 350 in 9a7519f

	case Instruction::Select: {
	if (isKnownNegation(I->getOperand(1), I->getOperand(2), /*NeedNSW=*/false,
	/*AllowPoison=*/false)) {
	// Of one hand of select is known to be negation of another hand,
	// just swap the hands around.
	auto *NewSelect = cast<SelectInst>(I->clone());
	// Just swap the operands of the select.
	NewSelect->swapValues();
	// Don't swap prof metadata, we didn't change the branch behavior.
	NewSelect->setName(I->getName() + ".neg");
	Builder.Insert(NewSelect);
	return NewSelect;
	}
	// `select` is negatible if both hands of `select` are negatible.
	Value *NegOp1 = negate(I->getOperand(1), IsNSW, Depth + 1);
	if (!NegOp1) // Early return.
	return nullptr;
	Value *NegOp2 = negate(I->getOperand(2), IsNSW, Depth + 1);
	if (!NegOp2)
	return nullptr;
	// Do preserve the metadata!
	return Builder.CreateSelect(I->getOperand(0), NegOp1, NegOp2,
	I->getName() + ".neg", /*MDFrom=*/I);
	}

Alive2 report: https://alive2.llvm.org/ce/z/tdHHuq

----------------------------------------
define <2 x i32> @negate_select_of_negation_poison.2(<2 x i1> %c, <2 x i32> %x) {
#0:
  %#1 = srem <2 x i32> { poison, 0 }, %x
  %neg = sub nsw <2 x i32> %#1, %x
  %sel = select <2 x i1> %c, <2 x i32> %neg, <2 x i32> %x
  %neg2 = sub <2 x i32> %x, %sel
  ret <2 x i32> %neg2
}
=>
define <2 x i32> @negate_select_of_negation_poison.2(<2 x i1> %c, <2 x i32> %x) {
#0:
  %neg = sub nsw <2 x i32> { 0, 0 }, %x
  %#1 = select <2 x i1> %c, <2 x i32> %x, <2 x i32> %neg
  %neg2 = add <2 x i32> %#1, %x
  ret <2 x i32> %neg2
}
Transformation doesn't verify!

ERROR: Target is more poisonous than source

Example:
<2 x i1> %c = < #x0 (0), #x0 (0) >
<2 x i32> %x = < #x80000000 (2147483648, -2147483648), #x0000000d (13) >

Source:
<2 x i32> %#1 = < poison, #x00000000 (0) >
<2 x i32> %neg = < poison, #xfffffff3 (4294967283, -13) >
<2 x i32> %sel = < #x80000000 (2147483648, -2147483648), #x0000000d (13) >
<2 x i32> %neg2 = < #x00000000 (0), #x00000000 (0) >

Target:
<2 x i32> %neg = < poison, #xfffffff3 (4294967283, -13) >
<2 x i32> %#1 = < poison, #xfffffff3 (4294967283, -13) >
<2 x i32> %neg2 = < poison, #x00000000 (0) >
Source value: < #x00000000 (0), #x00000000 (0) >
Target value: < poison, #x00000000 (0) >

Summary:
  0 correct transformations
  1 incorrect transformations
  0 failed-to-prove transformations
  0 Alive2 errors

[dtcxzyw]

Duplicate of #112666