[InstCombine] wrong folding of an extension and a binary operator for undefined inputs

[bongjunj]

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

Lines 2336 to 2346 in a33fd61

	// bo (ext X), (ext Y) --> ext (bo X, Y)
	// bo (ext X), C --> ext (bo X, C')
	Value *NarrowBO = Builder.CreateBinOp(BO.getOpcode(), X, Y, "narrow");
	if (auto *NewBinOp = dyn_cast<BinaryOperator>(NarrowBO)) {
	if (IsSext)
	NewBinOp->setHasNoSignedWrap();
	else
	NewBinOp->setHasNoUnsignedWrap();
	}
	return CastInst::Create(CastOpc, NarrowBO, BO.getType());
	}

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

----------------------------------------
define i32 @test8_vec.3(i16 %V) {
#0:
  %ashr = ashr i16 %V, 8
  %sext = sext i16 %ashr to i32
  %#1 = sext i16 %ashr to i32
  %mul = mul i32 %sext, %#1
  ret i32 %mul
}
=>
define i32 @test8_vec.3(i16 %V) {
#0:
  %ashr = ashr i16 %V, 8
  %narrow = mul nsw i16 %ashr, %ashr
  %mul = zext nneg i16 %narrow to i32
  ret i32 %mul
}
Transformation doesn't verify!

ERROR: Target is more poisonous than source

Example:
i16 %V = undef

Source:
i16 %ashr = #x0000 (0)	[based on undef value]
i32 %sext = #x00000000 (0)
i32 %#1 = #x00000000 (0)	[based on undef value]
i32 %mul = #x00000000 (0)

Target:
i16 %ashr = #x0018 (24)
i16 %narrow = #xf400 (62464, -3072)
i32 %mul = poison
Source value: #x00000000 (0)
Target value: poison


----------------------------------------
define <2 x i32> @test8_vec.2(<2 x i16> %V) {
#0:
  %ashr = ashr <2 x i16> %V, { 8, 8 }
  %sext = sext <2 x i16> %ashr to <2 x i32>
  %#1 = sext <2 x i16> %ashr to <2 x i32>
  %mul = mul <2 x i32> %sext, %#1
  ret <2 x i32> %mul
}
=>
define <2 x i32> @test8_vec.2(<2 x i16> %V) {
#0:
  %ashr = ashr <2 x i16> %V, { 8, 8 }
  %narrow = mul nsw <2 x i16> %ashr, %ashr
  %mul = zext nneg <2 x i16> %narrow to <2 x i32>
  ret <2 x i32> %mul
}
Transformation doesn't verify!

ERROR: Target is more poisonous than source

Example:
<2 x i16> %V = < #x0000 (0), undef >

Source:
<2 x i16> %ashr = < #x0000 (0), #x0000 (0)	[based on undef value] >
<2 x i32> %sext = < #x00000000 (0), #x00000000 (0) >
<2 x i32> %#1 = < #x00000000 (0), #x00000000 (0)	[based on undef value] >
<2 x i32> %mul = < #x00000000 (0), #x00000000 (0) >

Target:
<2 x i16> %ashr = < #x0000 (0), #xff80 (65408, -128) >
<2 x i16> %narrow = < #x0000 (0), #xfc00 (64512, -1024) >
<2 x i32> %mul = < #x00000000 (0), poison >
Source value: < #x00000000 (0), #x00000000 (0) >
Target value: < #x00000000 (0), poison >

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