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ConstantFold, InstSimplify: undef >>a x can be either -1 or 0, choose 0

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Zero is usually a nicer constant to have than -1.

llvm-svn: 223969
This commit is contained in:
David Majnemer 2014-12-10 21:58:15 +00:00
parent 56c5d273bf
commit cde1ba6638
3 changed files with 7 additions and 6 deletions
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4
lib/Analysis/InstructionSimplify.cpp
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@ -1422,11 +1422,11 @@ static Value *SimplifyAShrInst(Value *Op0, Value *Op1, bool isExact,
if (match(Op0, m_AllOnes()))
return Op0;
// undef >>a X -> all ones
// undef >>a X -> 0
// undef >>a X -> undef (if it's exact)
if (match(Op0, m_Undef()))
return isExact ? UndefValue::get(Op0->getType())
: Constant::getAllOnesValue(Op0->getType());
: Constant::getNullValue(Op0->getType());
// (X << A) >> A -> X
Value *X;

5
lib/IR/ConstantFold.cpp
View File

@ -960,8 +960,9 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode,
// X >>a undef -> undef
if (isa<UndefValue>(C2))
return C2;
// undef >>a X -> all ones
return Constant::getAllOnesValue(C1->getType());
// TODO: undef >>a X -> undef if the shift is exact
// undef >>a X -> 0
return Constant::getNullValue(C1->getType());
case Instruction::Shl:
// X << undef -> undef
if (isa<UndefValue>(C2))

4
test/Transforms/InstCombine/shift.ll
View File

@ -84,14 +84,14 @@ define <4 x i32> @test5a_non_splat_vector(<4 x i32> %A) {
define i32 @test5b() {
; CHECK-LABEL: @test5b(
; CHECK: ret i32 -1
; CHECK: ret i32 0
%B = ashr i32 undef, 2 ;; top two bits must be equal, so not undef
ret i32 %B
}
define i32 @test5b2(i32 %A) {
; CHECK-LABEL: @test5b2(
; CHECK: ret i32 -1
; CHECK: ret i32 0
%B = ashr i32 undef, %A ;; top %A bits must be equal, so not undef
ret i32 %B
}