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InstSimplify: [al]shr exact undef, %X -> undef

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Exact shifts always keep the non-zero bits of their input.  This means
it keeps it's undef bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223923 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2014-12-10 09:14:52 +00:00
parent 89db49fb9b
commit 6578f1beb1
2 changed files with 20 additions and 2 deletions
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8
lib/Analysis/InstructionSimplify.cpp
View File

@ -1387,8 +1387,10 @@ static Value *SimplifyLShrInst(Value *Op0, Value *Op1, bool isExact,
return V; return V;
// undef >>l X -> 0 // undef >>l X -> 0
// undef >>l X -> undef (if it's exact)
if (match(Op0, m_Undef())) if (match(Op0, m_Undef()))
return Constant::getNullValue(Op0->getType()); return isExact ? UndefValue::get(Op0->getType())
: Constant::getNullValue(Op0->getType());
// (X << A) >> A -> X // (X << A) >> A -> X
Value *X; Value *X;
@ -1421,8 +1423,10 @@ static Value *SimplifyAShrInst(Value *Op0, Value *Op1, bool isExact,
return Op0; return Op0;
// undef >>a X -> all ones // undef >>a X -> all ones
// undef >>a X -> undef (if it's exact)
if (match(Op0, m_Undef())) if (match(Op0, m_Undef()))
return Constant::getAllOnesValue(Op0->getType()); return isExact ? UndefValue::get(Op0->getType())
: Constant::getAllOnesValue(Op0->getType());
// (X << A) >> A -> X // (X << A) >> A -> X
Value *X; Value *X;

14
test/Transforms/InstSimplify/undef.ll
View File

@ -174,3 +174,17 @@ define i32 @test21(i32 %a) {
%b = sdiv i32 %a, 0 %b = sdiv i32 %a, 0
ret i32 %b ret i32 %b
} }
; CHECK-LABEL: @test22
; CHECK: ret i32 undef
define i32 @test22(i32 %a) {
%b = ashr exact i32 undef, %a
ret i32 %b
}
; CHECK-LABEL: @test23
; CHECK: ret i32 undef
define i32 @test23(i32 %a) {
%b = lshr exact i32 undef, %a
ret i32 %b
}