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InstSimplify: shl nsw/nuw undef, %V -> undef 

We can always choose an value for undef which might cause %V to shift
out an important bit except for one case, when %V is zero.

However, shl behaves like an identity function when the right hand side
is zero.

llvm-svn: 224405
This commit is contained in:
David Majnemer 2014-12-17 01:54:33 +00:00
parent ee0a3a7a2f
commit 65c52ae8ca
2 changed files with 35 additions and 13 deletions
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20
llvm/lib/Analysis/InstructionSimplify.cpp
View File

@ -1338,6 +1338,11 @@ static Value *SimplifyRightShift(unsigned Opcode, Value *Op0, Value *Op1,
if (Op0 == Op1)
return Constant::getNullValue(Op0->getType());
// undef >> X -> 0
// undef >> X -> undef (if it's exact)
if (match(Op0, m_Undef()))
return isExact ? Op0 : Constant::getNullValue(Op0->getType());
// The low bit cannot be shifted out of an exact shift if it is set.
if (isExact) {
unsigned BitWidth = Op0->getType()->getScalarSizeInBits();
@ -1360,8 +1365,9 @@ static Value *SimplifyShlInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
return V;
// undef << X -> 0
// undef << X -> undef if (if it's NSW/NUW)
if (match(Op0, m_Undef()))
return Constant::getNullValue(Op0->getType());
return isNSW || isNUW ? Op0 : Constant::getNullValue(Op0->getType());
// (X >> A) << A -> X
Value *X;
@ -1386,12 +1392,6 @@ static Value *SimplifyLShrInst(Value *Op0, Value *Op1, bool isExact,
MaxRecurse))
return V;
// undef >>l X -> 0
// undef >>l X -> undef (if it's exact)
if (match(Op0, m_Undef()))
return isExact ? UndefValue::get(Op0->getType())
: Constant::getNullValue(Op0->getType());
// (X << A) >> A -> X
Value *X;
if (match(Op0, m_NUWShl(m_Value(X), m_Specific(Op1))))
@ -1422,12 +1422,6 @@ static Value *SimplifyAShrInst(Value *Op0, Value *Op1, bool isExact,
if (match(Op0, m_AllOnes()))
return Op0;
// undef >>a X -> 0
// undef >>a X -> undef (if it's exact)
if (match(Op0, m_Undef()))
return isExact ? UndefValue::get(Op0->getType())
: Constant::getNullValue(Op0->getType());
// (X << A) >> A -> X
Value *X;
if (match(Op0, m_NSWShl(m_Value(X), m_Specific(Op1))))

28
llvm/test/Transforms/InstSimplify/undef.ll
View File

@ -216,3 +216,31 @@ define i32 @test27() {
%b = shl i32 0, undef
ret i32 %b
}
; CHECK-LABEL: @test28
; CHECK: ret i32 undef
define i32 @test28(i32 %a) {
%b = shl nsw i32 undef, %a
ret i32 %b
}
; CHECK-LABEL: @test29
; CHECK: ret i32 undef
define i32 @test29(i32 %a) {
%b = shl nuw i32 undef, %a
ret i32 %b
}
; CHECK-LABEL: @test30
; CHECK: ret i32 undef
define i32 @test30(i32 %a) {
%b = shl nsw nuw i32 undef, %a
ret i32 %b
}
; CHECK-LABEL: @test31
; CHECK: ret i32 0
define i32 @test31(i32 %a) {
%b = shl i32 undef, %a
ret i32 %b
}