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InstCombine: Don't fold (X <<s log(INT_MIN)) /s INT_MIN to X

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Consider the case where X is 2.  (2 <<s 31)/s-2147483648 is zero but we
would fold to X.  Note that this is valid when we are in the unsigned
domain because we require NUW: 2 <<u 31 results in poison.

This fixes PR21245.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219568 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2014-10-11 10:20:04 +00:00
parent 9043f74acb
commit 171825a8ce
2 changed files with 19 additions and 1 deletions
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3
lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -762,7 +762,8 @@ Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
}
}
if ((IsSigned && match(LHS, m_NSWShl(m_Value(X), m_APInt(C1)))) ||
if ((IsSigned && match(LHS, m_NSWShl(m_Value(X), m_APInt(C1))) &&
*C1 != C1->getBitWidth() - 1) ||
(!IsSigned && match(LHS, m_NUWShl(m_Value(X), m_APInt(C1))))) {
APInt Quotient(C1->getBitWidth(), /*Val=*/0ULL, IsSigned);
APInt C1Shifted = APInt::getOneBitSet(

17
test/Transforms/InstCombine/div.ll
View File

@ -247,3 +247,20 @@ define i32 @test28(i32 %a) {
; CHECK-NEXT: %div = mul nuw i32 %a, 12
; CHECK-NEXT: ret i32 %div
}
define i32 @test29(i32 %a) {
%mul = shl nsw i32 %a, 31
%div = sdiv i32 %mul, -2147483648
ret i32 %div
; CHECK-LABEL: @test29(
; CHECK-NEXT: %[[and:.*]] = and i32 %a, 1
; CHECK-NEXT: ret i32 %[[and]]
}
define i32 @test30(i32 %a) {
%mul = shl nuw i32 %a, 31
%div = udiv i32 %mul, -2147483648
ret i32 %div
; CHECK-LABEL: @test30(
; CHECK-NEXT: ret i32 %a
}