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InstCombine: Don't miscompile (x lshr C1) udiv C2

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We have a transform that changes:
  (x lshr C1) udiv C2
into:
  x udiv (C2 << C1)

However, it is unsafe to do so if C2 << C1 discards any of C2's bits.

This fixes PR21255.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219634 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2014-10-13 21:48:30 +00:00
parent 2061a56b8a
commit dfe81adbce
4 changed files with 40 additions and 13 deletions
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3
include/llvm/ADT/APInt.h
View File

@ -945,7 +945,8 @@ public:
APInt sdiv_ov(const APInt &RHS, bool &Overflow) const;
APInt smul_ov(const APInt &RHS, bool &Overflow) const;
APInt umul_ov(const APInt &RHS, bool &Overflow) const;
APInt sshl_ov(unsigned Amt, bool &Overflow) const;
APInt sshl_ov(const APInt &Amt, bool &Overflow) const;
APInt ushl_ov(const APInt &Amt, bool &Overflow) const;
/// \brief Array-indexing support.
///

20
lib/Support/APInt.cpp
View File

@ -2064,19 +2064,29 @@ APInt APInt::umul_ov(const APInt &RHS, bool &Overflow) const {
return Res;
}
APInt APInt::sshl_ov(unsigned ShAmt, bool &Overflow) const {
Overflow = ShAmt >= getBitWidth();
APInt APInt::sshl_ov(const APInt &ShAmt, bool &Overflow) const {
Overflow = ShAmt.uge(getBitWidth());
if (Overflow)
ShAmt = getBitWidth()-1;
return APInt(BitWidth, 0);
if (isNonNegative()) // Don't allow sign change.
Overflow = ShAmt >= countLeadingZeros();
Overflow = ShAmt.uge(countLeadingZeros());
else
Overflow = ShAmt >= countLeadingOnes();
Overflow = ShAmt.uge(countLeadingOnes());
return *this << ShAmt;
}
APInt APInt::ushl_ov(const APInt &ShAmt, bool &Overflow) const {
Overflow = ShAmt.uge(getBitWidth());
if (Overflow)
return APInt(BitWidth, 0);
Overflow = ShAmt.ugt(countLeadingZeros());
return *this << ShAmt;
}

14
lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -965,11 +965,17 @@ Instruction *InstCombiner::visitUDiv(BinaryOperator &I) {
return Common;
// (x lshr C1) udiv C2 --> x udiv (C2 << C1)
if (Constant *C2 = dyn_cast<Constant>(Op1)) {
{
Value *X;
Constant *C1;
if (match(Op0, m_LShr(m_Value(X), m_Constant(C1))))
return BinaryOperator::CreateUDiv(X, ConstantExpr::getShl(C2, C1));
const APInt *C1, *C2;
if (match(Op0, m_LShr(m_Value(X), m_APInt(C1))) &&
match(Op1, m_APInt(C2))) {
bool Overflow;
APInt C2ShlC1 = C2->ushl_ov(*C1, Overflow);
if (!Overflow)
return BinaryOperator::CreateUDiv(
X, ConstantInt::get(X->getType(), C2ShlC1));
}
}
// (zext A) udiv (zext B) --> zext (A udiv B)

16
test/Transforms/InstCombine/div.ll
View File

@ -132,11 +132,11 @@ define i32 @test15(i32 %a, i32 %b) nounwind {
}
define <2 x i64> @test16(<2 x i64> %x) nounwind {
%shr = lshr <2 x i64> %x, <i64 3, i64 5>
%div = udiv <2 x i64> %shr, <i64 4, i64 6>
%shr = lshr <2 x i64> %x, <i64 5, i64 5>
%div = udiv <2 x i64> %shr, <i64 6, i64 6>
ret <2 x i64> %div
; CHECK-LABEL: @test16(
; CHECK-NEXT: udiv <2 x i64> %x, <i64 32, i64 192>
; CHECK-NEXT: udiv <2 x i64> %x, <i64 192, i64 192>
; CHECK-NEXT: ret <2 x i64>
}
@ -264,3 +264,13 @@ define i32 @test30(i32 %a) {
; CHECK-LABEL: @test30(
; CHECK-NEXT: ret i32 %a
}
define <2 x i32> @test31(<2 x i32> %x) nounwind {
%shr = lshr <2 x i32> %x, <i32 31, i32 31>
%div = udiv <2 x i32> %shr, <i32 2147483647, i32 2147483647>
ret <2 x i32> %div
; CHECK-LABEL: @test31(
; CHECK-NEXT: %[[shr:.*]] = lshr <2 x i32> %x, <i32 31, i32 31>
; CHECK-NEXT: udiv <2 x i32> %[[shr]], <i32 2147483647, i32 2147483647>
; CHECK-NEXT: ret <2 x i32>
}