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Fix a mistake in the computation of leading zeros for udiv. 

llvm-svn: 50591
This commit is contained in:
Dan Gohman 2008-05-02 21:30:02 +00:00
parent 82b9e96274
commit 1962c2be6a
4 changed files with 49 additions and 6 deletions
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8
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
View File

@ -1257,7 +1257,7 @@ void SelectionDAG::ComputeMaskedBits(SDOperand Op, const APInt &Mask,
case ISD::UDIV: {
// For the purposes of computing leading zeros we can conservatively
// treat a udiv as a logical right shift by the power of 2 known to
// be greater than the denominator.
// be less than the denominator.
APInt AllOnes = APInt::getAllOnesValue(BitWidth);
ComputeMaskedBits(Op.getOperand(0),
AllOnes, KnownZero2, KnownOne2, Depth+1);
@ -1267,8 +1267,10 @@ void SelectionDAG::ComputeMaskedBits(SDOperand Op, const APInt &Mask,
KnownZero2.clear();
ComputeMaskedBits(Op.getOperand(1),
AllOnes, KnownZero2, KnownOne2, Depth+1);
LeadZ = std::min(BitWidth,
LeadZ + BitWidth - KnownOne2.countLeadingZeros());
unsigned RHSUnknownLeadingOnes = KnownOne2.countLeadingZeros();
if (RHSUnknownLeadingOnes != BitWidth)
LeadZ = std::min(BitWidth,
LeadZ + BitWidth - RHSUnknownLeadingOnes - 1);
KnownZero = APInt::getHighBitsSet(BitWidth, LeadZ) & Mask;
return;

8
llvm/lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -780,7 +780,7 @@ void InstCombiner::ComputeMaskedBits(Value *V, const APInt &Mask,
case Instruction::UDiv: {
// For the purposes of computing leading zeros we can conservatively
// treat a udiv as a logical right shift by the power of 2 known to
// be greater than the denominator.
// be less than the denominator.
APInt AllOnes = APInt::getAllOnesValue(BitWidth);
ComputeMaskedBits(I->getOperand(0),
AllOnes, KnownZero2, KnownOne2, Depth+1);
@ -790,8 +790,10 @@ void InstCombiner::ComputeMaskedBits(Value *V, const APInt &Mask,
KnownZero2.clear();
ComputeMaskedBits(I->getOperand(1),
AllOnes, KnownZero2, KnownOne2, Depth+1);
LeadZ = std::min(BitWidth,
LeadZ + BitWidth - KnownOne2.countLeadingZeros());
unsigned RHSUnknownLeadingOnes = KnownOne2.countLeadingZeros();
if (RHSUnknownLeadingOnes != BitWidth)
LeadZ = std::min(BitWidth,
LeadZ + BitWidth - RHSUnknownLeadingOnes - 1);
KnownZero = APInt::getHighBitsSet(BitWidth, LeadZ) & Mask;
return;

14
llvm/test/Transforms/InstCombine/udiv-simplify-bug-0.ll Normal file
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@ -0,0 +1,14 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {ret i64 0} | count 2
define i64 @foo(i32 %x) nounwind {
%y = lshr i32 %x, 1
%r = udiv i32 %y, -1
%z = sext i32 %r to i64
ret i64 %z
}
define i64 @bar(i32 %x) nounwind {
%y = lshr i32 %x, 31
%r = udiv i32 %y, 3
%z = sext i32 %r to i64
ret i64 %z
}

25
llvm/test/Transforms/InstCombine/udiv-simplify-bug-1.ll Normal file
View File

@ -0,0 +1,25 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis > %t1.ll
; RUN: grep udiv %t1.ll | count 3
; RUN: grep zext %t1.ll | count 3
; PR2274
; The udiv instructions shouldn't be optimized away, and the
; sext instructions should be optimized to zext.
define i64 @foo(i32 %x) nounwind {
%r = udiv i32 %x, -1
%z = sext i32 %r to i64
ret i64 %z
}
define i64 @bar(i32 %x) nounwind {
%y = lshr i32 %x, 30
%r = udiv i32 %y, 3
%z = sext i32 %r to i64
ret i64 %z
}
define i64 @qux(i32 %x, i32 %v) nounwind {
%y = lshr i32 %x, 31
%r = udiv i32 %y, %v
%z = sext i32 %r to i64
ret i64 %z
}