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InstCombine, InstSimplify: (%X /s C1) /s C2 isn't always 0 when C1 * C2 overflow

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consider:
C1 = INT_MIN
C2 = -1

C1 * C2 overflows without a doubt but consider the following:
%x = i32 INT_MIN

This means that (%X /s C1) is 1 and (%X /s C1) /s C2 is -1.

N. B.  Move the unsigned version of this transform to InstSimplify, it
doesn't create any new instructions.

This fixes PR21243.

llvm-svn: 219567
This commit is contained in:
David Majnemer 2014-10-11 10:20:01 +00:00
parent 2d1353223b
commit 475b056c9a
4 changed files with 30 additions and 19 deletions
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10
lib/Analysis/InstructionSimplify.cpp
View File

@ -1057,6 +1057,16 @@ static Value *SimplifyDiv(Instruction::BinaryOps Opcode, Value *Op0, Value *Op1,
(!isSigned && match(Op0, m_URem(m_Value(), m_Specific(Op1)))))
return Constant::getNullValue(Op0->getType());
// (X /u C1) /u C2 -> 0 if C1 * C2 overflow
ConstantInt *C1, *C2;
if (!isSigned && match(Op0, m_UDiv(m_Value(X), m_ConstantInt(C1))) &&
match(Op1, m_ConstantInt(C2))) {
bool Overflow;
C1->getValue().umul_ov(C2->getValue(), Overflow);
if (Overflow)
return Constant::getNullValue(Op0->getType());
}
// If the operation is with the result of a select instruction, check whether
// operating on either branch of the select always yields the same value.
if (isa<SelectInst>(Op0) || isa<SelectInst>(Op1))

9
lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -727,11 +727,10 @@ Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
// (X / C1) / C2 -> X / (C1*C2)
if (Instruction::BinaryOps(LHS->getOpcode()) == I.getOpcode())
if (ConstantInt *LHSRHS = dyn_cast<ConstantInt>(LHS->getOperand(1))) {
if (MultiplyOverflows(RHS, LHSRHS,
I.getOpcode() == Instruction::SDiv))
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
return BinaryOperator::Create(I.getOpcode(), LHS->getOperand(0),
ConstantExpr::getMul(RHS, LHSRHS));
if (!MultiplyOverflows(RHS, LHSRHS,
I.getOpcode() == Instruction::SDiv))
return BinaryOperator::Create(I.getOpcode(), LHS->getOperand(0),
ConstantExpr::getMul(RHS, LHSRHS));
}
Value *X;

14
test/Transforms/InstCombine/2008-02-16-SDivOverflow.ll
View File

@ -1,14 +0,0 @@
; RUN: opt < %s -instcombine -S | grep "ret i.* 0" | count 2
; PR2048
define i32 @i(i32 %a) {
%tmp1 = sdiv i32 %a, -1431655765
%tmp2 = sdiv i32 %tmp1, 3
ret i32 %tmp2
}
define i8 @j(i8 %a) {
%tmp1 = sdiv i8 %a, 64
%tmp2 = sdiv i8 %tmp1, 3
ret i8 %tmp2
}

16
test/Transforms/InstSimplify/exact-nsw-nuw.ll
View File

@ -42,3 +42,19 @@ define i32 @shift5(i32 %A, i32 %B) {
%D = ashr i32 %C, %B
ret i32 %D
}
; CHECK-LABEL: @div1(
; CHECK: ret i32 0
define i32 @div1(i32 %V) {
%A = udiv i32 %V, -2147483648
%B = udiv i32 %A, -2147483648
ret i32 %B
}
; CHECK-LABEL: @div2(
; CHECK-NOT: ret i32 0
define i32 @div2(i32 %V) {
%A = sdiv i32 %V, -1
%B = sdiv i32 %A, -2147483648
ret i32 %B
}