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ConstantFolding: Add a missing folding that leads to a miscompile. 

We use constant folding to see if an intrinsic evaluates to the same value as a
constant that we know. If we don't take the undefinedness into account we get a
value that doesn't match the actual implementation, and miscompiled code.

This was uncovered by Chandler's simplifycfg changes.

llvm-svn: 173356
This commit is contained in:
Benjamin Kramer 2013-01-24 16:28:28 +00:00
parent cbc86bcbb5
commit 435eba09b7
3 changed files with 41 additions and 5 deletions
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8
llvm/lib/Analysis/ConstantFolding.cpp
View File

@ -1474,12 +1474,12 @@ llvm::ConstantFoldCall(Function *F, ArrayRef<Constant *> Operands,
return ConstantStruct::get(cast<StructType>(F->getReturnType()), Ops);
}
case Intrinsic::cttz:
// FIXME: This should check for Op2 == 1, and become unreachable if
// Op1 == 0.
if (Op2->isOne() && Op1->isZero()) // cttz(0, 1) is undef.
return UndefValue::get(Ty);
return ConstantInt::get(Ty, Op1->getValue().countTrailingZeros());
case Intrinsic::ctlz:
// FIXME: This should check for Op2 == 1, and become unreachable if
// Op1 == 0.
if (Op2->isOne() && Op1->isZero()) // ctlz(0, 1) is undef.
return UndefValue::get(Ty);
return ConstantInt::get(Ty, Op1->getValue().countLeadingZeros());
}
}

2
llvm/test/Transforms/ConstProp/2007-11-23-cttz.ll
View File

@ -3,6 +3,6 @@
declare i13 @llvm.cttz.i13(i13, i1)
define i13 @test() {
%X = call i13 @llvm.cttz.i13(i13 0, i1 true)
%X = call i13 @llvm.cttz.i13(i13 0, i1 false)
ret i13 %X
}

36
llvm/test/Transforms/InstCombine/intrinsics.ll
View File

@ -220,3 +220,39 @@ define i32 @cttz_simplify1b(i32 %x) nounwind readnone ssp {
; CHECK: @cttz_simplify1b
; CHECK-NEXT: ret i32 0
}
define i32 @ctlz_undef(i32 %Value) nounwind {
%ctlz = call i32 @llvm.ctlz.i32(i32 0, i1 true)
ret i32 %ctlz
; CHECK: @ctlz_undef
; CHECK-NEXT: ret i32 undef
}
define i32 @cttz_undef(i32 %Value) nounwind {
%cttz = call i32 @llvm.cttz.i32(i32 0, i1 true)
ret i32 %cttz
; CHECK: @cttz_undef
; CHECK-NEXT: ret i32 undef
}
define i32 @ctlz_select(i32 %Value) nounwind {
%tobool = icmp ne i32 %Value, 0
%ctlz = call i32 @llvm.ctlz.i32(i32 %Value, i1 true)
%s = select i1 %tobool, i32 %ctlz, i32 32
ret i32 %s
; CHECK: @ctlz_select
; CHECK: select i1 %tobool, i32 %ctlz, i32 32
}
define i32 @cttz_select(i32 %Value) nounwind {
%tobool = icmp ne i32 %Value, 0
%cttz = call i32 @llvm.cttz.i32(i32 %Value, i1 true)
%s = select i1 %tobool, i32 %cttz, i32 32
ret i32 %s
; CHECK: @cttz_select
; CHECK: select i1 %tobool, i32 %cttz, i32 32
}