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[ValueTracking] Out of range shifts might be undef 

If it is possible for the RHS of a shift operation to be greater than or equal
to the bit-width, then the result might be undef, and we can't report any known
bits.

In some cases, this was allowing a transformation in instcombine which widened
an undef value from i1 to i32, increasing the range of values that a function
could return.

Differential revision: https://reviews.llvm.org/D30781

llvm-svn: 297724
This commit is contained in:
Oliver Stannard 2017-03-14 10:13:17 +00:00
parent d0bee506a0
commit 062041113f
2 changed files with 38 additions and 0 deletions
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8
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -848,6 +848,14 @@ static void computeKnownBitsFromShiftOperator(
computeKnownBits(I->getOperand(1), KnownZero, KnownOne, Depth + 1, Q);
// If the shift amount could be greater than or equal to the bit-width of the LHS, the
// value could be undef, so we don't know anything about it.
if ((~KnownZero).uge(BitWidth)) {
KnownZero.clearAllBits();
KnownOne.clearAllBits();
return;
}
// Note: We cannot use KnownZero.getLimitedValue() here, because if
// BitWidth > 64 and any upper bits are known, we'll end up returning the
// limit value (which implies all bits are known).

30
llvm/test/Transforms/InstCombine/zext-or-icmp.ll
View File

@ -19,3 +19,33 @@ define i8 @zext_or_icmp_icmp(i8 %a, i8 %b) {
; CHECK-NEXT: ret i8 %zext
}
; Here, widening the or from i1 to i32 and removing one of the icmps would
; widen an undef value (created by the out-of-range shift), increasing the
; range of valid values for the return, so we can't do it.
define i32 @dont_widen_undef() {
entry:
br label %block2
block1:
br label %block2
block2:
%m.011 = phi i32 [ 33, %entry ], [ 0, %block1 ]
%cmp.i = icmp ugt i32 %m.011, 1
%m.1.op = lshr i32 1, %m.011
%sext.mask = and i32 %m.1.op, 65535
%cmp115 = icmp ne i32 %sext.mask, 0
%cmp1 = or i1 %cmp.i, %cmp115
%conv2 = zext i1 %cmp1 to i32
ret i32 %conv2
; CHECK-LABEL: dont_widen_undef(
; CHECK: %m.011 = phi i32 [ 33, %entry ], [ 0, %block1 ]
; CHECK-NEXT: %cmp.i = icmp ugt i32 %m.011, 1
; CHECK-NEXT: %m.1.op = lshr i32 1, %m.011
; CHECK-NEXT: %sext.mask = and i32 %m.1.op, 65535
; CHECK-NEXT: %cmp115 = icmp ne i32 %sext.mask, 0
; CHECK-NEXT: %cmp1 = or i1 %cmp.i, %cmp115
; CHECK-NEXT: %conv2 = zext i1 %cmp1 to i32
; CHECK-NEXT: ret i32 %conv2
}