diff options
| -rw-r--r-- | llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp | 20 | ||||
| -rw-r--r-- | llvm/test/Transforms/InstCombine/shift-amount-reassociation-in-bittest.ll | 11 |
2 files changed, 26 insertions, 5 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp index f38dc436722..e49e6cec65c 100644 --- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp +++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp @@ -3494,7 +3494,8 @@ foldShiftIntoShiftInAnotherHandOfAndInICmp(ICmpInst &I, const SimplifyQuery SQ, Instruction *NarrowestShift = XShift; Type *WidestTy = WidestShift->getType(); - assert(NarrowestShift->getType() == I.getOperand(0)->getType() && + Type *NarrowestTy = NarrowestShift->getType(); + assert(NarrowestTy == I.getOperand(0)->getType() && "We did not look past any shifts while matching XShift though."); bool HadTrunc = WidestTy != I.getOperand(0)->getType(); @@ -3533,6 +3534,23 @@ foldShiftIntoShiftInAnotherHandOfAndInICmp(ICmpInst &I, const SimplifyQuery SQ, if (XShAmt->getType() != YShAmt->getType()) return nullptr; + // As input, we have the following pattern: + // icmp eq/ne (and ((x shift Q), (y oppositeshift K))), 0 + // We want to rewrite that as: + // icmp eq/ne (and (x shift (Q+K)), y), 0 iff (Q+K) u< bitwidth(x) + // While we know that originally (Q+K) would not overflow + // (because 2 * (N-1) u<= iN -1), we have looked past extensions of + // shift amounts. so it may now overflow in smaller bitwidth. + // To ensure that does not happen, we need to ensure that the total maximal + // shift amount is still representable in that smaller bit width. + unsigned MaximalPossibleTotalShiftAmount = + (WidestTy->getScalarSizeInBits() - 1) + + (NarrowestTy->getScalarSizeInBits() - 1); + APInt MaximalRepresentableShiftAmount = + APInt::getAllOnesValue(XShAmt->getType()->getScalarSizeInBits()); + if (MaximalRepresentableShiftAmount.ult(MaximalPossibleTotalShiftAmount)) + return nullptr; + // Can we fold (XShAmt+YShAmt) ? auto *NewShAmt = dyn_cast_or_null<Constant>( SimplifyAddInst(XShAmt, YShAmt, /*isNSW=*/false, diff --git a/llvm/test/Transforms/InstCombine/shift-amount-reassociation-in-bittest.ll b/llvm/test/Transforms/InstCombine/shift-amount-reassociation-in-bittest.ll index 0386d8042f9..97506e193e6 100644 --- a/llvm/test/Transforms/InstCombine/shift-amount-reassociation-in-bittest.ll +++ b/llvm/test/Transforms/InstCombine/shift-amount-reassociation-in-bittest.ll @@ -688,13 +688,16 @@ entry: ret i1 %tobool } -; FIXME: this is a miscompile. We should not transform this. ; See https://bugs.llvm.org/show_bug.cgi?id=44802 define i1 @pr44802(i3 %a, i3 %x, i3 %y) { ; CHECK-LABEL: @pr44802( -; CHECK-NEXT: [[TMP1:%.*]] = and i3 [[X:%.*]], [[Y:%.*]] -; CHECK-NEXT: [[TMP2:%.*]] = icmp ne i3 [[TMP1]], 0 -; CHECK-NEXT: ret i1 [[TMP2]] +; CHECK-NEXT: [[T0:%.*]] = icmp ne i3 [[A:%.*]], 0 +; CHECK-NEXT: [[T1:%.*]] = zext i1 [[T0]] to i3 +; CHECK-NEXT: [[T2:%.*]] = lshr i3 [[X:%.*]], [[T1]] +; CHECK-NEXT: [[T3:%.*]] = shl i3 [[Y:%.*]], [[T1]] +; CHECK-NEXT: [[T4:%.*]] = and i3 [[T2]], [[T3]] +; CHECK-NEXT: [[T5:%.*]] = icmp ne i3 [[T4]], 0 +; CHECK-NEXT: ret i1 [[T5]] ; %t0 = icmp ne i3 %a, 0 %t1 = zext i1 %t0 to i3 |
