[InstCombine] foldShiftIntoShiftInAnotherHandOfAndInICmp(): fix miscompile (PR44802)

Much like with reassociateShiftAmtsOfTwoSameDirectionShifts(),
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 may 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 bitwidth.
If the overflow would happen, (Q+K) u< bitwidth(x) check would be bogus.

https://bugs.llvm.org/show_bug.cgi?id=44802
(cherry picked from commit 2855c8fed9326ec44526767f1596a4fe4e55dc70)

1 files changed, 19 insertions, 1 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,