[llvm] [InstCombine] Narrow trunc(lshr) in more cases (PR #139645)

[Yingwei Zheng via llvm-commits]

================
@@ -319,13 +321,21 @@ static bool canEvaluateTruncated(Value *V, Type *Ty, InstCombinerImpl &IC,
     //       zero - use AmtKnownBits.getMaxValue().
     uint32_t OrigBitWidth = OrigTy->getScalarSizeInBits();
     uint32_t BitWidth = Ty->getScalarSizeInBits();
-    KnownBits AmtKnownBits =
-        llvm::computeKnownBits(I->getOperand(1), IC.getDataLayout());
+    KnownBits AmtKnownBits = IC.computeKnownBits(I->getOperand(1), 0, CxtI);
+    APInt MaxShiftAmt = AmtKnownBits.getMaxValue();
     APInt ShiftedBits = APInt::getBitsSetFrom(OrigBitWidth, BitWidth);
-    if (AmtKnownBits.getMaxValue().ult(BitWidth) &&
-        IC.MaskedValueIsZero(I->getOperand(0), ShiftedBits, 0, CxtI)) {
-      return canEvaluateTruncated(I->getOperand(0), Ty, IC, CxtI) &&
-             canEvaluateTruncated(I->getOperand(1), Ty, IC, CxtI);
+    if (MaxShiftAmt.ult(BitWidth)) {
+      // If the only user is a trunc then we can narrow the shift if any new
+      // MSBs are not going to be used.
+      if (auto *Trunc = dyn_cast<TruncInst>(V->user_back())) {
+        auto DemandedBits = Trunc->getType()->getScalarSizeInBits();
+        if ((MaxShiftAmt + DemandedBits).ule(BitWidth))
----------------
dtcxzyw wrote:

Oh, this is enough.


https://github.com/llvm/llvm-project/pull/139645