[PATCH] D138814: [InstCombine] Combine a/lshr of add -> uadd.with.overflow

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

Thanks.



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Comment at: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp:847-848
+//      - Only the K leading bits of X and Y can be non-zero.
+//      - The add is only used by the shr, or by iK truncates.
+//      - The lshr type has more than 2 bits.
+Instruction *InstCombinerImpl::foldLShrToOverflow(BinaryOperator &I) {
----------------



================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp:880
+      TruncInst *Trunc = dyn_cast<TruncInst>(U);
+      if (!Trunc || Trunc->getType()->getScalarSizeInBits() > ShAmt)
+        return nullptr;
----------------
Is there test coverage when the shift amt isn't the half of the original width?


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp:902-905
+  // Replace the uses of the original add with a zext of the
+  // uaddo's result.
+  if (!Add->hasOneUse())
+    replaceInstUsesWith(*AddInst, Builder.CreateZExt(UAdd, Ty));
----------------
alive2 proof please? 


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  https://reviews.llvm.org/D138814/new/

https://reviews.llvm.org/D138814