[PATCH] D36443: [InstCombine] Support pulling left shifts through a subtract with constant LHS

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

For reference, http://rise4fun.com/Alive/8l5 .

This transform throws nsw/nuw flags; can we preserve them in some situations?



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Comment at: lib/Transforms/InstCombine/InstCombineShifts.cpp:525
+        return BinaryOperator::CreateSub(NewRHS, NewShift);
+      }
     }
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Maybe add a comment writing out the transform in mathematical terms?  (e.g. "2 * (A - B)" -> "2 * A - 2 * B").


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Comment at: test/Transforms/InstCombine/sub.ll:1082
+  %D = sub i32 2, %C
+  ret i32 %D
+}
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This could use a couple more tests, to specifically show the transform this patch adds.


https://reviews.llvm.org/D36443