[PATCH] D141773: [InstCombine] Generalize (icmp sgt (1 << Y), -1) -> (icmp ne Y, BitWidth-1) to any negative constant.

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


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Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:2051
+    // (1 << Y) >  C -> Y != 31 if C is negative.
+    if (Pred == ICmpInst::ICMP_SGT && C.sle(0))
       return new ICmpInst(ICmpInst::ICMP_NE, Y, BitWidthMinusOne);
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Nitpick: either change the proof to check *this* precondition, or use the same check as in the proof.


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Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:2057-2058
+    // (1 << Y) <  C -> Y == 31 if C is negative and not signed min.
+    // Exclude signed min by subtracting 1 and lower the upper bound to 0.
+    if (Pred == ICmpInst::ICMP_SLT && (C-1).sle(0))
       return new ICmpInst(ICmpInst::ICMP_EQ, Y, BitWidthMinusOne);
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Same nitpick. Not what the proof shows.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D141773/new/

https://reviews.llvm.org/D141773