[PATCH] D88316: [InstCombine] recognizeBSwapOrBitReverseIdiom - recognise zext(bswap(trunc(x))) patterns (PR39793)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri accepted this revision.
lebedev.ri added a comment.
This revision is now accepted and ready to land.

Looks good to me. @spatel ?



================
Comment at: llvm/lib/Transforms/Utils/Local.cpp:3026-3037
+  // If the upper bits are zero, then attempt to perform as a truncated op.
+  if (BitProvenance[BitProvenance.size() - 1] < 0) {
+    unsigned NumBits = BitProvenance.size();
+    unsigned NumUpperZeroBits = 1;
+    while (NumBits > NumUpperZeroBits &&
+           BitProvenance[NumBits - 1 - NumUpperZeroBits] < 0)
+      ++NumUpperZeroBits;
----------------
Suggestion:
```
while(!BitProvenance.empty() && BitProvenance.back() < 0)
  BitProvenance = BitProvenance.drop_back();
if(BitProvenance.empty())
  ???
DemandedTy = IntegerType::get(I->getContext(), BitProvenance.size());
```



Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D88316/new/

https://reviews.llvm.org/D88316