[llvm] [LoopIdiom] Use HashRecognize to optimize CRC (PR #143208)

[Ramkumar Ramachandra via llvm-commits]

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@@ -1473,6 +1491,160 @@ bool LoopIdiomRecognize::avoidLIRForMultiBlockLoop(bool IsMemset,
   return false;
 }
 
+bool LoopIdiomRecognize::optimizeCRCLoop(const PolynomialInfo &Info) {
+  // FIXME: Hexagon has a special HexagonLoopIdiom that optimizes CRC using
+  // carry-less multiplication instructions, which is more efficient than our
+  // Sarwate table-lookup optimization. Hence, until we're able to emit
+  // target-specific instructions for Hexagon, subsuming HexagonLoopIdiom,
+  // disable the optimization for Hexagon.
+  Module &M = *CurLoop->getHeader()->getModule();
+  Triple TT(M.getTargetTriple());
+  if (TT.getArch() == Triple::hexagon)
+    return false;
+
+  // First, create a new GlobalVariable corresponding to the
+  // Sarwate-lookup-table.
+  Type *CRCTy = Info.LHS->getType();
+  unsigned CRCBW = CRCTy->getIntegerBitWidth();
+  std::array<Constant *, 256> CRCConstants;
+  transform(HashRecognize::genSarwateTable(Info.RHS, Info.ByteOrderSwapped),
+            CRCConstants.begin(),
+            [CRCTy](const APInt &E) { return ConstantInt::get(CRCTy, E); });
+  Constant *ConstArray =
+      ConstantArray::get(ArrayType::get(CRCTy, 256), CRCConstants);
+  GlobalVariable *GV =
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artagnon wrote:

I would expect Identical COMDAT folding to work on any two identical global variables, and I'm not sure what that has to do with the patch? Does it depend on some parameter of the GlobalVariable?

https://github.com/llvm/llvm-project/pull/143208