[llvm] [LoopIdiom] Use narrower bit widths where possible in `optimizeCRCLoopUsingClmul` (PR #210139)

[Piotr Fusik via llvm-commits]

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@@ -1628,32 +1625,40 @@ bool LoopIdiomRecognize::optimizeCRCLoop(const PolynomialInfo &Info) {
 // The algorithm used in this optimization is a Polynomial (GF(2)) Barrett
 // Reduction based on Intel's "Fast CRC Computation for Generic Polynomials
 // Using PCLMULQDQ Instruction" white paper (December 2009).
-void LoopIdiomRecognize::optimizeCRCLoopUsingClmul(const PolynomialInfo &Info,
-                                                   IntegerType *ClmulTy) {
+void LoopIdiomRecognize::optimizeCRCLoopUsingClmul(const PolynomialInfo &Info) {
   Type *CRCTy = Info.LHS->getType();
   LLVMContext &Ctx = CRCTy->getContext();
   unsigned CRCBW = CRCTy->getIntegerBitWidth();
   // The loop's TripCount determines how many bits of the data are processed,
   // regardless of whether the actual data bit width matches (if auxiliary data
   // is even used at all).
   unsigned TC = Info.TripCount;
-  unsigned ClmulBW = ClmulTy->getBitWidth();
+  // Based on the clmul inputs, the first clmul needs 2*TC bits, and the second
+  // needs CRCBW+TC bits.
+  IntegerType *ClmulMuTy = IntegerType::get(Ctx, 2 * TC);
+  IntegerType *ClmulGPTy = IntegerType::get(Ctx, CRCBW + TC);
 
   // First, generate the constants required for GF(2) Barrett reduction.
   auto [Mu, FullGenPoly] = HashRecognize::genBarrettConstants(Info);
-  Value *MuConst = ConstantInt::get(Ctx, Mu.zext(ClmulBW));
-  Value *GenPolyConst = ConstantInt::get(Ctx, FullGenPoly.zext(ClmulBW));
+  Value *MuConst = ConstantInt::get(Ctx, Mu.zext(ClmulMuTy->getBitWidth()));
+  Value *GenPolyConst =
+      ConstantInt::get(Ctx, FullGenPoly.zext(ClmulGPTy->getBitWidth()));
 
   IRBuilder<> Builder(CurLoop->getLoopPreheader()->getTerminator());
 
   auto LoTCBits = [TC, &Builder, &Ctx](Value *Op, const Twine &Name) {
     unsigned OpBW = Op->getType()->getIntegerBitWidth();
-    assert(OpBW >= TC && "Bit width should be at least TripCount");
+    if (OpBW <= TC)
+      return Op;
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pfusik wrote:

Oh. Let's keep the `return`.

https://github.com/llvm/llvm-project/pull/210139