[llvm] [LoopIdiom] Use costing to determine CRC strategy (PR #211040)

[Sean Clarke via llvm-commits]

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@@ -1589,43 +1603,110 @@ bool LoopIdiomRecognize::optimizeCRCLoop(const PolynomialInfo &Info) {
   if (TT.getArch() == Triple::hexagon)
     return false;
 
-  // The force-crc-clmul flag should cause the clmul optimization to run
-  // unconditionally.
-  if (ForceCRCClmul) {
+  // The Sarwate lookup table optimization requires a byte-multiple trip count,
+  // and should not be applied under any circumstances if optimizing for size.
+  bool TableStrategyPossible =
+      Info.TripCount % 8 == 0 && !ApplyCodeSizeHeuristics;
+
+  switch (CRCStrategy) {
+  default:
+    break;
+  case CRCStrategyKind::Table: {
+    if (TableStrategyPossible) {
+      optimizeCRCLoopUsingTableLookup(Info);
+      return true;
+    }
+    return false;
+  }
+  case CRCStrategyKind::Clmul:
     optimizeCRCLoopUsingClmul(Info);
     return true;
   }
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xarkenz wrote:

The `-loop-idiom-crc-strategy=table` case still needs to be "conditional" on `TC % 8 == 0` (since it would be applied incorrectly otherwise), but not `ApplyCodeSizeHeuristics`.

https://github.com/llvm/llvm-project/pull/211040