[llvm] [LoopIdiom] Select llvm.experimental.memset.pattern intrinsic rather than memset_pattern16 libcall (PR #126736)

[Craig Topper via llvm-commits]

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@@ -1064,53 +1073,95 @@ bool LoopIdiomRecognize::processLoopStridedStore(
     return Changed;
 
   // Okay, everything looks good, insert the memset.
+  // MemsetArg is the number of bytes for the memset libcall, and the number
+  // of pattern repetitions if the memset.pattern intrinsic is being used.
+  Value *MemsetArg;
+  std::optional<int64_t> BytesWritten = std::nullopt;
+
+  if (PatternValue && (HasMemsetPattern || ForceMemsetPatternIntrinsic)) {
+    const SCEV *TripCountS =
+        SE->getTripCountFromExitCount(BECount, IntIdxTy, CurLoop);
+    if (!Expander.isSafeToExpand(TripCountS))
+      return Changed;
+    const SCEVConstant *ConstStoreSize = dyn_cast<SCEVConstant>(StoreSizeSCEV);
+    if (!ConstStoreSize)
+      return Changed;
+    Value *TripCount = Expander.expandCodeFor(TripCountS, IntIdxTy,
+                                              Preheader->getTerminator());
+    uint64_t PatternRepsPerTrip =
+        (ConstStoreSize->getValue()->getZExtValue() * 8) /
+        DL->getTypeSizeInBits(PatternValue->getType());
+    // If ConstStoreSize is not equal to the width of PatternValue, then
+    // MemsetArg is TripCount * (ConstStoreSize/PatternValueWidth). Else
+    // MemSetArg is just TripCount.
+    MemsetArg =
+        PatternRepsPerTrip == 1
+            ? TripCount
+            : Builder.CreateMul(TripCount,
+                                Builder.getIntN(IntIdxTy->getIntegerBitWidth(),
+                                                PatternRepsPerTrip));
+    if (auto CI = dyn_cast<ConstantInt>(TripCount))
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topperc wrote:

I believe this should be `auto *CI` by coding standards.

https://github.com/llvm/llvm-project/pull/126736