[llvm] [PreISelIntrinsicLowering] Produce a memset_pattern16 libcall for llvm.experimental.memset.pattern when available (PR #120420)

[Philip Reames via llvm-commits]

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@@ -233,6 +234,60 @@ static bool canEmitLibcall(const TargetMachine *TM, Function *F,
   return TLI->getLibcallName(LC) != nullptr;
 }
 
+// Return a value appropriate for use with the memset_pattern16 libcall, if
+// possible and if we know how. (Adapted from equivalent helper in
+// LoopIdiomRecognize).
+static Constant *getMemSetPattern16Value(MemSetPatternInst *Inst,
+                                         const TargetLibraryInfo &TLI) {
+  // FIXME: This could check for UndefValue because it can be merged into any
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preames wrote:

Replace FIXME w/TODO.  Usually TODO means there's a bug, not a missed optimization.  

https://github.com/llvm/llvm-project/pull/120420