[llvm] [MemProf] Optionally update hints on existing hot/cold new calls (PR #91047)

[Snehasish Kumar via llvm-commits]

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@@ -1722,45 +1730,118 @@ Value *LibCallSimplifier::optimizeNew(CallInst *CI, IRBuilderBase &B,
   uint8_t HotCold;
   if (CI->getAttributes().getFnAttr("memprof").getValueAsString() == "cold")
     HotCold = ColdNewHintValue;
+  else if (CI->getAttributes().getFnAttr("memprof").getValueAsString() ==
+           "notcold")
+    HotCold = NotColdNewHintValue;
   else if (CI->getAttributes().getFnAttr("memprof").getValueAsString() == "hot")
     HotCold = HotNewHintValue;
   else
     return nullptr;
 
+  // For calls that already pass a hot/cold hint, only update the hint if
+  // directed by OptimizeExistingHotColdNew. For other calls to new, add a hint
+  // if cold or hot, and leave as-is for default handling if "notcold" aka warm.
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snehasish wrote:

If an existing hinted allocation is to be marked with a notcold hint, there should be a slight performance advantage to delegate to an unhinted variant rather than passing in the new hinted value. It's not a very important use case right now  but perhaps worth noting in the comment?

https://github.com/llvm/llvm-project/pull/91047