[llvm] [SampleFDO] Read call-graph matching recovered top-level function profile (PR #101053)

[via llvm-commits]

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@@ -782,6 +782,26 @@ bool SampleProfileMatcher::functionMatchesProfileHelper(
   float Similarity = 0.0;
 
   const auto *FSFlattened = getFlattenedSamplesFor(ProfFunc);
+  // Check if the function is top-level function. For extended profile format,
+  // if a function profile is unused and it's top-level, even if the profile is
+  // matched, it's not found in the profile. This is because sample reader only
+  // read the used profile at the beginning, we need to read the profile
+  // on-demand. Also save it into the FlattenedProfiles for future look-up.
+  if (!FSFlattened) {
+    DenseSet<StringRef> TopLevelFunc;
+    TopLevelFunc.insert(ProfFunc.stringRef());
+    SampleProfileMap TopLevelProfile;
+    Reader.readOnDemand(TopLevelFunc, TopLevelProfile);
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WenleiHe wrote:

How about we add a switch to on/off this feature? Just in case this (loading profile for matched functions) overall becomes expensive.  

https://github.com/llvm/llvm-project/pull/101053