[clang][CodeGenPGO] Don't use an invalid index when region counts disagree

[lanza]

Summary

If we're using an old instrprof profile and the user passes
-Wno-profile-instr-out-of-date then we can get Decls with children
decl counts not matching the what the profile was written against. In
a particular case I was debugging we have 24 decls in the AST and 22
decls in the profile.

There probably isn't a "right" thing to do other than override the users
wish to force using the profiles, but that't the point of that flag in
the first place.

[lanza]

Actually, this still is consumed even without that flag being passed. Anybody know the rational for not having at least an llvm major version check here?

[smeenai]

Seems pretty reasonable to add a bounds check here. You should probably add a comment explaining the reasoning though.

clang/lib/CodeGen/CodeGenPGO.h

void emitCounterIncrement(CGBuilderTy &Builder, const Stmt *S,
                             llvm::Value *StepV);
 
   /// Return the region count for the counter at the given index.
   uint64_t getRegionCount(const Stmt *S) {
     if (!RegionCounterMap)
       return 0;
     if (!haveRegionCounts())
       return 0;
-    return RegionCounts[(*RegionCounterMap)[S]];
+    // With profiles from a differing version of clang we can have mismatched
+    // decl counts. Don't crash in such a case.
+    auto Index = (*RegionCounterMap)[S];
+    if (Index >= RegionCounts.size())
+      return 0;
+    return RegionCounts[Index];
   }
 };
 
 }  // end namespace CodeGen
 }  // end namespace clang
 
 #endif