[llvm] [PGO] Ensure non-zero entry-count after `populateCounters` (PR #112029)

[Michael O'Farrell via llvm-commits]

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@@ -1624,8 +1624,12 @@ void PGOUseFunc::populateCounters() {
     FuncMaxCount = std::max(FuncMaxCount, *BI->Count);
   }
 
-  // Fix the obviously inconsistent entry count.
-  if (FuncMaxCount > 0 && FuncEntryCount == 0)
+  // Fix the obviously inconsistent entry count.  A function that has all zero
+  // counters will skip populating the counters so a minimum entry count of 1
+  // makes sense.  Note that the presence of a non-zero counter does not imply
+  // FuncMaxCount is greater than zero because the only non-zero counts could
+  // be for select instrumentation which is handled later.
+  if (FuncEntryCount == 0)
----------------
mofarrell wrote:

FWIW I went this route because there are already similar feeble attempts elsewhere.  Like when loading the profile (but it only works if the entry block was chosen for instrumentation): https://github.com/llvm/llvm-project/blob/abfdcb663983eaa8c7ce0d03a3c6440c5fc97e9a/llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp#L1279-L1280

That does factor in to the MaxCount at the end of the day though.

https://github.com/llvm/llvm-project/pull/112029