[compiler-rt] [llvm] [clang] [InstrProf] Single byte counters in coverage (PR #75425)

[Ellis Hoag via cfe-commits]

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@@ -821,15 +822,23 @@ void InstrProfRecord::merge(InstrProfRecord &Other, uint64_t Weight,
 
   for (size_t I = 0, E = Other.Counts.size(); I < E; ++I) {
     bool Overflowed;
-    uint64_t Value =
-        SaturatingMultiplyAdd(Other.Counts[I], Weight, Counts[I], &Overflowed);
-    if (Value > getInstrMaxCountValue()) {
-      Value = getInstrMaxCountValue();
-      Overflowed = true;
+    uint64_t Value;
+    // When a profile has single byte coverage, use || to merge counters.
+    if (HasSingleByteCoverage)
+      Value = Other.Counts[I] || Counts[I];
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ellishg wrote:

As I understand, this is needed so that when we merge multiple raw coverage profiles we get an indexed profile with counters that are either 0 or 1.
If we instead merge by adding counters, we would get technically inaccurate counts. However, they would be accurate if interpreted for coverage, i.e., 0 is not covered and >0 is covered. In addition, indexed profiles would have more information because if a counter is large, we know it is commonly executed because it was covered in many raw profiles.
Do you think it makes sense to merge these as counters for the indexed profile and simply interpret them as coverage?

https://github.com/llvm/llvm-project/pull/75425