[llvm] [PGO] Fix incorrect count threshold calculation when 0% cutoff (PR #117359)

[Ellis Hoag via llvm-commits]

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@@ -77,9 +77,18 @@ static const uint32_t DefaultCutoffsData[] = {
 const ArrayRef<uint32_t> ProfileSummaryBuilder::DefaultCutoffs =
     DefaultCutoffsData;
 
+// An entry for the 0th percentile to correctly calculate hot/cold count
+// thresholds when -profile-summary-cutoff-hot/cold is 0.  If the hot cutoff is
+// 0, no sample counts are treated as hot.  If the cold cutoff is 0, all sample
+// counts are treated as cold.  Assumes there is no UINT64_MAX sample counts.
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ellishg wrote:

It seems like this can be accomplished with `-profile-summary-cutoff-hot=1000000`. A function will be hot if its count is larger than 100% of functions, which is impossible. So no functions are hot. Likewise, `-profile-summary-cutoff-cold=1000000` should make all functions cold. Why do we need a special case for these? With this patch, how do I specify `-profile-summary-cutoff-hot` to make all functions hot?

https://github.com/llvm/llvm-project/pull/117359