[llvm] [llvm-profgen] Improve sample profile density (PR #92144)

[via llvm-commits]

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@@ -768,9 +747,94 @@ void ProfileGenerator::populateBoundarySamplesForAllFunctions(
   }
 }
 
+void ProfileGeneratorBase::calculateBodySamplesAndSize(
+    const FunctionSamples &FSamples, uint64_t &TotalBodySamples,
+    uint64_t &FuncBodySize) {
+  // Note that ideally the size should be the number of function instruction.
+  // However, for probe-based profile, we don't have the accurate instruction
+  // count for each probe, instead, the probe sample is the samples count for
+  // the block, which is equivelant to
+  // total_instruction_samples/num_of_instruction in one block. Hence, we use
+  // the number of probe as a proxy for the function's size.
+  FuncBodySize += FSamples.getBodySamples().size();
+
+  // The accumulated body samples re-calculated here could be different from the
+  // TotalSamples(getTotalSamples) field of FunctionSamples for line-number
+  // based profile. The reason is that TotalSamples is the sum of all the
+  // samples of the machine instruction in one source-code line, however, the
+  // entry of Bodysamples is the only max number of them, so the TotalSamples is
+  // usually much bigger than the accumulated body samples as one souce-code
+  // line can emit many machine instructions. We observed a regression when we
+  // switched to use the accumulated body samples(by using
+  // -update-total-samples). Hence, it's safer to re-calculate here to avoid
+  // such discrepancy.
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WenleiHe wrote:

worth pointing not explicitly that it makes no difference for probe-based profile? 

https://github.com/llvm/llvm-project/pull/92144