[llvm] [PGO] Sampled instrumentation in PGO to speed up instrumentation binary (PR #69535)

[via llvm-commits]

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@@ -635,33 +664,161 @@ PreservedAnalyses InstrProfilingLoweringPass::run(Module &M,
   return PreservedAnalyses::none();
 }
 
+//
+// Perform instrumentation sampling.
+//
+// There are 3 favors of sampling:
+// (1) Full burst sampling: We transform:
+//   Increment_Instruction;
+// to:
+//   if (__llvm_profile_sampling__ < SampledInstrBurstDuration) {
+//     Increment_Instruction;
+//   }
+//   __llvm_profile_sampling__ += 1;
+//   if (__llvm_profile_sampling__ >= SampledInstrPeriod) {
+//     __llvm_profile_sampling__ = 0;
+//   }
+//
+// "__llvm_profile_sampling__" is a thread-local global shared by all PGO
+// counters (value-instrumentation and edge instrumentation).
+//
+// (2) Fast burst sampling:
+// The value is an unsigned type, meaning it will wrap around to zero when
+// overflows. In this case, a second check (check2) is unnecessary, so we
+// won't generate check2 when the SampledInstrPeriod is set to 65535 (64K - 1).
+// The code after:
+//   if (__llvm_profile_sampling__ < SampledInstrBurstDuration) {
+//     Increment_Instruction;
+//   }
+//   __llvm_profile_sampling__ += 1;
+//
+// (3) Simple sampling:
+// When SampledInstrBurstDuration sets to 1, we do a simple sampling:
+//   __llvm_profile_sampling__ += 1;
+//   if (__llvm_profile_sampling__ >= SampledInstrPeriod) {
+//     __llvm_profile_sampling__ = 0;
+//     Increment_Instruction;
+//   }
+//
+// Note that, the code snippet after the transformation can still be counter
+// promoted. However, with sampling enabled, counter updates are expected to
+// be infrequent, making the benefits of counter promotion negligible.
+// Moreover, counter promotion can potentially cause issues in server
+// applications, particularly when the counters are dumped without a clean
+// exit. To mitigate this risk, counter promotion is disabled by default when
+// sampling is enabled. This behavior can be overridden using the internal
+// option.
+void InstrLowerer::doSampling(Instruction *I) {
+  if (!isSamplingEnabled())
+    return;
+
+  unsigned SampledBurstDuration = SampledInstrBurstDuration.getValue();
+  unsigned SampledPeriod = SampledInstrPeriod.getValue();
+  assert(SampledBurstDuration < SampledPeriod);
+  bool UseShort = (SampledPeriod <= USHRT_MAX);
+  bool IsSimpleSampling = (SampledBurstDuration == 1);
+  bool IsFastSampling = (!IsSimpleSampling && SampledPeriod == 65535);
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WenleiHe wrote:

The condition for `SampledBurstDuration` is generated for non-simple sampling, regardless whether it's fast sampling. My question was why do we need to include `!IsSimpleSampling` in `IsFastSampling`..

https://github.com/llvm/llvm-project/pull/69535