[CSSPGO][llvm-profgen] Do not duplicate context profiles into base profile when converting CS flat profile to nested.

[hoy]

Summary

Recent experiments with our two large internal services showed that duplicating context profiles into base profile caused code size inflation and didn't deliver good performance compared to no such duplication. It was a trick we made to catch up with the CS flat profile and I'm now turning it off by default.

The code size inflation mainly comes from the enriched based profiles. A base profile for a function represents the uninlined (or outlined) portion of the whole function running time. Such portion could be very small if a function is inlined into most of its hot callsites. Duplicating context profiles of the function into its base profiles could cause the outlined body to be hot enough and in turn get many of its callees inlined, thus increases the code size. The size inflation could further cause perf regression.

[wenlei]

lgtm, thanks.

llvm/lib/ProfileData/SampleProf.cpp

37

Perhaps this is something we could completely remove after everything stabilizes..

llvm/lib/ProfileData/SampleProf.cpp

 using namespace sampleprof;
 
 static cl::opt<uint64_t> ProfileSymbolListCutOff(
     "profile-symbol-list-cutoff", cl::Hidden, cl::init(-1), cl::ZeroOrMore,
     cl::desc("Cutoff value about how many symbols in profile symbol list "
              "will be used. This is very useful for performance debugging"));
 
 cl::opt<bool> GenerateMergedBaseProfiles(
-    "generate-merged-base-profiles", cl::init(true), cl::ZeroOrMore,
+    "generate-merged-base-profiles", cl::init(false), cl::ZeroOrMore,
     cl::desc("When generating nested context-sensitive profiles, always "
              "generate extra base profile for function with all its context "
              "profiles merged into it."));
 
 namespace llvm {
 namespace sampleprof {
 bool FunctionSamples::ProfileIsProbeBased = false;
 bool FunctionSamples::ProfileIsCS = false;

llvm/test/tools/llvm-profgen/cs-preinline.test

 ; CHECK-PREINL-NEST:     foo:393:0
 ; CHECK-PREINL-NEST-NEXT: 2.1: 14
 ; CHECK-PREINL-NEST-NEXT: 3: 15
 ; CHECK-PREINL-NEST-NEXT: 3.1: 14 bar:14
 ; CHECK-PREINL-NEST-NEXT: 3.2: 1
 ; CHECK-PREINL-NEST-NEXT: 65526: 14
 ; CHECK-PREINL-NEST-NEXT: 3.1: bar:84
 ; CHECK-PREINL-NEST-NEXT:  1: 14
-; CHECK-PREINL-NEST-NEXT:  !Attributes: 7
+; CHECK-PREINL-NEST-NEXT:  !Attributes: 3
 
 
 ; CHECK-PREINL-FLAG: ProfileSummarySection {{.*}} Flags: {{{.*}}preInlined}