52c583b3f9
This change extends SampleFDO ext-binary and text format to record the vtable symbols and their counts for virtual calls inside a function. The vtable profiles will allow the compiler to annotate vtable types on IR instructions and perform vtable-based indirect call promotion. An RFC is in https://discourse.llvm.org/t/rfc-vtable-type-profiling-for-samplefdo/87283 Given a function below, the before vs after of a function's profile is illustrated in text format in the table: ``` __attribute__((noinline)) int loop_func(int i, int a, int b) { Base *ptr = createType(i); int sum = ptr->func(a, b); delete ptr; return sum; } ``` | before | after | | --- | --- | | Samples collected in the function's body { <br> 0: 636241 <br> 1: 681458, calls: _Z10createTypei:681458 <br> 3: 543499, calls: _ZN12_GLOBAL__N_18Derived24funcEii:410621 _ZN8Derived14funcEii:132878 <br> 5.1: 602201, calls: _ZN12_GLOBAL__N_18Derived2D0Ev:454635 _ZN8Derived1D0Ev:147566 <br> 7: 511057 <br> } | Samples collected in the function's body { <br> 0: 636241 <br> 1: 681458, calls: _Z10createTypei:681458 <br> 3: 543499, calls: _ZN12_GLOBAL__N_18Derived24funcEii:410621 _ZN8Derived14funcEii:132878 <br> 3: vtables: _ZTV8Derived1:1377 _ZTVN12_GLOBAL__N_18Derived2E:4250 <br> 5.1: 602201, calls: _ZN12_GLOBAL__N_18Derived2D0Ev:454635 _ZN8Derived1D0Ev:147566 <br> 5.1: vtables: _ZTV8Derived1:227 _ZTVN12_GLOBAL__N_18Derived2E:765 <br> 7: 511057 <br> } | Key points for this change: 1. In-memory representation of vtable profiles * A field of type `map<LineLocation, map<FunctionId, uint64_t>>` is introduced in a function's in-memory representation [FunctionSamples](ccc416312e/llvm/include/llvm/ProfileData/SampleProf.h (L749-L754)). 2. The vtable counters for one LineLocation represents the relative frequency among vtables for this LineLocation. They are not required to be comparable across LineLocations. 3. For backward compatibility of ext-binary format, we take one bit from ProfSummaryFlag as illustrated in the enum class `SecProfSummaryFlags`. The ext-binary profile reader parses the integer type flag and reads this bit. If it's set, the profile reader will parse vtable profiles. 4. The vtable profiles are optional in ext-binary format, and not serialized out by default, we introduce an LLVM boolean option (named `-extbinary-write-vtable-type-prof`). The ext-binary profile writer reads the boolean option and decide whether to set the section flag bit and serialize the in-memory class members corresponding to vtables. 5. This change doesn't implement `llvm-profdata overlap --sample` for the vtable profiles. A subsequent change will do it to keep this one focused on the profile format change. We don't plan to add the vtable support to non-extensible format mainly because of the maintenance cost to keep backward compatibility for prior versions of profile data. * Currently, the [non-extensible binary format](5c28af4099/llvm/lib/ProfileData/SampleProfWriter.cpp (L899-L900)) does not have feature parity with extensible binary format today, for instance, the former doesn't support [profile symbol list](41e22aa31b/llvm/include/llvm/ProfileData/SampleProf.h (L1518-L1522)) or context-sensitive PGO, both of which give measurable performance boost. Presumably the non-extensible format is not in wide use. --------- Co-authored-by: Paschalis Mpeis <paschalis.mpeis@arm.com>
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33 KiB