e517cfc531
This patch adds support for cold function coverage instrumentation based on sampling PGO counts. The major motivation is to detect dead functions for the services that are optimized with sampling PGO. If a function is covered by sampling profile count (e.g., those with an entry count > 0), we choose to skip instrumenting those functions, which significantly reduces the instrumentation overhead. More details about the implementation and flags: - Added a flag `--pgo-instrument-cold-function-only` in `PGOInstrumentation.cpp` as the main switch to control skipping the instrumentation. - Built the extra instrumentation passes(a bundle of passes in `addPGOInstrPasses`) under sampling PGO pipeline. This is controlled by `--instrument-cold-function-only-path` flag. - Added a driver flag `-fprofile-generate-cold-function-coverage`: - 1) Config the flags in one place, i,e. adding `--instrument-cold-function-only-path=<...>` and `--pgo-function-entry-coverage`. Note that the instrumentation file path is passed through `--instrument-sample-cold-function-path`, because we cannot use the `PGOOptions.ProfileFile` as it's already used by `-fprofile-sample-use=<...>`. - 2) makes linker to link `compiler_rt.profile` lib(see [ToolChain.cpp#L1125-L1131](https://github.com/llvm/llvm-project/blob/main/clang/lib/Driver/ToolChain.cpp#L1125-L1131) ). - Added a flag(`--pgo-cold-instrument-entry-threshold`) to config entry count to determine cold function. Overall, the full command is like: ``` clang++ -O2 -fprofile-generate-cold-function-coverage=<...> -fprofile-sample-use=<...> code.cc -o code ```
20 lines
565 B
C
20 lines
565 B
C
// Test -fprofile-generate-cold-function-coverage
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// RUN: rm -rf %t && split-file %s %t
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// RUN: %clang -O2 -fprofile-generate-cold-function-coverage=/xxx/yyy/ -fprofile-sample-accurate -fprofile-sample-use=%t/pgo-cold-func.prof -S -emit-llvm -o - %t/pgo-cold-func.c | FileCheck %s
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// CHECK: @__llvm_profile_filename = {{.*}} c"/xxx/yyy/default_%m.profraw\00"
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// CHECK: store i8 0, ptr @__profc_bar, align 1
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// CHECK-NOT: @__profc_foo
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//--- pgo-cold-func.prof
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foo:1:1
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1: 1
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//--- pgo-cold-func.c
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int bar(int x) { return x;}
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int foo(int x) {
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return x;
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}
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