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...profile context sensitivity, GCC is used in the experiment. Note that GCC PGO improves clang performance a lot more than clang PGO. First we summarize the methodology used in the experiment: 0) build clang with GCC O2 without early inlining and measure clang's performance. GCC early inlining (einline) is similar to pre-inline used by late instrumentation. 1) build clang with GCC O2 with early inlining and measure performance. The performance difference of 1) and 0) is denoted as E which measures the contribution of early inlining. 2) build clang with GCC O2 + PGO without early inlining. 3) b...

...GCC is used > in the experiment. Note that GCC PGO improves clang performance a lot more > than clang PGO. > > First we summarize the methodology used in the experiment: > 0) build clang with GCC O2 without early inlining and measure clang's > performance. GCC early inlining (einline) is similar to pre-inline used by > late instrumentation. > 1) build clang with GCC O2 with early inlining and measure performance. > > The performance difference of 1) and 0) is denoted as E which measures the > contribution of early inlining. > > 2) build clang with GCC O2 +...

One aspect of this that I have not seen discussed is that middle-end instrumentation enables PGO optimizations to front-ends other than Clang. While I agree that FE instrumentation could be improved, it still requires every FE to implement essentially the same common functionality. Having PGO instrumentation generated in the middle-end, allows us every FE to automatically take advantage of PGO.