问题
Which information does GCC collect when I enable -fprofile-generate and which optimization does in fact uses the collected information (when setting the -fprofile-use flag) ?
I need citations here. I've searched for a while but didn't found anything documented.
Information regarding link-time optimization (LTO) would be a plus! =D
回答1:
-fprofile-generate enables -fprofile-arcs, -fprofile-values and -fvpt.
-fprofile-use enables -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops and -ftracer
Source: http://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Optimize-Options.html#Optimize-Options
PS. Information about LTO also on that page.
回答2:
"What Every Programmer Should Know About Memory" by Ulrich Drepper https://people.freebsd.org/~lstewart/articles/cpumemory.pdf http://www.akkadia.org/drepper/cpumemory.pdf
In section 7.4
- compilation with --profile-generate generates .gcno file for each object file. (the same file that is used for gcov coverage reports)
- then you must run a few tests, during runtime it records coverage data into .gcda files
- recompile with --profile-use : it will gather the coverage data and infer if an branch is likely (__builtin_expect( .. , 1 ) or unlikely (__builtin_expect( .. , 0)
The result should run faster as it should be better at prefetching code into the processor instruction cache.
来源：https://stackoverflow.com/questions/13881292/what-information-does-gcc-profile-guided-optimization-pgo-collect-and-which-op