[cfe-dev] RFC - Stop ignoring -fprofile-generate and -fprofile-use

[Duncan P. N. Exon Smith]

+cfe-dev

> On 2015 Jun 17, at 13:53, Diego Novillo <dnovillo at google.com> wrote:
> 
> 
> The flags -fprofile-generate and -fprofile-use are currently ignored for GCC compatibility.  I would like to enable them and give them similar semantics to GCC.  These flags are baked pretty deeply into our build environment, so supporting them at the driver level will make our lives a lot simpler.
> 
> From https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html:
> 
> -fprofile-generate
> -fprofile-generate=path
> Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program.
> The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.
> 
> If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir. 
> 
> -fprofile-use
> -fprofile-use=path
> Enable profile feedback-directed optimizations, and the following optimizations which are generally profitable only with profile feedback available: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, -ftracer, -ftree-vectorize, and ftree-loop-distribute-patterns.
> By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wcoverage-mismatch. Note this may result in poorly optimized code.
> 
> If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir. 
> 
> 
> Note that the argument to -fprofile-generate and -fprofile-use is not a file name. It is a path prefix used to store the generated profile (in GCC's case, each object file in the binary generates its own profile file). In Clang, the flags would do the following:
> 	• -fprofile-generate=path-prefix would cause the instrumented binary to write the profile <path-prefix>/default.profraw. If <path-prefix> does not exist, it is created by the runtime.
> 	• -fprofile-use=path-prefix would cause the compiler to read from <path-prefix>/default.profile.
> 	• I could also add support for -fprofile-dir, but we don't really use it internally.
> As with -fprofile-instr-generate, LLVM_PROFILE_FILE would override the path prefix and name of the profile file.
> 
> Does this sound reasonable?
> 
> 
> Thanks.  Diego.
> 
>