MaskRay added a comment. > This approach was already used prior to 9a041a75221ca, but we changed it to > always generate the llvm_profile_runtime due to a TAPI limitation.
D43794 <https://reviews.llvm.org/D43794> (rG9a041a75221ca <https://reviews.llvm.org/rG9a041a75221cadaafbf902e8ec6b73f6d2612d89>) does not seem to affect the ELF behavior. > We can stop passing -u__llvm_profile_runtime to the linker on Linux and > Fuchsia since the generated undefined symbol in each translation unit that > needed it serves the same purpose. This restores the behavior before @davidxl's rG170cd100ed6f38ec5826dbd1bd6930ddfd3490a4 <https://reviews.llvm.org/rG170cd100ed6f38ec5826dbd1bd6930ddfd3490a4> (2015). While having less code in the clang driver side is pros to me, I don't know whether it should come with the cost of conditional presence/absence of the .profraw file. (you can specify LLVM_PROFILE_FILE without %m; -fprofile-profile-generate by default does not use %m) >> There are also going to be binaries in our system build that don't use foo.h >> and ideally shouldn't have any overhead since nothing inside them is >> instrumented (they should behave as if -fprofile-instr-generate wasn't set >> for them at all). That's not the case today because if you set >> -fprofile-instr-generate, driver passes -u__llvm_profile_runtime to the >> linker which "pulls in" the profile runtime introducing some extra bloat and >> startup overhead I described earlier. >> >> The overhead is just __llvm_profile_write_file, right? It just writes a 100+ >> bytes file which has very little overhead. > > It could be more if you use the continuous mode where you'd also need to mmap > the profile and do some additional setup. Can the cost be avoided in the runtime? ================ Comment at: llvm/test/Instrumentation/InstrProfiling/linkage.ll:13 ; MACHO: @__llvm_profile_runtime = external global i32 -; ELF-NOT: @__llvm_profile_runtime = external global i32 ---------------- If the symbol is now present, a CHECK line should be kept. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D98061/new/ https://reviews.llvm.org/D98061 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
