Change Details

When using -fprofile-list to selectively apply instrumentation only to certain files or functions, we may end up with a binary that doesn't have any counters in the case where no files were selected. However, because on Linux and Fuchsia, we pass -u__llvm_profile_runtime, the runtime would still be pulled in and incur some non-trivial overhead, especially in the case when the continuous or runtime counter relocation mode is being used. A better way would be to pull in the profile runtime only when needed by declaring the __llvm_profile_runtime symbol in the translation unit only when needed. This approach was already used prior to 9a041a75221ca, but we changed it to always generate the __llvm_profile_runtime due to a TAPI limitation. Since TAPI is only used on Mach-O platforms, we could use the early emission of __llvm_profile_runtime there, and on other platforms we could change back to the earlier approach where the symbol is generated later only when needed. 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.

When none of the translation units in the binary have been instrumented we shouldn't need to link the profile runtime. However, because we pass -u__llvm_profile_runtime on Linux and Fuchsia, the runtime would still be pulled in and incur some overhead. On Fuchsia which uses runtime counter relocation, it also means that we cannot reference the bias variable unconditionally. This change modifies the InstrProfiling pass to pull in the profile runtime only when needed by declaring the __llvm_profile_runtime symbol in the translation unit only when needed. For now we restrict this only for Fuchsia, but this can be later expanded to other platforms. This approach was already used prior to 9a041a75221ca, but we changed it to always generate the __llvm_profile_runtime due to a TAPI limitation, but that limitation may no longer apply, and it certainly doesn't apply on platforms like Fuchsia.

When using -fprofile-list to selectively applynone of the translation units in the binary have been instrumentation onlyted to certain files or functionswe shouldn't need to link the profile runtime. However, we may end up with a binary that doesn'tbecause we pass have any counters in the case where no files were selected.-u__llvm_profile_runtime on Linux and Fuchsia, However,the runtime would still because on Linuxbe pulled in and Fuchsia, we pass -u__llvm_profile_runtime,incur some overhead. theOn Fuchsia which uses runtime runtime would still be pulled in and incur some non-trivial overhead,counter relocation, it also means that we cannot reference the bias especially in the case when the continuous or runtime counter relocation mode is being used. A better way would be to pull in the profile runtime only when needed by declaring the __llvm_profile_runtime symbol in the translation unit only when neededvariable unconditionally. This approach was already used prior to 9a041a75221ca, but we changed itchange modifies the InstrProfiling pass to pull in the profile to always generateruntime only when needed by declaring the __llvm_profile_runtime due to a TAPI limitation.e symbol Since TAPI is only used on Mach-O platforms,in the translation unit only when needed. we could use the earlyFor now we restrict this only emission of __llvm_profile_runtime there,for Fuchsia, but this can be later expanded to other platforms. and on other platforms weThis could change back to the earlier approach where the symbol is generatedapproach was already used prior to 9a041a75221ca, but we changed it later only when needed. We can stop passing -uto always generate the __llvm_profile_runtime toe due to a TAPI limitation, the linker on Linux and Fuchsia since the generated undefined symbol inbut that limitation may no longer apply, and it certainly doesn't apply each translation unit that needed it serves the same purposeon platforms like Fuchsia.