On Mon, Aug 17, 2020 at 03:02:08PM -0700, Nick Desaulniers wrote: > -ffreestanding typically inhibits "libcall optimizations" where calls to > certain library functions can be replaced by the compiler in certain > cases to calls to other library functions that may be more efficient. > This can be problematic for embedded targets that don't provide full > libc implementations. > > -ffreestanding inhibits all such optimizations, which is the safe > choice, but generally we want the optimizations that are performed. The > Linux kernel does implement a fair amount of libc routines. Instead of > -ffreestanding (which makes more sense in smaller images like kexec's > purgatory image), prefer -fno-builtin-* flags to disable the compiler > from emitting calls to functions which may not be defined. > > If you see a linkage failure due to a missing symbol that's typically > defined in a libc, and not explicitly called from the source code, then > the compiler may have done such a transform. You can either implement > such a function (ie. in lib/string.c) or disable the transform outright > via -fno-builtin-* flag (where * is the name of the library routine, ie. > -fno-builtin-bcmp). > > Patch 1 unbreaks the build with ToT clang, which has been red all > weekend, by adding -fno-builtin-stpcpy. > Patch 2 is a revert but adds -fno-builtin-bcmp. > Patch 3 does the same for x86 purgatory. > Patch 4 removes -ffreestanding from i386. > > The first patch makes sense for Kbuild, the second maybe akpm@, the > third and forth for x86. Not sure who should pick up the series (they > can be merged out of order, technically) but I really need the first > patch soon. The 3 latter patches are cleanups. > > Nick Desaulniers (4): > Makefile: add -fno-builtin-stpcpy > Revert "lib/string.c: implement a basic bcmp" > x86/boot: use -fno-builtin-bcmp > x86: don't build CONFIG_X86_32 as -ffreestanding > > Makefile | 7 +++++++ > arch/x86/Makefile | 3 --- > arch/x86/boot/Makefile | 1 + > arch/x86/boot/string.c | 8 -------- > include/linux/string.h | 3 --- > lib/string.c | 20 -------------------- > 6 files changed, 8 insertions(+), 34 deletions(-) > > -- > 2.28.0.220.ged08abb693-goog > Another thing that needs to be fixed is that at least lib/string.c needs to be compiled with -ffreestanding. gcc-10 optimizes the generic memset implementation in there into a call to memset. Now that's on x86 which doesn't use the generic implementation, but this is just waiting to bite us. https://godbolt.org/z/6EhG15
