On Mon, 23 Oct 2023 13:54:01 +0100 Iain Sandoe <i...@sandoe.co.uk> wrote:
> hi Sergei, > > > On 23 Oct 2023, at 13:43, Sergei Trofimovich <sly...@gmail.com> wrote: > > > > From: Sergei Trofimovich <siarh...@google.com> > > > > To build `libc` for a target one needs to build `gcc` without `libc` > > support first. Commit r14-4823-g8abddb187b3348 "libgcc: support > > heap-based trampolines" added unconditional `libc` dependency and broke > > libc-less `gcc` builds. > > > > An example failure on `x86_64-unknown-linux-gnu`: > > > > $ mkdir -p /tmp/empty > > $ ../gcc/configure \ > > --disable-multilib \ > > --without-headers \ > > --with-newlib \ > > --enable-languages=c \ > > --disable-bootstrap \ > > --disable-gcov \ > > --disable-threads \ > > --disable-shared \ > > --disable-libssp \ > > --disable-libquadmath \ > > --disable-libgomp \ > > --disable-libatomic \ > > --with-build-sysroot=/tmp/empty > > $ make > > ... > > /tmp/gb/./gcc/xgcc -B/tmp/gb/./gcc/ > > -B/usr/local/x86_64-pc-linux-gnu/bin/ -B/usr/local/x86_64-pc-linux-gnu/lib/ > > -isystem /usr/local/x86_64-pc-linux-gnu/include -isystem > > /usr/local/x86_64-pc-linux-gnu/sys-include --sysroot=/tmp/empty -g -O2 > > -O2 -g -O2 -DIN_GCC -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual > > -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition -isystem > > ./include -fpic -mlong-double-80 -DUSE_ELF_SYMVER -fcf-protection -mshstk > > -g -DIN_LIBGCC2 -fbuilding-libgcc -fno-stack-protector -Dinhibit_libc -fpic > > -mlong-double-80 -DUSE_ELF_SYMVER -fcf-protection -mshstk -I. -I. > > -I../.././gcc -I/home/slyfox/dev/git/gcc/libgcc > > -I/home/slyfox/dev/git/gcc/libgcc/. > > -I/home/slyfox/dev/git/gcc/libgcc/../gcc > > -I/home/slyfox/dev/git/gcc/libgcc/../include -DHAVE_CC_TLS -DUSE_TLS -o > > heap-trampoline.o -MT heap-trampoline.o -MD -MP -MF heap-trampoline.dep -c > > .../gcc/libgcc/config/i386/heap-trampoline.c -fvisibility=hidden > > -DHIDE_EXPORTS > > ../gcc/libgcc/config/i386/heap-trampoline.c:3:10: fatal error: unistd.h: > > No such file or directory > > 3 | #include <unistd.h> > > | ^~~~~~~~~~ > > compilation terminated. > > make[2]: *** [.../gcc/libgcc/static-object.mk:17: heap-trampoline.o] > > Error 1 > > make[2]: Leaving directory '/tmp/gb/x86_64-pc-linux-gnu/libgcc' > > make[1]: *** [Makefile:13307: all-target-libgcc] Error 2 > > > > The change inhibits any heap-based trampoline code. > > That looks reasonable to me (I was considering using __has_include(), but the > inhibit_libc is neater). > > The fact that this first compiler is buit without heap-trampoline support, > would become relevant, I guess if libc wanted to use them, it would need > another iteration. > > so, it looks fine, but I cannot actually approve it. Sounds good. Let's wait for others to chime in. Maybe Richard? :) AFAIU libcs (like `glibc`) try hard not to use link tests and uses mainly preprocessor and code generator to specifically accommodate this case. Maybe there is a way to pass the support flag to libc without the reliance on code presence in libgcc. Otherwise we could use __builtin_trap() as an implementation for exposed symbols. > > > > > libgcc/ > > > > * libgcc/config/aarch64/heap-trampoline.c: Disable when libc is > > not present. > > --- > > libgcc/config/aarch64/heap-trampoline.c | 5 +++++ > > libgcc/config/i386/heap-trampoline.c | 5 +++++ > > 2 files changed, 10 insertions(+) > > > > diff --git a/libgcc/config/aarch64/heap-trampoline.c > > b/libgcc/config/aarch64/heap-trampoline.c > > index c8b83681ed7..f22233987ca 100644 > > --- a/libgcc/config/aarch64/heap-trampoline.c > > +++ b/libgcc/config/aarch64/heap-trampoline.c > > @@ -1,5 +1,8 @@ > > /* Copyright The GNU Toolchain Authors. */ > > > > +/* libc is required to allocate trampolines. */ > > +#ifndef inhibit_libc > > + > > #include <unistd.h> > > #include <sys/mman.h> > > #include <stdint.h> > > @@ -170,3 +173,5 @@ __builtin_nested_func_ptr_deleted (void) > > tramp_ctrl_curr = prev; > > } > > } > > + > > +#endif /* !inhibit_libc */ > > diff --git a/libgcc/config/i386/heap-trampoline.c > > b/libgcc/config/i386/heap-trampoline.c > > index 96e13bf828e..4b9f4365868 100644 > > --- a/libgcc/config/i386/heap-trampoline.c > > +++ b/libgcc/config/i386/heap-trampoline.c > > @@ -1,5 +1,8 @@ > > /* Copyright The GNU Toolchain Authors. */ > > > > +/* libc is required to allocate trampolines. */ > > +#ifndef inhibit_libc > > + > > #include <unistd.h> > > #include <sys/mman.h> > > #include <stdint.h> > > @@ -170,3 +173,5 @@ __builtin_nested_func_ptr_deleted (void) > > tramp_ctrl_curr = prev; > > } > > } > > + > > +#endif /* !inhibit_libc */ > > -- > > 2.42.0 > > > -- Sergei
