[cfe-dev] --gcc-toolchain (GCC_INSTALL_PREFIX) and --prefix(-B)

Thu Mar 4 14:30:40 PST 2021

On Thu, Mar 4, 2021 at 1:54 PM Fāng-ruì Sòng <maskray at google.com> wrote:

> On 2021-03-04, Hubert Tong wrote:
> >On Thu, Mar 4, 2021 at 3:19 AM Fāng-ruì Sòng via cfe-dev <
> >cfe-dev at lists.llvm.org> wrote:
> >
> >> If --gcc-toolchain is specified, its value overrides the cmake variable
> >> GCC_INSTALL_PREFIX.
> >> When the value is non-empty: the value is appended to the --prefix list
> >> and is used to detect GCC installations.
> >> The GCC installation is used to provide include directories/library
> >> directories and some startup files (e.g. crtbegin).
> >>
> >> Problem 1.
> >>
> >> --prefix(-B) does more than --gcc-toolchain:
> >> clang::driver::Driver::GetProgramPath basically searches for
> >> $prefix/$triple-$file and $prefix$file,
> >> where $prefix is taken from the list of --prefix(-B). --gcc-toolchain
> does
> >> not participate in the search. <have attached a summary of the
> algorithm at
> >> the bottom>
> >> The result is that 'ld' and 'as' may come from the system (more
> precisely,
> >> sysroot):
> >>
> >> cd clang/test/Driver
> >> # Make sure Inputs/opensuse_42.2_aarch64_tree/usr/bin/ld exists.
> >> clang -target aarch64-suse-linux
> >> --gcc-toolchain=Inputs/opensuse_42.2_aarch64_tree/usr '-###'
> >> gcc-toolchain.cpp -v
> >>
> >>    # I have ld in my /usr/local/bin and it takes precedence over
> >> /usr/bin/ld
> >>  "/usr/local/bin/ld" "-EL" "--eh-frame-hdr" "-m" "aarch64linux"
> >> "-dynamic-linker" "/lib/ld-linux-aarch64.so.1" "-o" "a.out"
> >>
> "Inputs/opensuse_42.2_aarch64_tree/usr/lib64/gcc/aarch64-suse-linux/4.8/../../../../lib64/crt1.o"
> >>
> "Inputs/opensuse_42.2_aarch64_tree/usr/lib64/gcc/aarch64-suse-linux/4.8/../../../../lib64/crti.o"
> >>
> "Inputs/opensuse_42.2_aarch64_tree/usr/lib64/gcc/aarch64-suse-linux/4.8/crtbegin.o"
> >>
> "-LInputs/opensuse_42.2_aarch64_tree/usr/lib64/gcc/aarch64-suse-linux/4.8"
> >>
> "-LInputs/opensuse_42.2_aarch64_tree/usr/lib64/gcc/aarch64-suse-linux/4.8/../../../../lib64"
> >> "-L/lib/aarch64-linux-gnu" "-L/lib/../lib64"
> "-L/usr/lib/aarch64-linux-gnu"
> >> "-L/usr/lib/../lib64"
> >>
> "-LInputs/opensuse_42.2_aarch64_tree/usr/lib64/gcc/aarch64-suse-linux/4.8/../../.."
> >> "-L/tmp/RelA/bin/../lib" "-L/lib" "-L/usr/lib"
> >> "/tmp/gcc-toolchain-f87f08.o" "-lgcc" "--as-needed" "-lgcc_s"
> >> "--no-as-needed" "-lc" "-lgcc" "--as-needed" "-lgcc_s" "--no-as-needed"
> >>
> "Inputs/opensuse_42.2_aarch64_tree/usr/lib64/gcc/aarch64-suse-linux/4.8/crtend.o"
> >>
> "Inputs/opensuse_42.2_aarch64_tree/usr/lib64/gcc/aarch64-suse-linux/4.8/../../../../lib64/crtn.o"
> >>
> >> The -L and crt* files are indeed from
> >> Inputs/opensuse_42.2_aarch64_tree/usr, but ld (and as if
> >> -fno-integrated-as) is from the system.
> >> On many Linux distributions you can normally assume that the system ld
> and
> >> as only support the host architecture.
> >> This means --gcc-toolchain can only be used to specify a GCC
> installation
> >> with the same architecture.
> >>
> >... and specifying a GCC installation with the same architecture (e.g., to
> >use a newer libstdc++) is something that people do:
> >$ clang++ -fuse-ld=ld --gcc-toolchain=/opt/rh/devtoolset-7/root -xc -
> >-fno-integrated-as <<<'#include <stdio.h>'$'\n''int main(void) {
> >printf("Hello, world!\n"); }' && echo OKAY
> >OKAY
> >... but it does seem ld is coming from --gcc-toolchain for me anyway:
> >$ ./build/bootstrap/stage1/build/bin/clang++ -fuse-ld=ld
> >--gcc-toolchain=/opt/rh/devtoolset-7/root -xc - -fno-integrated-as
> ><<<'#include <stdio.h>'$'\n''int main(void) { printf("Hello, world!\n");
> }'
> >-### 2>&1 | grep /ld
> >
> "/opt/rh/devtoolset-7/root/lib/gcc/ppc64le-redhat-linux/7/../../../../bin/ld"
> >"--hash-style=gnu" "--eh-frame-hdr" "-m" "elf64lppc" "-dynamic-linker"
> >"/lib64/ld64.so.2" "-o" "a.out" "/lib/../lib64/crt1.o"
> >"/lib/../lib64/crti.o"
> >"/opt/rh/devtoolset-7/root/lib/gcc/ppc64le-redhat-linux/7/crtbegin.o"
> >"-L/opt/rh/devtoolset-7/root/lib/gcc/ppc64le-redhat-linux/7"
>
> >"-L/opt/rh/devtoolset-7/root/lib/gcc/ppc64le-redhat-linux/7/../../../../lib64"
> >"-L/lib/powerpc64le-linux-gnu" "-L/lib/../lib64"
> >"-L/usr/lib/powerpc64le-linux-gnu" "-L/usr/lib/../lib64"
> >"-L/opt/rh/devtoolset-7/root/lib/gcc/ppc64le-redhat-linux/7/../../.."
> >"-L/data1/hstong.local/wybuild/build/bootstrap/stage1/build/bin/../lib"
> >"-L/lib" "-L/usr/lib" "/tmp/--8f8c6a.o" "-lstdc++" "-lm" "-lgcc_s" "-lgcc"
> >"-lc" "-lgcc_s" "-lgcc"
> >"/opt/rh/devtoolset-7/root/lib/gcc/ppc64le-redhat-linux/7/crtend.o"
> >"/lib/../lib64/crtn.o"
>
> 'ld' path is computed by clang/lib/Driver/Driver.cpp
> Driver::GetProgramPath.
> Did you set PATH to the devtoolset-7 bin directory?
>
No, I did not.

>
>
> If --gcc-toolchain and multiple -B are specified, the search order is:
>
> -B -B -B --gcc-toolchain
>
> Currently the GCC installation with the largest version wins.
> Do you think we should stop search after one -B provides a GCC
> installation?
>
Well, the ordering where --gcc-toolchain follows all of the -B seems okay
in the context of looking for ld or as. For changing "largest version
wins", it might make some sense to order --gcc-toolchain first for the
resolution of the GCC installation (which allows "largest version wins" to
continue with -B). I suppose it means people using builds preconfigured
with a GCC install prefix will be somewhat inconvenienced when they don't
want to use the preconfigured path; however, I think this retains most
functionality (there is a way to limit the search for GCC installations but
also a way to compose search paths for the latest GCC version).

>
> >> --prefix can make as and ld paths correct, but: if another --prefix is
> >> needed, why do we use --gcc-toolchain?
> >>
> >Well, I'm not sure that another --prefix is needed. Also, --prefix doesn't
> >help to pick up the GCC header and library paths and --sysroot overrides
> >too much:
> >$ clang++ -fuse-ld=ld --sysroot=/opt/rh/devtoolset-7/root -xc -
> >-fno-integrated-as <<<'#include <stdio.h>'$'\n''int main(void) {
> >printf("Hello, world!\n"); }'
> ><stdin>:1:10: fatal error: 'stdio.h' file not found
> >#include <stdio.h>
> >         ^~~~~~~~~
> >1 error generated.
>
> (The example used --sysroot instead of --prefix)
>
Yes.

>
> OK, your other reply clarified that --prefix does pick up include/library
> directories.
>
Yes.
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