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

Fāng-ruì Sòng via cfe-dev cfe-dev at lists.llvm.org
Thu Mar 4 10:53:57 PST 2021

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
>> 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>'$'\n''int main(void) {
>printf("Hello, world!\n"); }' && echo OKAY
>... but it does seem ld is coming from --gcc-toolchain for me anyway:
>$ ./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"
>"-L/lib/powerpc64le-linux-gnu" "-L/lib/../lib64"
>"-L/usr/lib/powerpc64le-linux-gnu" "-L/usr/lib/../lib64"
>"-L/lib" "-L/usr/lib" "/tmp/--8f8c6a.o" "-lstdc++" "-lm" "-lgcc_s" "-lgcc"
>"-lc" "-lgcc_s" "-lgcc"

'ld' path is computed by clang/lib/Driver/Driver.cpp Driver::GetProgramPath.
Did you set PATH to the devtoolset-7 bin directory?

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?

>> --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)

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

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