On Wed, Jul 22, 2020 at 9:14 AM Fangrui Song <maskray@xxxxxxxxxx> wrote: > > On 2020-07-22, Masahiro Yamada wrote: > >On Wed, Jul 22, 2020 at 2:31 AM 'Fangrui Song' via Clang Built Linux > ><clang-built-linux@xxxxxxxxxxxxxxxx> wrote: > >> > >> When CROSS_COMPILE is set (e.g. aarch64-linux-gnu-), if > >> $(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-elfedit, > >> GCC_TOOLCHAIN_DIR will be set to /usr/bin/. --prefix= will be set to > >> /usr/bin/ and Clang as of 11 will search for both > >> $(prefix)aarch64-linux-gnu-$needle and $(prefix)$needle. > >> > >> GCC searchs for $(prefix)aarch64-linux-gnu/$version/$needle, > >> $(prefix)aarch64-linux-gnu/$needle and $(prefix)$needle. In practice, > >> $(prefix)aarch64-linux-gnu/$needle rarely contains executables. > >> > >> To better model how GCC's -B/--prefix takes in effect in practice, newer > >> Clang (since > >> https://github.com/llvm/llvm-project/commit/3452a0d8c17f7166f479706b293caf6ac76ffd90) > >> only searches for $(prefix)$needle. Currently it will find /usr/bin/as > >> instead of /usr/bin/aarch64-linux-gnu-as. > >> > >> Set --prefix= to $(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE) > >> (/usr/bin/aarch64-linux-gnu-) so that newer Clang can find the > >> appropriate cross compiling GNU as (when -no-integrated-as is in > >> effect). > >> > >> Cc: stable@xxxxxxxxxxxxxxx > >> Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > >> Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx> > >> Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > >> Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > >> Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > >> Link: https://github.com/ClangBuiltLinux/linux/issues/1099 > >> --- > >> Changes in v2: > >> * Updated description to add tags and the llvm-project commit link. > >> * Fixed a typo. > >> > >> Changes in v3: > >> * Add Cc: stable@xxxxxxxxxxxxxxx > >> --- > >> Makefile | 2 +- > >> 1 file changed, 1 insertion(+), 1 deletion(-) > >> > >> diff --git a/Makefile b/Makefile > >> index 0b5f8538bde5..3ac83e375b61 100644 > >> --- a/Makefile > >> +++ b/Makefile > >> @@ -567,7 +567,7 @@ ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),) > >> ifneq ($(CROSS_COMPILE),) > >> CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%)) > >> GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit)) > >> -CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR) > >> +CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE) > > > > > > > >CROSS_COMPILE may contain the directory path > >to the cross toolchains. > > > > > >For example, I use aarch64-linux-gnu-* > >installed in > >/home/masahiro/tools/aarch64-linaro-7.5/bin > > > > > > > >Basically, there are two ways to use it. > > > >[1] > >PATH=$PATH:/home/masahiro/tools/aarch64-linaro-7.5/bin > >CROSS_COMPILE=aarch64-linux-gnu- > > > > > >[2] > >Without setting PATH, > >CROSS_COMPILE=~/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu- > > > > > > > >I usually do [2] (and so does intel's 0day bot). > > > > > > > >This patch works for the use-case [1] > >but if I do [2], --prefix is set to a strange path: > > > >--prefix=/home/masahiro/tools/aarch64-linaro-7.5/bin//home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu- > > Thanks. I did not know the use-case [2]. > This explains why there is a `$(notdir ...)` in > `CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))` > > > > > > > >Interestingly, the build is still successful. > >Presumably Clang searches for more paths > >when $(prefix)$needle is not found ? > > The priority order is: > > -B(--prefix), COMPILER_PATH, detected gcc-cross paths > > (In GCC, -B paths get prepended to the COMPILER_PATH list. Clang<=11 incorrectly > adds -B to the COMPILER_PATH list. I have fixed it for 12.0.0) > > If -B fails, the detected gcc-cross paths may still be able to find > /usr/bin/aarch64-linux-gnu- > > For example, on my machine (a variant of Debian testing), Clang finds > $(realpath > /usr/lib/gcc-cross/aarch64-linux-gnu/9/../../../../aarch64-linux-gnu/bin/as), > which is /usr/bin/aarch64-linux-gnu-as > > > > >I applied your patch and added -v option > >to see which assembler was internally invoked: > > > > "/home/masahiro/tools/aarch64-linaro-7.5/lib/gcc/aarch64-linux-gnu/7.5.0/../../../../aarch64-linux-gnu/bin/as" > >-EL -I ./arch/arm64/include -I ./arch/arm64/include/generated -I > >./include -I ./arch/arm64/include/uapi -I > >./arch/arm64/include/generated/uapi -I ./include/uapi -I > >./include/generated/uapi -o kernel/smp.o /tmp/smp-2ec2c7.s > > > > > >Ok, it looks like Clang found an alternative path > >to the correct 'as'. > > > > > > > > > >But, to keep the original behavior for both [1] and [2], > >how about this? > > > >CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE)) > > > > > > > >Then, I can get this: > > > > "/home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-as" > >-EL -I ./arch/arm64/include -I ./arch/arm64/include/generated -I > >./include -I ./arch/arm64/include/uapi -I > >./arch/arm64/include/generated/uapi -I ./include/uapi -I > >./include/generated/uapi -o kernel/smp.o /tmp/smp-16d76f.s > > This looks good. > > Agreed that `--prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))` should work for both [1] and [2]. > > Shall I send a v4? Or you are kind enough that you'll just add your Signed-off-by: tag > and fix that for me? :) > I fixed it up and applied to linux-kbuild/fixes. Thanks. While I am here, could you teach me a bit more? The top Makefile sets the following option as well: CLANG_GCC_TC := --gcc-toolchain=$(GCC_TOOLCHAIN) I checked the manual: https://clang.llvm.org/docs/ClangCommandLineReference.html -B<dir>, --prefix <arg>, --prefix=<arg> Add <dir> to search path for binaries and object files used implicitly --gcc-toolchain=<arg>, -gcc-toolchain <arg> Use the gcc toolchain at the given directory It is not clear to me how they work differently when clang searches for toolchains. If I delete --gcc-toolchain from the top Makefile, clang fails to link standalone programs because it wrongly invokes /usr/bin/ld instead of aarch64-linux-gnu-ld. Does Clang search for gnu assembler based on --prefix option? And, searches for a linker based on --gcc-toolchain ? -- Best Regards Masahiro Yamada
