Messages in this thread | | | Date | Tue, 21 Jul 2020 17:14:24 -0700 | From | Fangrui Song <> | Subject | Re: [PATCH v3] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation |
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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@googlegroups.com> 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@vger.kernel.org >> Reported-by: Nathan Chancellor <natechancellor@gmail.com> >> Signed-off-by: Fangrui Song <maskray@google.com> >> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com> >> Tested-by: Nathan Chancellor <natechancellor@gmail.com> >> Tested-by: Nick Desaulniers <ndesaulniers@google.com> >> 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@vger.kernel.org >> --- >> 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? :)
> > >> GCC_TOOLCHAIN := $(realpath $(GCC_TOOLCHAIN_DIR)/..) >> endif >> ifneq ($(GCC_TOOLCHAIN),) >> -- >> 2.28.0.rc0.105.gf9edc3c819-goog >> >> -- >> You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. >> To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com. >> To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20200721173125.1273884-1-maskray%40google.com. > > > >-- >Best Regards >Masahiro Yamada
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