Re: [PATCH v3] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation

[Fangrui Song]

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? :)

> >  GCC_TOOLCHAIN  := $(realpath $(GCC_TOOLCHAIN_DIR)/..)
> >  endif
> >  ifneq ($(GCC_TOOLCHAIN),)
> > --
> > 2.28.0.rc0.105.gf9edc3c819-goog
> >
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>
>
>
> --
> Best Regards
> Masahiro Yamada