Re: [PATCH v2 7/9] LoongArch: Tweak CFLAGS for Clang compatibility

[WANG Xuerui]

On 2023/6/25 10:13, Huacai Chen wrote:
> Hi, Ruoyao,
>
> On Sun, Jun 25, 2023 at 2:42 AM WANG Xuerui <kernel@xxxxxxxxxx> wrote:
> > From: WANG Xuerui <git@xxxxxxxxxx>
> >
> > Now the arch code is mostly ready for LLVM/Clang consumption, it is time
> > to re-organize the CFLAGS a little to actually enable the LLVM build.
> >
> > In particular, -mexplicit-relocs and -mdirect-extern-access are not
> > necessary nor supported on Clang; feature detection via cc-option would
> > not work, because that way the broken combo of "new GNU as + old GCC"
> > would seem to get "fixed", but actually produce broken kernels.
> > Explicitly depending on CONFIG_CC_IS_CLANG is thus necessary to not
> > regress UX for those building their own kernels.
> >
> > A build with !RELOCATABLE && !MODULE is confirmed working within a QEMU
> > environment; support for the two features are currently blocked on
> > LLVM/Clang, and will come later.
> >
> > Signed-off-by: WANG Xuerui <git@xxxxxxxxxx>
> > ---
> >   arch/loongarch/Makefile | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> > index 366771016b99..82c619791a63 100644
> > --- a/arch/loongarch/Makefile
> > +++ b/arch/loongarch/Makefile
> > @@ -51,7 +51,9 @@ LDFLAGS_vmlinux                       += -static -n -nostdlib
> >
> >   # When the assembler supports explicit relocation hint, we must use it.
> >   # GCC may have -mexplicit-relocs off by default if it was built with an old
> > -# assembler, so we force it via an option.
> > +# assembler, so we force it via an option. For LLVM/Clang the desired behavior
> > +# is the default, and the flag is not supported, so don't pass it if Clang is
> > +# being used.
> >   #
> >   # When the assembler does not supports explicit relocation hint, we can't use
> >   # it.  Disable it if the compiler supports it.
> > @@ -61,8 +63,10 @@ LDFLAGS_vmlinux                      += -static -n -nostdlib
> >   # combination of a "new" assembler and "old" compiler is not supported.  Either
> >   # upgrade the compiler or downgrade the assembler.
> >   ifdef CONFIG_AS_HAS_EXPLICIT_RELOCS
> > +ifndef CONFIG_CC_IS_CLANG
> >   cflags-y                       += -mexplicit-relocs
> >   KBUILD_CFLAGS_KERNEL           += -mdirect-extern-access
> > +endif
> I prefer to drop CONFIG_CC_IS_CLANG and use
> cflags-y                       += $(call cc-option,-mexplicit-relocs)
> KBUILD_CFLAGS_KERNEL           += $(call cc-option,-mdirect-extern-access)
>
> Then Patch-6 can be merged in this.
>
> What's your opinion?

FYI: with this approach the build no longer instantly dies with binutils 
2.40 + gcc 12.3, but there are also tons of warnings that say the model 
attribute is being ignored. I checked earlier discussions and this means 
modules are silently broken at runtime, which is not particularly good UX.

But after more thought, I find it possible to not regress UX nor 
explicitly check for Clang: the special point about Clang is that it 
emits explicit relocs *without* support for the CLI -mexplicit-relocs 
flag. So assuming:

* CC_HAS_EXPLICIT_RELOCS means "-mexplicit-relocs" is supported by $CC,
* CC_EMITS_EXPLICIT_RELOCS means explicit relocs are emitted by $CC, 
with -mexplicit-relocs in CFLAGS if supported,

We then have:

* gcc 12.x: !CC_HAS_EXPLICIT_RELOCS && !CC_EMITS_EXPLICIT_RELOCS
* gcc 13.x: CC_HAS_EXPLICIT_RELOCS && CC_EMITS_EXPLICIT_RELOCS
* clang: !CC_HAS_EXPLICIT_RELOCS && CC_EMITS_EXPLICIT_RELOCS

So in this case (inside the AS_HAS_EXPLICIT_RELOCS=y block), as long as 
CC_EMITS_EXPLICIT_RELOCS we are okay. We can $(error) out in the first 
case and provide helpful diagnostics too.

What do you people think about this alternative approach?

--
WANG "xen0n" Xuerui

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