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

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Message-ID: <cc8c2f2d-b242-7488-3b5a-33828ee38b36@xen0n.name>
Date:   Sun, 25 Jun 2023 15:16:44 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     Huacai Chen <chenhuacai@...nel.org>
Cc:     WANG Rui <wangrui@...ngson.cn>, Xi Ruoyao <xry111@...111.site>,
        loongarch@...ts.linux.dev, linux-kbuild@...r.kernel.org,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        WANG Xuerui <git@...0n.name>
Subject: Re: [PATCH v2 7/9] LoongArch: Tweak CFLAGS for Clang compatibility

On 2023/6/25 10:13, Huacai Chen wrote:
> Hi, Ruoyao,
> 
> On Sun, Jun 25, 2023 at 2:42 AM WANG Xuerui <kernel@...0n.name> wrote:
>>
>> From: WANG Xuerui <git@...0n.name>
>>
>> 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@...0n.name>
>> ---
>>   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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