Re: Android 11 clang toochain: kernel 5.18 build error with 32 bit target

From: Nick Desaulniers <ndesaulniers@google.com>
To: Nathan Chancellor <nathan@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Mauro Rossi <issor.oruam@gmail.com>,
	 Chih-Wei Huang <cwhuang@android-x86.org>,
	clang-built-linux <llvm@lists.linux.dev>,
	 Mark Brown <broonie@kernel.org>, Brian Gerst <brgerst@gmail.com>
Subject: Re: Android 11 clang toochain: kernel 5.18 build error with 32 bit target
Date: Tue, 26 Apr 2022 12:23:51 -0700	[thread overview]
Message-ID: <CAKwvOdkvg+jKVqetfrNPn5yNCxfZYHAhu2Cvc+dGLFG3+kOGUg@mail.gmail.com> (raw)
In-Reply-To: <Ymg43Ed/Sg8szdKq@dev-arch.thelio-3990X>

On Tue, Apr 26, 2022 at 11:24 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Tue, Apr 26, 2022 at 02:21:16PM +0200, Arnd Bergmann wrote:
> > On Mon, Apr 25, 2022 at 9:13 PM Mauro Rossi <issor.oruam@gmail.com> wrote:
> > > I have checked also with kernel-5.17 and the same build error is
> > > happening also there, so it is not due to commit 34737e2698 ("uaccess: add generic
> > > __{get,put}_kernel_nofault")
> >
> > Ok, that's good to know, thanks for checking! I suspect the problem is
> > using a slightly too-old prerelease build of clang, where the released
> > clang-11 build had the asm-goto-with-output feature enabled.
>
> Correct, the build failure can be reproduced with tip of tree LLVM if
> CONFIG_CC_HAS_ASM_GOTO_OUTPUT is manually disabled, such as:

That begs the question though; not all supported versions of GCC have
asm goto w/ outputs. Why is this not an issue for older gcc versions?
Perhaps it's because clang is validating inline asm constraints in
dead code again? I thought I had fixed that in
commit 158807de5822 ("x86/uaccess: Make __get_user_size() Clang
compliant on 32-bit")
in v5.9-rc1.

I don't know why `=a` can't be used, yet.
void foo(void *a) {
    asm("":"=a"(*((unsigned long*)a)));
}
Compiles w/ -m32 just fine.  Perhaps creduce is needed.
-- 
Thanks,
~Nick Desaulniers