Re: clang fails on linux-next since commit 8bf705d13039

[Dmitry Vyukov <dvyukov@google.com>]

On Sun, May 6, 2018 at 12:41 PM,  <sedat.dilek@gmail.com> wrote:
>
>
> Am Montag, 19. März 2018 18:29:04 UTC+1 schrieb Matthias Kaehlcke:
>>
>> El Mon, Mar 19, 2018 at 09:43:25AM +0300 Dmitry Vyukov ha dit:
>>
>> > On Sat, Mar 17, 2018 at 2:13 PM, Lukas Bulwahn <lukas....@gmail.com>
>> > wrote:
>> > > Hi Dmitry, hi Ingo,
>> > >
>> > > since commit 8bf705d13039 ("locking/atomic/x86: Switch atomic.h to use
>> > > atomic-instrumented.h")
>> > > on linux-next (tested and bisected from tag next-20180316), compiling
>> > > the
>> > > kernel with clang fails with:
>> > >
>> > > In file included from arch/x86/entry/vdso/vdso32/vclock_gettime.c:33:
>> > > In file included from
>> > > arch/x86/entry/vdso/vdso32/../vclock_gettime.c:15:
>> > > In file included from ./arch/x86/include/asm/vgtod.h:6:
>> > > In file included from ./include/linux/clocksource.h:13:
>> > > In file included from ./include/linux/timex.h:56:
>> > > In file included from ./include/uapi/linux/timex.h:56:
>> > > In file included from ./include/linux/time.h:6:
>> > > In file included from ./include/linux/seqlock.h:36:
>> > > In file included from ./include/linux/spinlock.h:51:
>> > > In file included from ./include/linux/preempt.h:81:
>> > > In file included from ./arch/x86/include/asm/preempt.h:7:
>> > > In file included from ./include/linux/thread_info.h:38:
>> > > In file included from ./arch/x86/include/asm/thread_info.h:53:
>> > > In file included from ./arch/x86/include/asm/cpufeature.h:5:
>> > > In file included from ./arch/x86/include/asm/processor.h:21:
>> > > In file included from ./arch/x86/include/asm/msr.h:67:
>> > > In file included from ./arch/x86/include/asm/atomic.h:279:
>> > > ./include/asm-generic/atomic-instrumented.h:295:10: error: invalid
>> > > output size for constraint '=a'
>> > >                 return arch_cmpxchg((u64 *)ptr, (u64)old, (u64)new);
>> > >                        ^
>> > > ./arch/x86/include/asm/cmpxchg.h:149:2: note: expanded from macro
>> > > 'arch_cmpxchg'
>> > >         __cmpxchg(ptr, old, new, sizeof(*(ptr)))
>> > >         ^
>> > > ./arch/x86/include/asm/cmpxchg.h:134:2: note: expanded from macro
>> > > '__cmpxchg'
>> > >         __raw_cmpxchg((ptr), (old), (new), (size), LOCK_PREFIX)
>> > >         ^
>> > > ./arch/x86/include/asm/cmpxchg.h:95:17: note: expanded from macro
>> > > '__raw_cmpxchg'
>> > >                              : "=a" (__ret), "+m" (*__ptr)
>> > > \
>> > >                                      ^
>> > >
>> > > (... and some more similar and closely related errors)
>> >
>> >
>> > Thanks for reporting, Lukas.
>> >
>> > +more people who are more aware of the current state of clang for
>> > kernel.
>> >
>> > Are there are known issues in '=a' constraint handling between gcc and
>> > clang? Is there a recommended way to resolve them?
>> >
>> > Also, Lukas what's your version of clang? Potentially there are some
>> > fixes for kernel in the very latest versions of clang.
>>
>> My impression is that the problem only occurs in code built for
>> 32-bit (like arch/x86/entry/vdso/vdso32/*), where the use of a 64-bit
>> address with a '=a' constraint is indeed invalid. I think the 'root
>> cause' is that clang parses unreachable code before it discards it:
>>
>> static __always_inline unsigned long
>> cmpxchg_local_size(volatile void *ptr, unsigned long old, unsigned long
>> new,
>>                    int size)
>> {
>>         ...
>>         switch (size) {
>>         ...
>>         case 8:
>>                 BUILD_BUG_ON(sizeof(unsigned long) != 8);
>>                 return arch_cmpxchg_local((u64 *)ptr, (u64)old, (u64)new);
>>         }
>>            ...
>> }
>>
>> For 32-bit builds size is 4 and the code in the 'offending' branch is
>> unreachable, however clang still parses it.
>>
>> d135b8b5060e ("arm64: uaccess: suppress spurious clang warning") fixes
>> a similar issue.
>
>
> When forcing to build with '-O0' instead of default '-O2' I can see this...
>
>  ./include/asm-generic/atomic-instrumented.h:364:3: error: array size is
> negative
>                 BUILD_BUG_ON(sizeof(unsigned long) != 8);
>                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./include/linux/build_bug.h:66:52: note: expanded from macro 'BUILD_BUG_ON'
> #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
>                                                    ^~~~~~~~~~~~~~~~~~~


With clang or gcc?