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SubjectRe: [PATCH v2] x86/asm: fix assembly constraints in bitops

* Alexander Potapenko <glider@google.com> wrote:

> 1. Use memory clobber in bitops that touch arbitrary memory
>
> Certain bit operations that read/write bits take a base pointer and an
> arbitrarily large offset to address the bit relative to that base.
> Inline assembly constraints aren't expressive enough to tell the
> compiler that the assembly directive is going to touch a specific memory
> location of unknown size, therefore we have to use the "memory" clobber
> to indicate that the assembly is going to access memory locations other
> than those listed in the inputs/outputs.
> To indicate that BTR/BTS instructions don't necessarily touch the first
> sizeof(long) bytes of the argument, we also move the address to assembly
> inputs.
>
> This particular change leads to size increase of 124 kernel functions in
> a defconfig build. For some of them the diff is in NOP operations, other
> end up re-reading values from memory and may potentially slow down the
> execution. But without these clobbers the compiler is free to cache
> the contents of the bitmaps and use them as if they weren't changed by
> the inline assembly.
>
> 2. Use byte-sized arguments for operations touching single bytes.
>
> Passing a long value to ANDB/ORB/XORB instructions makes the compiler
> treat sizeof(long) bytes as being clobbered, which isn't the case. This
> may theoretically lead to worse code in the case of heavy optimization.
>
> Signed-off-by: Alexander Potapenko <glider@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Paul E. McKenney <paulmck@linux.ibm.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: James Y Knight <jyknight@google.com>
> ---
> v2:
> -- renamed the patch
> -- addressed comment by Peter Zijlstra: don't use "+m" for functions
> returning void
> -- fixed input types for operations touching single bytes
> ---
> arch/x86/include/asm/bitops.h | 41 +++++++++++++++--------------------
> 1 file changed, 18 insertions(+), 23 deletions(-)

I'm wondering what the primary motivation for the patch is:

- Does it fix an actual miscompilation, or only a theoretical miscompilation?

- If it fixes an existing miscompilation:

- Does it fix a miscompilation triggered by current/future versions of GCC?
- Does it fix a miscompilation triggered by current/future versions of Clang?

- Also, is the miscompilation triggered by 'usual' kernel configs, or
does it require exotics such as weird debug options or GCC plugins,
etc?

I.e. a bit more context would be useful.

Thanks,

Ingo

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