[RFC] Add asm constraint modifier to mark strict memory accesses

[Yury Gribov <y dot gribov at samsung dot com>]

Hi all,

Current semantics of memory constraints in GCC inline asm (i.e. "m", 
"v", etc.) is somewhat loosy in that it tells GCC that asm code _may_ 
access given amount of bytes but is not guaranteed to do so. This is 
(ab)used by e.g. glibc (and also some pieces of kernel):
__STRING_INLINE void *
__rawmemchr (const void *__s, int __c)
{
...
  __asm__ __volatile__
    ("cld\n\t"
     "repne; scasb\n\t"
...
       "m" ( *(struct { char __x[0xfffffff]; } *)__s)

Imprecise size specification prevents code analysis tools from 
understanding semantics of inline asm (without parsing inline asm 
instructions which e.g. Asan in Clang tries to do). In particular we 
can't automatically instrument inline asm in kernel with Kasan because 
we can not determine exact access size (see e.g. discussion in 
https://gcc.gnu.org/ml/gcc-patches/2014-05/msg02530.html).

Would it make sense to add another constraint modifier (like "=", "&", 
etc.) that would tell compiler/tool that memory access in asm is 
_guaranteed_ to have the specified size?

-Y