Support six-argument syscalls from C for 32-bit x86, use generic lowlevellock-futex.h (bug 18138)

[Joseph Myers]

On Mon, 23 Mar 2015, Richard Henderson wrote:

> On 03/20/2015 05:28 PM, Joseph Myers wrote:
> > +#define INTERNAL_SYSCALL_MAIN_6(name, err, arg1, arg2, arg3,		\
> > +				arg4, arg5, arg6)			\
> > +  struct libc_do_syscall_args _xv =					\
> > +    {									\
> > +      (int) (arg1),							\
> > +      (int) (arg5),							\
> > +      (int) (arg6)							\
> > +    };									\
> > +    asm volatile (							\
> > +    "movl %1, %%eax\n\t"						\
> > +    "call __libc_do_syscall"						\
> > +    : "=a" (resultvar)							\
> > +    : "i" (__NR_##name), "c" (arg2), "d" (arg3), "S" (arg4), "D" (&_xv) \
> > +    : "memory", "cc")
> 
> Is this really so much better than
> 
> extern int __attribute__((regcall(3))) attribute_hidden
> __libc_do_syscall(int nr_eax, int arg3_edx, int arg2_ecx, int arg1_ebx,
>                   int arg4_esi, int arg5_edi, int arg6_ebp);
> 
> and performing the call itself in C?  It's only one more register you need to
> read from the stack in the function, but it avoids the compiler seeing a leaked
> pointer to the caller's stack frame.  The later could well influence
> optimization negatively.

I don't see the leaked pointer as relevant to optimization.  GCC knows 
that a pointer to one stack object can't be a basis for a pointer to 
another stack object, and that the variable's lifetime ends at end of 
scope.

-- 
Joseph S. Myers
joseph@codesourcery.com