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SubjectRe: [BUG mm] "fixed" i386 memcpy inlining buggy
Date
On Tuesday 05 April 2005 19:34, Christophe Saout wrote:
> Hi Denis,
>
> the new i386 memcpy macro is a ticking timebomb.
>
> I've been debugging a new mISDN crash, just to find out that a memcpy
> was not inlined correctly.
>
> Andrew, you should drop the fix-i386-memcpy.patch (or have it fixed).
>
> This source code:
>
> mISDN_pid_t pid;
> [...]
> memcpy(&st->mgr->pid, &pid, sizeof(mISDN_pid_t));
>
> was compiled as:
>
> lea 0xffffffa4(%ebp),%esi <---- %esi is loaded
> ( add $0x10,%ebx )
> ( mov %ebx,%eax ) something else
> ( call 1613 <test_stack_protocol+0x83> ) %esi preserved
> mov 0xffffffa0(%ebp),%edx
> mov 0x74(%edx),%edi <---- %edi is loaded
> add $0x20,%edi offset in structure added
> ! mov $0x14,%esi !!!!!! <---- %esi overwritten!
> mov %esi,%ecx <---- %ecx loaded
> repz movsl %ds:(%esi),%es:(%edi)
>
> Apparently the compiled decided that the value 0x14 could be reused
> afterwards (which it does for an inlined memset of the same size some
> instructions below) and clobbers %esi.
>
> Looking at the macro:
>
> __asm__ __volatile__(
> ""
> : "=&D" (edi), "=&S" (esi)
> : "0" ((long) to),"1" ((long) from)
> : "memory"
> );
> }
> if (n >= 5*4) {
> /* large block: use rep prefix */
> int ecx;
> __asm__ __volatile__(
> "rep ; movsl"
> : "=&c" (ecx)
>
> it seems obvious that the compiled assumes it can reuse %esi and %edi
> for something else between the two __asm__ sections. These should
> probably be merged.

Oh shit. I was trying to be too clever. I still run with this patch,
so it must be happening very rarely.

Does this one compile ok?

static inline void * __constant_memcpy(void * to, const void * from, size_t n)
{
long esi, edi;
#if 1 /* want to do small copies with non-string ops? */
switch (n) {
case 0: return to;
case 1: *(char*)to = *(char*)from; return to;
case 2: *(short*)to = *(short*)from; return to;
case 4: *(int*)to = *(int*)from; return to;
#if 1 /* including those doable with two moves? */
case 3: *(short*)to = *(short*)from;
*((char*)to+2) = *((char*)from+2); return to;
case 5: *(int*)to = *(int*)from;
*((char*)to+4) = *((char*)from+4); return to;
case 6: *(int*)to = *(int*)from;
*((short*)to+2) = *((short*)from+2); return to;
case 8: *(int*)to = *(int*)from;
*((int*)to+1) = *((int*)from+1); return to;
#endif
}
#else
if (!n) return to;
#endif
{
/* load esi/edi */
__asm__ __volatile__(
""
: "=&D" (edi), "=&S" (esi)
: "0" ((long) to),"1" ((long) from)
: "memory"
);
}
if (n >= 5*4) {
/* large block: use rep prefix */
int ecx;
__asm__ __volatile__(
"rep ; movsl"
: "=&c" (ecx), "=&D" (edi), "=&S" (esi)
: "0" (n/4), "1" (edi),"2" (esi)
: "memory"
);
} else {
/* small block: don't clobber ecx + smaller code */
if (n >= 4*4) __asm__ __volatile__("movsl":"=&D"(edi),"=&S"(esi):"0"(edi),"1"(esi):"memory");
if (n >= 3*4) __asm__ __volatile__("movsl":"=&D"(edi),"=&S"(esi):"0"(edi),"1"(esi):"memory");
if (n >= 2*4) __asm__ __volatile__("movsl":"=&D"(edi),"=&S"(esi):"0"(edi),"1"(esi):"memory");
if (n >= 1*4) __asm__ __volatile__("movsl":"=&D"(edi),"=&S"(esi):"0"(edi),"1"(esi):"memory");
}
switch (n % 4) {
/* tail */
case 0: return to;
case 1: __asm__ __volatile__("movsb":"=&D"(edi),"=&S"(esi):"0"(edi),"1"(esi):"memory"); return to;
case 2: __asm__ __volatile__("movsw":"=&D"(edi),"=&S"(esi):"0"(edi),"1"(esi):"memory"); return to;
default: __asm__ __volatile__("movsw\n\tmovsb":"=&D"(edi),"=&S"(esi):"0"(edi),"1"(esi):"memory"); return to;
}
}
--
vda
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