On Jul 12 21:52, Christopher Faylor wrote:
> On Wed, Jul 08, 2009 at 01:11:53AM +0100, Dave Korn wrote:
> >Dave Korn wrote:
> >>It doesn't do anything about the reload failure, which is a bug in
> >>GCC-3, since the usage is a standard usage supported by the
> >>documentation. It's possible that it may disappear as a side-effect,
> >>in which case all the better.
> >
> >Nope, no such luck.
>
> I just bugged Dave in private email about this without doing my
> homework first. How embarrassing.
>
> There is a subtle difference in the generated code if you do this:
>
> --- winbase.h 7 Jul 2009 21:41:43 -0000 1.16
> +++ winbase.h 13 Jul 2009 01:46:17 -0000
> @@ -56,7 +56,7 @@
> {
> return
> ({
> - register long ret __asm ("%eax");
> + register long ret;
> __asm __volatile ("lock cmpxchgl %2, %1"
> : "=a" (ret), "=m" (*t)
> : "r" (v), "m" (*t), "0" (c)
>
> but does it really matter? This causes the esi register to be used
> rather than the edx register.
>
> with _asm ("%eax")
> 160e: 8b 5d 08 mov 0x8(%ebp),%ebx
> 1611: 8d 53 08 lea 0x8(%ebx),%edx
> 1614: f0 0f b1 0a lock cmpxchg %ecx,(%edx)
> 1618: 85 c0 test %eax,%eax
> 161a: 74 37 je 1653
> <pthread_mutex::_trylock(pthread*)+0x53>
>
> without
> 1616: 8b 5d 08 mov 0x8(%ebp),%ebx
> 1619: 8d 73 08 lea 0x8(%ebx),%esi
> 161c: f0 0f b1 0e lock cmpxchg %ecx,(%esi)
> 1620: 85 c0 test %eax,%eax
> 1622: 74 44 je 1668
> <pthread_mutex::_trylock(pthread*)+0x68>
>
>
> And, more crucially, it compiles with gcc 3.4.
>
> Should I check this variation in?
The affected operations have nothing to do with %eax. Why does the
compiler change the usage of some entirely unrelated register? This
looks suspicious.
Is there a chance that using the esi register obliterates data in the
calling function?
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
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