On Tue, Mar 17, 2015 at 01:38:06PM +0100, Laszlo Ersek wrote:
> On 03/16/15 15:15, Gabriel L. Somlo wrote:
> > 1. I can't for the life of me figure out how to stop gcc -O2
> > from assuming the if() test below is ALWAYS FALSE, and thus
> > optimizing it out completely. For now I've forced -O0 on
> > the entire function, but for some reason fw_cfg_read(&fcfile, ...)
> > does not appear to count as potentially modifying fcfile...
[...]
> > +static void
> > +fw_cfg_read(void *buf, int len)
> > +{
> > + insb(PORT_FW_CFG_DATA, buf, len);
> > +}
[...]
> I think fw_cfg_read() is inlined under -O2, and the insb() from that
> function is somehow confusing gcc.
>
> From "/usr/include/sys/io.h", on my RHEL-7.1 laptop:
>
> static __inline void
> insb (unsigned short int __port, void *__addr, unsigned long int __count)
> {
> __asm__ __volatile__ ("cld ; rep ; insb":"=D" (__addr), "=c" (__count)
> :"d" (__port), "0" (__addr), "1" (__count));
> }
My read of this is that gcc knows it must emit the instruction, and it
knows that __addr and __count can change. But, it doesn't know that
the memory at *__addr can change. I'd see if a barrier() fixes it.
See the section on "clobber" at:
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
In particular:
You can use a trick to avoid this if the size of the memory being
accessed is known at compile time. For example, if accessing ten bytes
of a string, use a memory input like:
{"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.
-Kevin
