Andrew Haley <aph <at> redhat.com> writes:
>
> On 10/13/2011 12:23 PM, MikeW wrote:
> > sh4-linux-gcc (GCC) 4.2.4 [unfortunately the version is tied to the
> > kernel build. ]
> >
> > In some kernel code where RAM is unavailable due to manipulation of
the MMU,
> > I wanted to place some 'got here' stops in the code so I could
^C break in gdb,
> > reset a register value and allow execution to continue.
> >
> > Accordingly I tried:
> > volatile register int stop_loop __asm("r5")__;
> > ...
> > stop_loop = 0x1234;
> > (disable MMU)
> > while (stop_loop != 0);
> > ...
> >
> > which seemed to generate code that checks the value of r5 only once:
> >
> > xxxx08: mov r5,r1
> > xxxx0a: tst r1,r1
> > xxxx0c: bf xxxx0c ;r5 never tested again!!
> > xxxx0e: (unrelated code)
> >
> > I also tried
> > while ((volatile)stop_loop != 0);
> > and
> > while ((volatile)(stop_loop != 0));
> >
> > which both gave the original asm code as above.
> >
> > So, in short, is there any way to persuade gcc to reload r5 - which could
> > in other non-debug situations be a global register variable updated
> > in an ISR, for example.
>
> I don't think so. If every there was a case for "use asm", it's surely
> this.
>
> Andrew.
>
Looks like the 'volatile' attribute does not work when registers are involved,
even though various language standard documents just mention "access to
an object" rather than stating that the qualifier only applies to
in-memory "objects".
The generated code would imply:
if (stop_loop != 0) {
while (1);
}
which is not equivalent to my source !
