Re: [Qemu-devel] qemu vs gcc4

[Avi Kivity]

Paul Brook wrote:
> > > That's exactly what my gcc4 hacks do.
> > >
> > > It gets complicated because a x86 uses variable length insn encodings so
> > > you don't know where insn boundaries are, and a jmp instruction is larger
> > > than a ret instruction so it's not always possible to do a straight
> > > replacement.
> > >       
> > how about
> >
> > void some_generated_instruction(u32 a1, u32 s2)
> > {
> >        // code
> >        asm volatile ( "" );
> > }
> >
> >
> > that will force the code to fall through to the null asm code, avoiding
> > premature returns.
> >
> > if the code uses 'return' explicitly, turn it to a goto just before the
> > 'asm volatile'.
> >     
>
> We already do that. It doesn't stop gcc putting the return in the middle of 
> the function.
>
> Paul
>   
void f1();
void f2();

void f(int *z, int x, int y)
{
   if (x) {
       *z = x;
       f1();
   } else {
       *z = y;
       f2();
   }
   asm volatile ("");
}

works, with gcc -O2 -fno-reorder-blocks. removing either the asm or the 
-f flag doesn't.  No idea if it's consistent across architectures.

(the function calls are there to prevent cmov optimizations)




--
error compiling committee.c: too many arguments to function