On Mon, Feb 20, 2012 at 07:10:26PM +0100, Uros Bizjak wrote:
> > So the above is right and needed, though perhaps we might want
> > a combine pattern or peephole to turn the
> > movl $-1, %eax
> > xbegin .+6
> > cmpl %eax, $-1
> > jne 1f
>
> The compiler can reverse the condition and exchange arms of if
> expression. This will void the assumption that false arm (and abort
> sequence) is at pc+6.
The xbegin .+6 instruction doesn't (from the compiler's POV) as any
JUMP_INSN, it just returns either -1 or some other value in %eax
(when coupled with setting %eax to -1 before it).
That is the semantics of the _xbegin () intrinsic.
We can (and at least in the combiner should) model it as
a JUMP_INSN with UNSPEC_VOLATILE in it, which will work similarly
to asm goto. The question is how happy cfgrtl.c etc. will be on
a conditional JUMP_INSN with UNSPEC_VOLATILE in it.
Jakub
