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Re: [PATCH, i386] RTM support


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


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