Re: [PATCH, rs6000, testsuite] Fix PR target/64579, __TM_end __builtin_tend failed to return transactional state

[Segher Boessenkool <segher@kernel.crashing.org>]

On Wed, Apr 22, 2015 at 06:08:26PM -0500, Peter Bergner wrote:
> > > > > +	  case HTM_BUILTIN_TTEST: /* Alias for: tabortwci. 0,r0,0  */
> > > > > +	    op[nopnds++] = GEN_INT (0);
> > > > > +	    op[nopnds++] = gen_rtx_REG (SImode, 0);
> > > > > +	    op[nopnds++] = GEN_INT (0);
> > > > 
> > > > Is that really r0, isn't that (0|rA)?  [Too lazy to read the docs myself
> > > > right now, sorry.]
> > > 
> > > The ISA doc shows:
> > 
> > [snip]
> > 
> > Thanks for looking it up!
> > 
> > I'm still a bit worried about putting a reg in the RTL (while the instruction
> > doesn't actually use one), but perhaps it's harmless.
> 
> I'm not sure what you mean by the "instruction doesn't use one".
> The hardware instruction does use a register for its second
> operand (even though its contents are ignored due to TO == 0)
> and the pattern requires us to pass in a reg rtx, so I'm not
> sure what you're referring to.

I mean the instruction doesn't actually use the value in the register
(if it did, you couldn't just pass in a non-fixed hard register in RTL).

Using a hard reg in the RTL like this has a few problems:
a) It might hinder register allocation.  Maybe it doesn't, not sure;
b) It does hinder scheduling;
c) It can make things ICE, maybe with register asm.

I no longer think c) will happen in this case.

The alternative is to write a separate define_insn for ttest, one
without inputs; the generated assembler can still be the same of
course.

Cheers,


Segher