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Re: [PATCH], Fix V1TI in Altivec regs on old systems

On Thu, Aug 29, 2019 at 12:02:05PM -0500, Segher Boessenkool wrote:
> Hi!
> On Tue, Aug 20, 2019 at 02:00:31PM -0400, Michael Meissner wrote:
> > I
> > noticed on power5 that the V1TImode mode is allowed in Altivec registers, even
> > though power5 doesn't have Altivec registers.
> > 
> > While it doesn't seem to effect anything (I couldn't create a test case that
> > failed), it is a small nit that should be fixed.  The test for TARGET_VADDUQM
> > matches a test earlier in the function where VSX registers are checked.
> Yeah, but does that test make any sense?

The test is copying the same test used earlier for VSX support.  As I said
without the patch, the reg_addr tables thinks that V1TImode can got in Altivec
registers, even if Altivec registers aren't allowed.  It is a nit, and other
things prevent the type from being allocated.

> Why p8 (or later) only?  Why vector only?  (Well that one is clear, and
> what this patch is about).  Why -mpowerpc64 only?  Because it has __int128
> maybe?  But then you should test for *that* (and why is it important?)

Well -mpower64 is needed because the only way you can get __int128 types is by
being 64-bit (VITImode is of course vector __int128).

The history is this mode is a hack.  It was added in the power8 time frame,
solely for the following built-in functions:


At the time, TImode (aka, __int128) was not allowed in VSX registers since we
were using the original register allocator (reload based).  There were several
issues that showed up in building large programs if we allowed TImode into
vector registers.  So we disallowed it (with a chicken switch to re-enable it
for debugging, -mvsx-timode).

Later when we switched over to the LRA register allocator, we allowed TImode to
go into vector registers.  But the GCC release had gone out that said if you
wanted to use those two power8 instructions you had to use a vector __int128
wrapper to allow the code.

> Where would V1TI go if not in vector regs?  Just in memory?

Since the only real use of it is for the built-ins, it likely won't matter.
But like all other modes, it could easily go in vector registers.

Personally except for the register allocation issue, I don't see any reason you
should use a vector type that is as large as the vector element inside.

> And, what happens on 970?  p5 doesn't have vector registers, but 970 does.

You can't declare the type using the normal sequence, since rs6000-c.c does not
allow 'vector __int128' until power8.  If instead, you write out:

	vector __int128
	__attribute__((altivec(vector__))) __int128

it will use GPRs to hold it on -mcpu=970 (with/without -maltivec).  On power7
it will use the vector register.  On power6 with -maltivec, it will use GPRs.

> > --- gcc/config/rs6000/rs6000.c	(revision 274635)
> > +++ gcc/config/rs6000/rs6000.c	(working copy)
> > @@ -1874,7 +1874,7 @@ rs6000_hard_regno_mode_ok_uncached (int
> >    /* AltiVec only in AldyVec registers.  */
> >    if (ALTIVEC_REGNO_P (regno))
> >      return (VECTOR_MEM_ALTIVEC_OR_VSX_P (mode)
> > -	    || mode == V1TImode);
> > +	    || (TARGET_VADDUQM && mode == V1TImode));
> Segher

Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email:, phone: +1 (978) 899-4797

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