[Beignet] [PATCH] GBE: avoid vector registers when there is high register pressure.

[Ruiling Song]

It makes sense to use short-live vector register under register pressure.
But please also remove the comment
"// If an element has very long interval, we don't want to put it into a
 // vector as it will add more pressure to the register allocation"

Thanks!
Ruiling
2015-09-17 8:39 GMT+08:00 Zhigang Gong <zhigang.gong at linux.intel.com>:

> Ping for review.
> Thanks.
>
> On Sun, Sep 06, 2015 at 05:21:29PM +0800, Zhigang Gong wrote:
> > If the reservedSpillRegs is not zero, it indicates we are in a
> > very high register pressure. Use register vector will likely
> > increase that pressure and will cause significant performance
> > problem which is much worse than use a short-live temporary
> > vector register with several additional MOVs.
> >
> > So let's simply avoid use vector registers and just use a
> > temporary short-live-interval vector.
> >
> > Signed-off-by: Zhigang Gong <zhigang.gong at intel.com>
> > ---
> >  backend/src/backend/gen_reg_allocation.cpp | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/backend/src/backend/gen_reg_allocation.cpp
> b/backend/src/backend/gen_reg_allocation.cpp
> > index 39f1934..36ad914 100644
> > --- a/backend/src/backend/gen_reg_allocation.cpp
> > +++ b/backend/src/backend/gen_reg_allocation.cpp
> > @@ -318,7 +318,7 @@ namespace gbe
> >        if (it == vectorMap.end() &&
> >            ctx.sel->isScalarReg(reg) == false &&
> >            ctx.isSpecialReg(reg) == false &&
> > -          (intervals[reg].maxID - intervals[reg].minID) < 2048)
> > +          ctx.reservedSpillRegs == 0 )
> >        {
> >          const VectorLocation location = std::make_pair(vector, regID);
> >          this->vectorMap.insert(std::make_pair(reg, location));
> > --
> > 1.9.1
> >
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