Re: Why high vsr registers [vsr32 - vsr63] are not used when -mvsx is specified on powerpc?

[Michael Meissner <meissner at linux dot vnet dot ibm dot com>]

On Fri, Jul 19, 2013 at 04:22:48PM -0700, Carrot Wei wrote:
> On Wed, Jul 17, 2013 at 6:01 PM, David Edelsohn <dje.gcc@gmail.com> wrote:
> > On Wed, Jul 17, 2013 at 7:27 PM, Carrot Wei <carrot@google.com> wrote:
> >> Hi
> >>
> >> When I tried to build 444.namd with options "-O2 -m64 -mvsx
> >> -mcpu=power7", I can see vsx instructions are actually used, there are
> >> many xs- started instructions, but none of them use high registers
> >> [vsr32 -vsr63], does anybody know the reason?
> >>
> >> One example is function calc_pair_energy_fullelect in file
> >> ComputeNonbondedUtil.o, there are many vsr register spilling but high
> >> vsr registers are never used.
> >
> > For scalar floating point, not vector floating point, GCC currently
> > uses only the lower VSRs because the upper registers only allow
> > indexed addressing modes (register + register) and not displacement
> > forms.  It's one register class with different valid addressing forms
> > depending on the register number, which is difficult for GCC. We did
> > not want to disable displacement address form in the initial support.
> 
> In insn patterns the register class is usually not directly used, instead
> different predicates and constraints are used. So can we use different
> predicates and constraints in memory access instructions and floating
> point arithmetic instructions?

Unfortunately, move patterns are special.  At the point of register allocation,
you need to have only one pattern that does a move of a given type.  Predicates
can only work on a single operand, so a a predicate operating on a memory
operand has no idea what register is being loaded or stored.  Constraints will
allow you to select the different patterns, and those are used.

Then in the secondary reload stage, you are given the move and have various
ways to 'fix' it.  Adding those fixes to the secondary reload hooks is way to
deal with machines that have different address formats for different
registers.

Unfortunately the first machines GCC originally targeted (68k, vax) had general
addressing formats that worked everywhere, and later machines you knew for a
given type what type of addressing could be used.

In the PowerPC you don't want register+offset if you are targetting VSX
registers, while you don't want register+register if you loading the value into
a GPR register and it is larger than a single register.

-- 
Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460, USA
email: meissner@linux.vnet.ibm.com, phone: +1 (978) 899-4797