Re: [Aarch64] Vector Function Application Binary Interface Specification for OpenMP

[Steve Ellcey <sellcey@cavium.com>]

On Wed, 2018-05-16 at 22:11 +0100, Richard Sandiford wrote:
> 
> TARGET_HARD_REGNO_CALL_PART_CLOBBERED is the only current way
> of saying that an rtl instruction preserves the low part of a
> register but clobbers the high part.  We would need something like
> Alan H's CLOBBER_HIGH patches to do it using explicit clobbers.
> 
> Another approach would be to piggy-back on the -fipa-ra
> infrastructure
> and record that vector PCS functions only clobber Q0-Q7.  If -fipa-ra
> knows that a function doesn't clobber Q8-Q15 then that should
> override
> TARGET_HARD_REGNO_CALL_PART_CLOBBERED.  (I'm not sure whether it does
> in practice, but it should :-)  And if it doesn't that's a bug that's
> worth fixing for its own sake.)
> 
> Thanks,
> Richard

Alan,

I have been looking at your CLOBBER_HIGH patches to see if they
might be helpful in implementing the ARM SIMD Vector ABI in GCC.
I have also been looking at the -fipa-ra flag and how it works.

I was wondering if you considered using the ipa-ra infrastructure
for the SVE work that you are currently trying to support with 
the CLOBBER_HIGH macro?

My current thought for the ABI work is to mark all the floating
point / vector registers as caller saved (the lower half of V8-V15
are currently callee saved) and remove
TARGET_HARD_REGNO_CALL_PART_CLOBBERED.
This should work but would be inefficient.

The next step would be to split get_call_reg_set_usage up into
two functions so that I don't have to pass in a default set of
registers.  One function would return call_used_reg_set by
default (but could return a smaller set if it had actual used
register information) and the other would return regs_invalidated
by_call by default (but could also return a smaller set).

Next I would add a 'largest mode used' array to call_cgraph_rtl_info
structure in addition to the current function_used_regs register
set.

Then I could turn the get_call_reg_set_usage replacement functions
into target specific functions and with the information in the
call_cgraph_rtl_info structure and any simd attribute information on
a function I could modify what registers are really being used/invalidated
without being saved.

If the called function only uses the bottom half of a register it would not
be marked as used/invalidated.  If it uses the entire register and the
function is not marked as simd, then the register would marked as
used/invalidated.  If the function was marked as simd the register would not
be marked because a simd function would save both the upper and lower halves
of a callee saved register (whereas a non simd function would only save the
lower half).

Does this sound like something that could be used in place of your 
CLOBBER_HIGH patch?

Steve Ellcey
sellcey@cavium.com