Re: [PATCH] Fix computation of register limit for -fsched-pressure

[Tamar Christina <Tamar dot Christina at arm dot com>]

Forwarding to list as well.
________________________________________
From: Tamar Christina
Sent: Friday, October 28, 2016 10:52:17 AM
To: Pat Haugen; Maxim Kuvyrkov
Cc: GCC Patches
Subject: Re: [PATCH] Fix computation of register limit for -fsched-pressure

Hi Pat,

The commit seems to be causing some odd stack spills on aarch64.

I've created a new ticket https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78142

Thanks,
Tamar

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From: gcc-patches-owner@gcc.gnu.org <gcc-patches-owner@gcc.gnu.org> on behalf of Pat Haugen <pthaugen@linux.vnet.ibm.com>
Sent: Tuesday, October 18, 2016 4:07:55 PM
To: Maxim Kuvyrkov
Cc: GCC Patches
Subject: Re: [PATCH] Fix computation of register limit for -fsched-pressure

On 10/18/2016 05:31 AM, Maxim Kuvyrkov wrote:
>> > I see your point and agree that current code isn't optimal.  However, I don't think your patch is accurate either.  Consider https://gcc.gnu.org/onlinedocs/gccint/Register-Basics.html and let's assume that FIXED_REGISTERS in class CL is set for a third of the registers, and CALL_USED_REGISTERS is set to "1" for another third of registers.  So we have a third available for zero-cost allocation (CALL_USED_REGISTERS-FIXED_REGISTERS), a third available for spill-cost allocation (ALL_REGISTERS-CALL_USED_REGISTERS) and a third non-available (FIXED_REGISTERS).
>> >
>> > For a non-loop-single-basic-block function we should be targeting only the third of register available at zero-cost -- correct?
Yes.

  This is what is done by the current code, but, apparently, by accident.  It seems that the right register count can be obtained with:
>> >
>> >      for (int i = 0; i < ira_class_hard_regs_num[cl]; ++i)
>> > -      if (call_used_regs[ira_class_hard_regs[cl][i]])
>> > -        ++call_used_regs_num[cl];
>> > +      if (!call_used_regs[ira_class_hard_regs[cl][i]]
>> > +               || fixed_regs[ira_class_hard_regs[cl][i]])
>> > +        ++call_saved_regs_num[cl];
>> >
>> > Does this look correct to you?
> Thinking some more, it seems like fixed_regs should not be available to the scheduler no matter what.  Therefore, the number of fixed registers should be subtracted from ira_class_hard_regs_num[cl] without any scaling (entry_freq / bb_freq).

Ahh, yes, I forgot about FIXED_REGISTERS being included in CALL_USED_REGISTERS. I agree they should be totally removed from the register limit calculation. I'll rework the patch.

Thanks,
Pat