Re: Wrap calculation of PIC address into a single instruction

[Maxim Kuvyrkov <maxim at codesourcery dot com>]

On 6/24/10 1:50 AM, Maxim Kuvyrkov wrote:
> On 6/24/10 1:22 AM, Steven Bosscher wrote:
> > On Wed, Jun 23, 2010 at 11:19 PM, Maxim
> > Kuvyrkov<maxim@codesourcery.com> wrote:
> > > This patch enables optimizations, particularly GCSE, handle
> > > calculation of
> > > PIC addresses. GCSE tracks only single instructions, so it can't handle
> > > two-instruction calculation of PIC address.
> > >
> > > With this patch, calculations of PIC addresses are represented as single
> > > instructions allowing GCSE eliminate all but the first address
> > > calculation
> > > for global variables.
> > >
> > > Any comments?
> >
> > Yes. This is what we added GCSE's ability to eliminate redundancies
> > from REG_EQUAL notes for. If your PIC addresses have a REG_EQUAL note,
> > GCSE is (or should be) already able to eliminate redundant address
> > calculations.
>
> You know, it turns out GCSE can eliminate calculation of PIC addresses
> on ARM. When I started working on improving code hoisting the example
> with PIC address wasn't fully optimized without this patch.

It was late in the night when I checked the generated code and, although 
GCSE of PIC addresses for ARM is now better, it is still not as good as 
with this patch.

GCSE cannot use (REG_EQUAL (symbol_ref)) note on the second instruction 
because can_assign_to_reg_without_clobbers returns false for symbol_ref 
when compiling PIC code.  (Symbol_ref) is not a 
LEGITIMATE_PIC_OPERAND_P, so it not a general_operand either.  The 
second check in can_assign_to_reg_without_clobbers returns false as (set 
(reg) (symbol_ref)) yields invalid instruction.

AFAICT, GCSE cannot optimize a bare symbol_ref for ARM PIC code because 
it has no guarantee that emit_move_insn (reg, symbol_ref) will generate 
simple enough code.

> Now, apparently, one of the other GCSE improvements (VBEout computation,
> probably) fixed the underlying problem.

Improvements to GCSE'ing constants were able to optimize half of second 
and subsequent address calculations.

> Richard, unless you think the patch may be valuable for some other
> reason, I'm dropping it.

The patch stands for now.

Thank you,

--
Maxim Kuvyrkov
CodeSourcery
maxim@codesourcery.com
(650) 331-3385 x724