Re: [PATCH][ARM] Remove an unused reload hook.

[Matthew Wahab <matthew dot wahab at arm dot com>]

On 27/02/15 09:41, Richard Earnshaw wrote:
> On 19/02/15 12:19, Matthew Wahab wrote:
> > The LEGITIMIZE_RELOAD_ADDRESS macro is only needed for reload. Since the
> > ARM backend no longer supports reload, this macro is not needed and this
> > patch removes it.
> >
> > gcc/
> > 2015-02-19  Matthew Wahab  <matthew.wahab@arm.com>
> >
> >      * config/arm/arm.h (LEGITIMIZE_RELOAD_ADDRESS): Remove.
> >      (ARM_LEGITIMIZE_RELOAD_ADDRESS): Remove.
> >      (THUMB_LEGITIMIZE_RELOAD_ADDRESS): Remove.
> >      * config/arm/arm.c (arm_legitimize_reload_address): Remove.
> >      (thumb_legitimize_reload_address): Remove.
> >      * config/arm/arm-protos.h (arm_legitimize_reload_address):
> >      Remove.
> >      (thumb_legitimize_reload_address): Remove.
>
> This is OK for stage 1.
>
> I have one open question: can LRA generate the optimizations that these
> hooks used to provide through reload?  If not, please could you file
> some bugzilla reports so that we don't lose them.
>
> Thanks,
> R.

arm_legitimize_reload_address was added by 
https://gcc.gnu.org/ml/gcc-patches/2011-04/msg00605.html. From 
config/arm/arm.c, the optimization turns
  	         add t1, r2, #4096
		 ldr r0, [t1, #4]
		 add t2, r2, #4096
		 ldr r1, [t2, #8]
into
	         add t1, r2, #4096
		 ldr r0, [t1, #4]
		 ldr r1, [t1, #8]

As far as I can tell, LRA does do this. Compiling the following with -O1:
----
int bar(int, int, int);
int test1(int* buf)
{
  int a = buf[41000];
  int b = buf[41004];
  int c = buf[41008];
  bar(a, b, c);
  return a +  b + c;
}
----
gcc version 4.5.1 (Sourcery G++ Lite 2010.09-51), which predates the 
optimization, produces
	ldr	r3, .L2
	ldr	r4, [r0, r3]
	add	r3, r3, #16
	ldr	r5, [r0, r3]
	add	r3, r3, #16
	ldr	r6, [r0, r3]

gcc version 4.9.3 20141119 with and without -mno-lra produce
	add	r0, r0, #163840
	ldr	r4, [r0, #160]
	ldr	r6, [r0, #176]
	ldr	r5, [r0, #192]
so it looks the better sequence gets generated.

thumb_legitimize_reload_address was added by 
https://gcc.gnu.org/ml/gcc-patches/2005-08/msg01140.html to fix PR 
23436. It replaces sequences like
	mov	r3, r9
	mov	r2, r10
	ldr	r0, [r3, r2]
with
	mov	r3, r9
	add	r3, r3, r10
	ldr	r0, [r3]

This looks like it's missing from trunk so I'll open a bugzilla report 
for it.

It's quite possible that I've got this all wrong so if I've missed 
something or you'd like me to open a bugzilla report for the ARM 
optimization as well, let me know.

Matthew