Re: ARM peephole2 from 2003 never merged, still valid

["Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>]

On 06/06/2019 15:55, Fredrik Hederstierna wrote:
>> From: Segher Boessenkool <segher@kernel.crashing.org>
>> Sent: Thursday, June 6, 2019 4:02 PM
>> To: Richard Earnshaw (lists)
>> Cc: Jeff Law; Fredrik Hederstierna; gcc@gcc.gnu.org
>> Subject: Re: ARM peephole2 from 2003 never merged, still valid
>   
>> That doesn't stop combine from considering it.  It does make that first SET
>> survive, so that you get a parallel as final insn.  It may not like that
>> one of the parallel SETs is just a move.  Needs testcase :-)
>  
> Hi all, thanks for investigating this,
> I added semi-stripped testcase in original issue taken from CSiBE teem sources
> 
> See attachment in
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=9663
> 
> Tested gcc-9.1.0 for ARM 32bit targets, first without peephole2 patch:
> 
> 00000000 <nrrdRangeSet>:
>    0:	e92d407f 	push	{r0, r1, r2, r3, r4, r5, r6, lr}
>    4:	e2504000 	subs	r4, r0, #0
>    8:	0a00003f 	beq	10c <nrrdRangeSet+0x10c>
>    c:	e3510000 	cmp	r1, #0
>   10:	e1a05001 	mov	r5, r1
> 
> then with new peephole2 patch:
> 
> 00000000 <nrrdRangeSet>:
>    0:	e92d407f 	push	{r0, r1, r2, r3, r4, r5, r6, lr}
>    4:	e2504000 	subs	r4, r0, #0
>    8:	0a00003e 	beq	108 <nrrdRangeSet+0x108>
>    c:	e2515000 	subs	r5, r1, #0
> 
> Thanks, Fredrik
> 

The reason combine doesn't catch this is because at the time it runs the
MOV is in a different basic block.  Later on it is sunk into the same
basic block, but it's then too late to do the merge.

R.