arm memcpy of aligned data

[Kyrill Tkachov]

On 29/05/15 10:08, Kyrill Tkachov wrote:
> Hi Mike,
>
> On 28/05/15 22:15, Mike Stump wrote:
>> So, the arm memcpy code of aligned data isn’t as good as it can be.
>>
>> void *memcpy(void *dest, const void *src, unsigned int n);
>>
>> void foo(char *dst, int i) {
>>     memcpy (dst, &i, sizeof (i));
>> }
>>
>> generates horrible code, but, it we are willing to notice the src or the destination are aligned, we can do much better:
>>
>> $ ./cc1 -fschedule-fusion -fdump-tree-all-all -da -march=armv7ve -mcpu=cortex-m4 -fomit-frame-pointer -quiet -O2 /tmp/t.c -o t.s
>> $ cat t.s
>> [ … ]
>> foo:
>> 	@ args = 0, pretend = 0, frame = 4
>> 	@ frame_needed = 0, uses_anonymous_args = 0
>> 	@ link register save eliminated.
>> 	sub	sp, sp, #4
>> 	str	r1, [r0]	@ unaligned
>> 	add	sp, sp, #4
> I think there's something to do with cpu tuning here as well.

That being said, I do think this is a good idea.
I'll give it a test.

Kyrill

> For the code you've given compiled with -O2 -mcpu=cortex-a53 I get:
>           sub     sp, sp, #8
>           mov     r2, r0
>           add     r3, sp, #8
>           str     r1, [r3, #-4]!
>           ldr     r0, [r3]        @ unaligned
>           str     r0, [r2]        @ unaligned
>           add     sp, sp, #8
>           @ sp needed
>           bx      lr
>
> whereas for -O2 -mcpu=cortex-a57 I get the much better:
>           sub     sp, sp, #8
>           str     r1, [r0]        @ unaligned
>           add     sp, sp, #8
>           @ sp needed
>           bx      lr
>
> Kyrill
>
>
>> Index: gcc/config/arm/arm.c
>> ===================================================================
>> --- gcc/config/arm/arm.c	(revision 223842)
>> +++ gcc/config/arm/arm.c	(working copy)
>> @@ -14376,7 +14376,10 @@ arm_block_move_unaligned_straight (rtx d
>>    				srcoffset + j * UNITS_PER_WORD - src_autoinc);
>>    	  mem = adjust_automodify_address (srcbase, SImode, addr,
>>    					   srcoffset + j * UNITS_PER_WORD);
>> -	  emit_insn (gen_unaligned_loadsi (regs[j], mem));
>> +	  if (src_aligned)
>> +	    emit_move_insn (regs[j], mem);
>> +	  else
>> +	    emit_insn (gen_unaligned_loadsi (regs[j], mem));
>>    	}
>>          srcoffset += words * UNITS_PER_WORD;
>>        }
>> @@ -14395,7 +14398,10 @@ arm_block_move_unaligned_straight (rtx d
>>    				dstoffset + j * UNITS_PER_WORD - dst_autoinc);
>>    	  mem = adjust_automodify_address (dstbase, SImode, addr,
>>    					   dstoffset + j * UNITS_PER_WORD);
>> -	  emit_insn (gen_unaligned_storesi (mem, regs[j]));
>> +	  if (dst_aligned)
>> +	    emit_move_insn (mem, regs[j]);
>> +	  else
>> +	    emit_insn (gen_unaligned_storesi (mem, regs[j]));
>>    	}
>>          dstoffset += words * UNITS_PER_WORD;
>>        }
>>
>>
>> Ok?
>>
>> Can someone spin this through an arm test suite run for me, I was doing this by inspection and cross compile on a system with no arm bits.  Bonus points if you can check it in with the test case above marked up as appropriate.
>>