Re: [PATCH][AArch64] Improve legitimize_address

[Christophe Lyon <christophe.lyon@linaro.org>]

Hi Wilco,

On 7 September 2016 at 14:43, Richard Earnshaw (lists)
<Richard.Earnshaw@arm.com> wrote:
> On 06/09/16 14:14, Wilco Dijkstra wrote:
>> Improve aarch64_legitimize_address - avoid splitting the offset if it is
>> supported.  When we do split, take the mode size into account.  BLKmode
>> falls into the unaligned case but should be treated like LDP/STP.
>> This improves codesize slightly due to fewer base address calculations:
>>
>> int f(int *p) { return p[5000] + p[7000]; }
>>
>> Now generates:
>>
>> f:
>>       add     x0, x0, 16384
>>       ldr     w1, [x0, 3616]
>>       ldr     w0, [x0, 11616]
>>       add     w0, w1, w0
>>       ret
>>
>> instead of:
>>
>> f:
>>       add     x1, x0, 16384
>>       add     x0, x0, 24576
>>       ldr     w1, [x1, 3616]
>>       ldr     w0, [x0, 3424]
>>       add     w0, w1, w0
>>       ret
>>
>> OK for trunk?
>>
>> ChangeLog:
>> 2016-09-06  Wilco Dijkstra  <wdijkstr@arm.com>
>>
>>     gcc/
>>       * config/aarch64/aarch64.c (aarch64_legitimize_address):
>>       Avoid use of base_offset if offset already in range.
>
> OK.
>
> R.

After this patch, I've noticed a regression:
FAIL: gcc.target/aarch64/ldp_vec_64_1.c scan-assembler ldp\td[0-9]+, d[0-9]
You probably need to adjust the scan pattern.

Thanks,

Christophe


>
>> --
>>
>> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
>> index 27bbdbad8cddc576f9ed4fd0670116bd6d318412..119ff0aecb0c9f88899fa141b2c7f9158281f9c3 100644
>> --- a/gcc/config/aarch64/aarch64.c
>> +++ b/gcc/config/aarch64/aarch64.c
>> @@ -5058,9 +5058,19 @@ aarch64_legitimize_address (rtx x, rtx /* orig_x  */, machine_mode mode)
>>        /* For offsets aren't a multiple of the access size, the limit is
>>        -256...255.  */
>>        else if (offset & (GET_MODE_SIZE (mode) - 1))
>> -     base_offset = (offset + 0x100) & ~0x1ff;
>> +     {
>> +       base_offset = (offset + 0x100) & ~0x1ff;
>> +
>> +       /* BLKmode typically uses LDP of X-registers.  */
>> +       if (mode == BLKmode)
>> +         base_offset = (offset + 512) & ~0x3ff;
>> +     }
>> +      /* Small negative offsets are supported.  */
>> +      else if (IN_RANGE (offset, -256, 0))
>> +     base_offset = 0;
>> +      /* Use 12-bit offset by access size.  */
>>        else
>> -     base_offset = offset & ~0xfff;
>> +     base_offset = offset & (~0xfff * GET_MODE_SIZE (mode));
>>
>>        if (base_offset != 0)
>>       {
>>
>