[Mesa-dev] [PATCH 14/32] i965/fs: Fix register coalesce not to lose track of the second half of 16-wide moves.

[Francisco Jerez]

Matt Turner <mattst88 at gmail.com> writes:

> On Fri, Feb 6, 2015 at 6:42 AM, Francisco Jerez <currojerez at riseup.net> wrote:
>> Fixes rewrite by the register coalesce pass of references to
>> individual halves of 16-wide coalesced registers.
>> ---
>>  src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp b/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp
>> index 09f0fad..2a26a46 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs_register_coalesce.cpp
>> @@ -211,9 +211,13 @@ fs_visitor::register_coalesce()
>>              continue;
>>           }
>>           reg_to_offset[offset] = inst->dst.reg_offset;
>> -         if (inst->src[0].width == 16)
>> -            reg_to_offset[offset + 1] = inst->dst.reg_offset + 1;
>>           mov[offset] = inst;
>> +
>> +         if (inst->exec_size * type_sz(inst->src[0].type) > REG_SIZE) {
>> +            reg_to_offset[offset + 1] = inst->dst.reg_offset + 1;
>> +            mov[offset + 1] = inst;
>> +         }
>> +
>>           channels_remaining -= inst->regs_written;
>>        }
>>
>> --
>> 2.1.3
>
> I can believe it. It would help me to understand if we had an example
> of a sequence of instructions that this code didn't handle properly.

The problem is in the "rewrite" phase of the register coalesce pass
(roughly lines 264-283).  It won't fix up instructions that reference
some specific offset of the coalesced register if mov[i] is NULL for
that offset, as is the case for the second half of a 16-wide move.  For
example:

| ADD (16) vgrf0:f, vgrf0:f, 1.0:f
| MOV (16) vgrf1:f, vgrf0:f
| MOV (8)  vgrf2:f, vgrf0+1:f { sechalf }

will get incorrectly register-coalesced into:

| ADD (16) vgrf1:f, vgrf1:f, 1.0:f
| MOV (8)  vgrf2:f, vgrf0+1:f { sechalf }
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