[Mesa-dev] [PATCH v2 3/6] i965/vec4: Don't emit scratch reads for a spilled register we have just written

[Francisco Jerez]

Francisco Jerez <currojerez at riseup.net> writes:

> Iago Toral Quiroga <itoral at igalia.com> writes:
>
>> When we have code such as this:
>>
>> mov vgrf1.0.x:F, vgrf2.xxxx:F
>> mov vgrf3.0.x:F, vgrf1.xxxx:F
>> ...
>> mov vgrf3.0.x:F, vgrf1.xxxx:F
>>
>> And vgrf1 is chosen for spilling, we can emit this:
>>
>> mov vgrf1.0.x:F, vgrf2.xxxx:F
>> gen4_scratch_write hw_reg0:F, vgrf1.xxxx:D, 22D
>> mov vgrf3.0.x:F, vgrf1.xxxx:F
>> ...
>> gen4_scratch_read vgrf4.0.x:F, 22D
>> mov vgrf3.0.x:F, vgrf4.xxxx:F
>>
>> Instead of this:
>>
>> mov vgrf1.0.x:F, vgrf2.xxxx:F
>> gen4_scratch_write hw_reg0:F, vgrf1.xxxx:D, 22D
>> gen4_scratch_read vgrf4.0.x:F, 22D
>> mov vgrf3.0.x:F, vgrf4.xxxx:F
>> ...
>> gen4_scratch_read vgrf5.0.x:F, 22D
>> mov vgrf3.0.x:F, vgrf5.xxxx:F
>>
>> And save one scratch read while still preserving the benefits of
>> spilling the register.
>>
>> In general, we avoid emitting scratch reads for as long as the next instruction
>> keeps reading the spilled register. This should not harm the benefit of
>> spilling the register because gains for register allocation only come when we
>> have chunks of program code where the register is alive but not really used
>> (because these are the points where we could effectively use that register for
>> another purpose if we spilled it), so as long as consecutive instructions use
>> that register we can avoid the scratch reads without losing anything.
>> ---
>>  .../drivers/dri/i965/brw_vec4_reg_allocate.cpp     | 37 +++++++++++++++++++++-
>>  1 file changed, 36 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_reg_allocate.cpp b/src/mesa/drivers/dri/i965/brw_vec4_reg_allocate.cpp
>> index cff5406..fd56dae 100644
>> --- a/src/mesa/drivers/dri/i965/brw_vec4_reg_allocate.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_vec4_reg_allocate.cpp
>> @@ -340,11 +340,43 @@ vec4_visitor::spill_reg(int spill_reg_nr)
>>     unsigned int spill_offset = last_scratch++;
>>  
>>     /* Generate spill/unspill instructions for the objects being spilled. */
>> +   vec4_instruction *spill_write_inst = NULL;
>>     foreach_block_and_inst(block, vec4_instruction, inst, cfg) {
>> +      /* We don't spill registers used for scratch */
>> +      if (inst->opcode == SHADER_OPCODE_GEN4_SCRATCH_READ ||
>> +          inst->opcode == SHADER_OPCODE_GEN4_SCRATCH_WRITE)
>> +         continue;
>> +
>>        int scratch_reg = -1;
>> +      bool spill_reg_was_read = false;
>>        for (unsigned int i = 0; i < 3; i++) {
>>           if (inst->src[i].file == GRF && inst->src[i].reg == spill_reg_nr) {
>> -            if (scratch_reg == -1) {
>> +            if (!spill_reg_was_read) {
>> +               spill_reg_was_read = (!inst->predicate ||
>> +                                     inst->opcode == BRW_OPCODE_SEL);
>> +            }
>> +
>> +            /* If we are reading the spilled register right after writing
>> +             * to it we can skip the scratch read and use directly the
>> +             * register we used as source for the scratch write. For this
>> +             * to work we must check that:
>> +             *
>> +             * 1) The write is inconditional, that is, it is not predicated or
>> +             *    it is a SEL.
>> +             * 2) All the channels that we read have been written in that
>> +             *    last write instruction.
>> +             *
>> +             * We keep doing this for as long as the next instruction
>> +             * keeps reading the spilled register and break as soon as we
>> +             * find an instruction that doesn't.
>> +             */
>> +            if (spill_write_inst &&
>> +                (!spill_write_inst->predicate ||
>> +                 spill_write_inst->opcode == BRW_OPCODE_SEL) &&
>> +                ((brw_mask_for_swizzle(inst->src[i].swizzle) &
>> +                 ~spill_write_inst->dst.writemask) == 0)) {
>> +               scratch_reg = spill_write_inst->dst.reg;
>> +            } else if (scratch_reg == -1) {
>
> One suggestion: You could factor out the rather complex caching logic
> into a separate function (e.g. 'bool can_reuse_scratch_for_source(const
> vec4_instruction *, unsigned i, unsigned scratch_reg)').  The function
> would simply compare scratch_reg with the sources of the current
> instruction (up to src) and the sources and destination of the previous

Huh, of course "up to i" is what I intended to write.

> non-scratch_read/write instruction.  If there's a match it would check
> that the regioning is compatible with the i-th source and return true in
> that case.  This would have several benefits:
>
>  - It would keep ::spill_reg() simple and the caching heuristic factored
>    out.  The only thing spill_reg() would still need to take care of is
>    keep track of the last spilling temporary (e.g. as you're doing it
>    now with the scratch_reg variable) and pass it to
>    can_reuse_scratch_for_source(), but it would no longer be necessary
>    to reset it to -1 when reuse is not possible.
>
>  - It would allow you to use a single implementation of the caching
>    policy to implement both spill_reg() and evaluate_spill_costs(), what
>    would make sure that things don't go out of sync in the likely case
>    that we want to change the caching heuristic in the future.
>
In case it's not obvious from my previous explanation
evaluate_spill_costs() could invoke the function I proposed like
"can_reuse_scratch_for_source(inst, i, inst->src[i].reg)" to find out if
the register would be cached by spill_reg().

>>                 scratch_reg = alloc.allocate(1);
>>                 src_reg temp = inst->src[i];
>>                 temp.reg = scratch_reg;
>> @@ -357,6 +389,9 @@ vec4_visitor::spill_reg(int spill_reg_nr)
>>  
>>        if (inst->dst.file == GRF && inst->dst.reg == spill_reg_nr) {
>>           emit_scratch_write(block, inst, spill_offset);
>> +         spill_write_inst = inst;
>> +      } else if (spill_write_inst && !spill_reg_was_read) {
>> +         spill_write_inst = NULL;
>>        }
>>     }
>>  
>> -- 
>> 1.9.1
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