[Mesa-dev] [PATCH v2 3/6] i965/vec4: Don't emit scratch reads for a spilled register we have just written
Francisco Jerez
currojerez at riseup.net
Tue Aug 4 06:40:35 PDT 2015
Iago Toral <itoral at igalia.com> writes:
> On Fri, 2015-07-31 at 13:12 +0300, Francisco Jerez wrote:
>> Iago Toral <itoral at igalia.com> writes:
>>
>> > On Thu, 2015-07-30 at 17:08 +0300, Francisco Jerez wrote:
>> >> 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
>> >> 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:
>> >
>> > I think this might need to be a bit more complex. The previous inst's
>> > src[i] might read only a subset of the channels that where loaded into
>> > scratch_reg so comparing only against that can lead us to think that we
>> > can't reuse scratch_reg when in fact we can.
>> >
>> > I think the process should be more like we loop back looking at the prev
>> > instruction for as long as the previous instruction reads the
>> > scratch_ref until we find the one that writes to scratch_reg (which must
>> > be a scratch read). At that point we check that the writemask in that
>> > instruction is compatible with our swizzle on src[i], in which case we
>> > can reuse scratch_reg. Does this make sense to you?
>> >
>> The thing is that there may be no scratch read in the program yet if
>> you're being called from evaluate_spill_costs(). I think you want to
>> iterate for as long as the previous instruction reads scratch_reg but
>> doesn't write it -- If after the end of the loop you end up at an
>> instruction that writes, fine, you take the writemask from it, otherwise
>> assume XYZW.
>>
>> Yes, we should probably change spill_reg() to read whole vec4s at once
>> when unspilling a register, reading a subset of the channels doesn't
>> save any memory bandwidth and makes the optimization you're implementing
>> less likely to succeed.
>>
>> > In this case, if we have a successful match, we probably want to keep a
>> > reference to that instruction so that for the next src of the current
>> > instruction (or for the next instruction in our program) we don't have
>> > to repeat that process.
>> >
>>
>> Keep in mind that the assumption behind this optimization is that a
>> given spill candidate is only re-used for a very short sequence of
>> consecutive instructions (otherwise it might in fact be harmful), so the
>> amount of CPU cycles you can save by doing that is likely to be tiny in
>> practice -- Feel free to do it if it's trivial but I wouldn't trade it
>> for any additional complexity in the spill_reg() and
>> evaluate_spill_costs() loops.
>>
>> One last concern I had about this series has to do with the possibility
>> of it leading to an infinite loop in the allocate-spill loop -- I guess
>> that at least in theory it would be possible for a register used *only*
>> in a consecutive sequence of instructions to score quite high if the
>> sequence is part of a region of high register pressure. If the register
>> allocator decides it's the best candidate we have a problem because this
>> heuristic will prevent us from making any progress. I guess the easiest
>> way around this would be to have an array of counters in
>> evaluate_spill_costs() keeping track of the number of individual
>> instructions each register had to be (un)spilled for. At the end of the
>> loop you'd check if it's less or equal to one and in that case mark it
>> as no_spill. (Note that in theory this situation is possible even
>> without your changes AFAICT, just far less likely)
>
> I think this can't happen. If a register is only used in consecutive
> instructions and is selected for spilling, we will at least emit one
> scratch read or write for it (in the first instruction that uses it).
> The code in evaluate_spill_costs will then mark as no_spill any register
> used in a scratch read/write, which will make this register
> automatically not selectable for spilling after the first time it is
> selected.
>
Ah, good point, disregard my comment in that case.
>> > Iago
>> >
>> >
>> >> - 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.
>> >>
>> >> > 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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