[Mesa-dev] [PATCH 4/4] i965/fs: Implement pass to lower instructions of unsupported SIMD width.
Francisco Jerez
currojerez at riseup.net
Thu Jul 23 02:49:46 PDT 2015
Jason Ekstrand <jason at jlekstrand.net> writes:
> On Wed, Jul 22, 2015 at 10:05 AM, Francisco Jerez <currojerez at riseup.net> wrote:
>> Jason Ekstrand <jason at jlekstrand.net> writes:
>>
>>> On Wed, Jul 22, 2015 at 12:31 AM, Francisco Jerez <currojerez at riseup.net> wrote:
>>>> Jason Ekstrand <jason at jlekstrand.net> writes:
>>>>
>>>>> A few comments below. Mostly just asking for explanation.
>>>>>
>>>>> 1-3 are
>>>>>
>>>>> Reviewed-by: Jason Ekstrand <jason.ekstrand at intel.com>
>>>>>
>>>>> Obviously, don't merge 4/4 until it actually has users.
>>>>> --Jason
>>>>
>>>> Thanks.
>>>>
>>>>>
>>>>> On Thu, Jul 16, 2015 at 8:35 AM, Francisco Jerez <currojerez at riseup.net> wrote:
>>>>>> This lowering pass implements an algorithm to expand SIMDN
>>>>>> instructions into a sequence of SIMDM instructions in cases where the
>>>>>> hardware doesn't support the original execution size natively for some
>>>>>> particular instruction. The most important use-cases are:
>>>>>>
>>>>>> - Lowering send message instructions that don't support SIMD16
>>>>>> natively into SIMD8 (several texturing, framebuffer write and typed
>>>>>> surface operations).
>>>>>>
>>>>>> - Lowering messages that don't support SIMD8 natively into SIMD16
>>>>>> (*cough*gen4*cough*).
>>>>>>
>>>>>> - 64-bit precision operations (e.g. FP64 and 64-bit integer
>>>>>> multiplication).
>>>>>>
>>>>>> - SIMD32.
>>>>>>
>>>>>> The algorithm works by splitting the sources of the original
>>>>>> instruction into chunks of width appropriate for the lowered
>>>>>> instructions, and then interleaving the results component-wise into
>>>>>> the destination of the original instruction. The pass is controlled
>>>>>> by the get_lowered_simd_width() function that currently just returns
>>>>>> the original execution size making the whole pass a no-op for the
>>>>>> moment until some user is introduced.
>>>>>> ---
>>>>>> src/mesa/drivers/dri/i965/brw_fs.cpp | 142 +++++++++++++++++++++++++++++++++++
>>>>>> src/mesa/drivers/dri/i965/brw_fs.h | 1 +
>>>>>> 2 files changed, 143 insertions(+)
>>>>>>
>>>>>> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
>>>>>> index d031352..eeb6938 100644
>>>>>> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
>>>>>> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
>>>>>> @@ -3204,6 +3204,147 @@ fs_visitor::lower_logical_sends()
>>>>>> return progress;
>>>>>> }
>>>>>>
>>>>>> +/**
>>>>>> + * Get the closest native SIMD width supported by the hardware for instruction
>>>>>> + * \p inst. The instruction will be left untouched by
>>>>>> + * fs_visitor::lower_simd_width() if the returned value is equal to the
>>>>>> + * original execution size.
>>>>>> + */
>>>>>> +static unsigned
>>>>>> +get_lowered_simd_width(const struct brw_device_info *devinfo,
>>>>>> + const fs_inst *inst)
>>>>>> +{
>>>>>> + switch (inst->opcode) {
>>>>>> + default:
>>>>>> + return inst->exec_size;
>>>>>> + }
>>>>>> +}
>>>>>> +
>>>>>> +/**
>>>>>> + * The \p rows array of registers represents a \p num_rows by \p num_columns
>>>>>> + * matrix in row-major order, write it in column-major order into the register
>>>>>> + * passed as destination. \p stride gives the separation between matrix
>>>>>> + * elements in the input in fs_builder::dispatch_width() units.
>>>>>> + */
>>>>>> +static void
>>>>>> +emit_transpose(const fs_builder &bld,
>>>>>> + const fs_reg &dst, const fs_reg *rows,
>>>>>> + unsigned num_rows, unsigned num_columns, unsigned stride)
>>>>>
>>>>> I'm not sure what I think about calling this "emit_transpose". I
>>>>> guess it is kind of a transpose, but maybe it's more of a "gather".
>>>>> I'm not going to quibble about it though.
>>>>
>>>> *Shrug*, it doesn't only gather the vectors of the rows array (that
>>>> would have been emit_collect :P), it copies them out in vertical, just
>>>> like a matrix transpose -- assuming you're not horrified by the idea of
>>>> considering the argument a matrix.
>>>
>>> That's fine. Feel free to ignore that suggestion.
>>>
>>>>>
>>>>>> +{
>>>>>> + fs_reg *const components = new fs_reg[num_rows * num_columns];
>>>>>> +
>>>>>> + for (unsigned i = 0; i < num_columns; ++i) {
>>>>>> + for (unsigned j = 0; j < num_rows; ++j)
>>>>>> + components[num_rows * i + j] = offset(rows[j], bld, stride * i);
>>>>>> + }
>>>>>> +
>>>>>> + bld.LOAD_PAYLOAD(dst, components, num_rows * num_columns, 0);
>>>>>> +
>>>>>> + delete[] components;
>>>>>> +}
>>>>>> +
>>>>>> +bool
>>>>>> +fs_visitor::lower_simd_width()
>>>>>> +{
>>>>>> + bool progress = false;
>>>>>> +
>>>>>> + foreach_block_and_inst_safe(block, fs_inst, inst, cfg) {
>>>>>> + const unsigned lower_width = get_lowered_simd_width(devinfo, inst);
>>>>>> +
>>>>>> + if (lower_width != inst->exec_size) {
>>>>>> + /* Builder matching the original instruction. */
>>>>>> + const fs_builder ibld = bld.at(block, inst)
>>>>>> + .exec_all(inst->force_writemask_all)
>>>>>> + .group(inst->exec_size, inst->force_sechalf);
>>>>>> +
>>>>>> + /* Split the copies in chunks of the execution width of either the
>>>>>> + * original or the lowered instruction, whichever is lower.
>>>>>> + */
>>>>>> + const unsigned copy_width = MIN2(lower_width, inst->exec_size);
>>>>>> + const unsigned n = inst->exec_size / copy_width;
>>>>>> + const unsigned dst_size = inst->regs_written * REG_SIZE /
>>>>>> + inst->dst.component_size(inst->exec_size);
>>>>>> + fs_reg dsts[4];
>>>>>> +
>>>>>> + assert(n > 0 && n <= ARRAY_SIZE(dsts) &&
>>>>>> + !inst->writes_accumulator && !inst->mlen);
>>>>>> +
>>>>>> + for (unsigned i = 0; i < n; i++) {
>>>>>> + /* Emit a copy of the original instruction with the lowered width.
>>>>>> + * If the EOT flag was set throw it away except for the last
>>>>>> + * instruction to avoid killing the thread prematurely.
>>>>>> + */
>>>>>> + fs_inst tmp_inst = *inst;
>>>>>> + tmp_inst.exec_size = lower_width;
>>>>>> + tmp_inst.eot = inst->eot && i == n - 1;
>>>>>> +
>>>>>> + /* Set exec_all if the lowered width is higher than the original
>>>>>> + * to avoid breaking the compiler invariant that no control
>>>>>> + * flow-masked instruction is wider than the shader's
>>>>>> + * dispatch_width. Then emit the lowered instruction.
>>>
>>> I don't think this is a standard invariant. The standard invariant is
>>> "set exec_all if you're doing something that doesn't match the natural
>>> exec mask". In fact, in the gen4 texturing code (which is what this
>>> is primarily for), we leave the writemask alone.
>>
>> The gen4 texturing code would have likely caused problems with
>> optimization passes that use the builder to emit new instructions based
>> on the channel enables of already existing instructions, precisely
>> because it used to break the invariant you mention (do something with an
>> exec mask higher than the natural without force_writemask_all).
>
> What do you mean by "higher than the natural"?
I was trying to use your own words :P. The Gen4 texturing code was
using an exec mask larger than the shader's dispatch_width-wide one, so
I assume it doesn't fall into the category of instructions matching the
"natural" exec mask.
> What is an optimization pass doing with exec_size that would get
> messed up?
>
> The optimization passes and most of the compiler beyond the initial
> NIR -> FS pass shouldn't care about dispatch_width. It should only
> ever care about the exec_sizes of particular instructions. Hey, look,
> a SIMD16 instruction! No big deal. I really don't know what these
> mythical optimization problems would be.
>
The problem is that optimization passes start off with a
dispatch_width-wide fs_builder, which isn't usable to emit non-exec_all
SIMD16 instructions in a SIMD8 shader, so they're going to hit an
assertion if they accidentally try to apply some transformation or emit
new instructions based on the exec_size of the original texturing
instruction.
A (somewhat disturbing) solution would be not to hand the original
dispatch_width-wide fs_builder to optimization passes, but a, say,
SIMD32 one, so they would be able to emit non-exec_all instructions of
arbitrary exec_size, with potentially undefined results. I can do that
as PATCH 3.5 if you insist in getting rid of the exec_all() call in this
case.
>>> If you do a SIMD16 instruction in SIMD8 mode without exec_all, you
>>> get garbage in the other 8 channels but it's otherwise fine. For
>>> instructions we need to "expand", this should be fine since we're
>>> throwing away the extra channels.
>>>
>>> On the other hand, we don't know what the other half of the writemask
>>> will be (experimentation indicates that it's *not* a duplicate of the
>>> first half) so not setting exec_all doesn't really gain us anything.
>>> Unless, of course, we could maybe save some memory bandwidth in
>>> texturing instructions if some of the channels are disabled. I doubt
>>> that matters much.
>>>
>> Yeah, in any case it can only have an effect on Gen4, and it will change
>> From writing garbage nondeterministically to some channels of the second
>> half to writing garbage deterministically.
>>
>>> My inclination would be to leave the exec_all alone since it's not
>>> needed, but I guess I'm not going to fight it too hard.
>>>
>> I wouldn't feel comfortable with doing that, it's surely going lead to
>> assertion failures in the future which will only be reproducible on Gen4
>> and will therefore not be obvious for the casual contributor unless it's
>> being tested on that specifically. I guess the alternative would be to
>> relax the invariant, but it's proven to catch legitimate mistakes and
>> what the Gen4 code was doing had rather dubious semantics anyway, so I
>> doubt it's justified to change it...
>>
>>>>>> + */
>>>>>> + const fs_builder lbld = ibld.exec_all(lower_width > inst->exec_size)
>>>>>> + .group(lower_width, i);
>>>>>> + fs_inst *split_inst = lbld.emit(tmp_inst);
>>>>>
>>>>> Why are you emitting the split instruction before the source
>>>>> transposes? Don't they need to go first? Maybe I'm missing
>>>>> something.
>>>>>
>>>> I guess it doesn't really matter in which order they are inserted as
>>>> long as they end up in the correct order inside the program (and as you
>>>> can see below there is an at() call making sure that the builder used to
>>>> emit the transposes is pointing before the split instruction).
>>>>
>>>> No important reason really, other than to be able to assign the
>>>> temporaries allocated below to the split instruction sources directly.
>>>
>>> That's fine. It would be nice if you left a comment to that effect.
>>> Otherwise, it looks like you're emitting the send and then the moves.
>>> It's really easy when reading the code to miss the at() call below.
>>>
>> Sure. It would also be easy to swap the order of the transposes and the
>> split_inst emission if you find it easier to understand that way. I'll
>> just go do that instead of the comment if you have no objection.
>>
>>>>>> +
>>>>>> + for (unsigned j = 0; j < inst->sources; j++) {
>>>>>> + if (inst->src[j].file != BAD_FILE &&
>>>>>> + !is_uniform(inst->src[j])) {
>>>>>> + /* Get the i-th copy_width-wide chunk of the source. */
>>>>>> + const fs_reg src = horiz_offset(inst->src[j], copy_width * i);
>>>>>> + const unsigned src_size = inst->regs_read(j) * REG_SIZE /
>>>>>> + inst->src[j].component_size(inst->exec_size);
>>>>>> +
>>>>>> + /* Use a trivial transposition to copy one every n
>>>>>> + * copy_width-wide components of the register into a
>>>>>> + * temporary passed as source to the lowered instruction.
>>>>>> + */
>>>>>> + split_inst->src[j] = lbld.vgrf(inst->src[j].type, src_size);
>>>>>> + emit_transpose(lbld.group(copy_width, 0).at(block, split_inst),
>>>>>> + split_inst->src[j], &src, 1, src_size, n);
>>>>>
>>>>> Shouldn't this be group(copy_width, i)?
>>>>>
>>>>
>>>> I'm already choosing the i-th channel group in the definition of lbld
>>>> while emitting the split instruction, this group() call is just to make
>>>> sure that the transposes don't copy data into the garbage channels of
>>>> the temporary in cases where copy_width is less than lower_width.
>>>
>>> Right. That makes sense.
>>>
>>>>>> + }
>>>>>> + }
>>>>>> +
>>>>>> + if (inst->regs_written) {
>>>>>> + /* Allocate enough space to hold the result of the lowered
>>>>>> + * instruction and fix up the number of registers written.
>>>>>> + */
>>>>>> + split_inst->dst = dsts[i] =
>>>>>> + lbld.vgrf(inst->dst.type, dst_size);
>>>>>> + split_inst->regs_written =
>>>>>> + DIV_ROUND_UP(inst->regs_written * lower_width,
>>>>>> + inst->exec_size);
>>>>>> + }
>>>>>> + }
>>>>>> +
>>>>>> + if (inst->regs_written) {
>>>>>> + /* Distance between useful channels in the temporaries, skipping
>>>>>> + * garbage if the lowered instruction is wider than the original.
>>>>>> + */
>>>>>> + const unsigned m = lower_width / copy_width;
>>>>>> +
>>>>>> + /* Interleave the components of the result from the lowered
>>>>>> + * instructions. We need to set exec_all() when copying more than
>>>>>> + * one half per component, because LOAD_PAYLOAD (in terms of which
>>>>>> + * emit_transpose is implemented) can only use the same channel
>>>>>> + * enable signals for all of its non-header sources.
>>>>>> + */
>>>>>> + emit_transpose(ibld.exec_all(inst->exec_size > copy_width)
>>>>>> + .group(copy_width, 0),
>>>>>> + inst->dst, dsts, n, dst_size, m);
>>>
>>> While we're on the exec_all topic, I don't think it's needed here
>>> either. In the case where exec_size > copy_width, we're throwing away
>>> the second half. Therefore, the channels with the wrong exec mask
>>> will either not get emitted at all or dead-code will delete them (I'm
>>> not sure which without thinking about it harder). In either case, the
>>> MOV's that do matter will have the right exec mask and exec_all
>>> doesn't gain us anything.
>>
>> Nope, the "exec_size > copy_width" case is the usual (non-gen4) case.
>> Nothing can be thrown away in that case because both halves have to be
>> copied interleaved into the destination register, so whatever execution
>> mask LOAD_PAYLOAD uses it has to work for both halves, which is only
>> possible if force_writemask_all is set.
>
> Right... Not a huge fan of exec_all'd LOAD_PAYLOAD, but we don't have
> a ZIP instruction so I won't complain too much.
>
>>>>>> + }
>>>>>> +
>>>>>> + inst->remove(block);
>>>>>> + progress = true;
>>>>>> + }
>>>>>> + }
>>>>>> +
>>>>>> + if (progress)
>>>>>> + invalidate_live_intervals();
>>>>>> +
>>>>>> + return progress;
>>>>>> +}
>>>>>> +
>>>>>> void
>>>>>> fs_visitor::dump_instructions()
>>>>>> {
>>>>>> @@ -3655,6 +3796,7 @@ fs_visitor::optimize()
>>>>>> int iteration = 0;
>>>>>> int pass_num = 0;
>>>>>>
>>>>>> + OPT(lower_simd_width);
>>>>>> OPT(lower_logical_sends);
>>>>>>
>>>>>> do {
>>>>>> diff --git a/src/mesa/drivers/dri/i965/brw_fs.h b/src/mesa/drivers/dri/i965/brw_fs.h
>>>>>> index f3850d1..9582648 100644
>>>>>> --- a/src/mesa/drivers/dri/i965/brw_fs.h
>>>>>> +++ b/src/mesa/drivers/dri/i965/brw_fs.h
>>>>>> @@ -184,6 +184,7 @@ public:
>>>>>> bool lower_load_payload();
>>>>>> bool lower_logical_sends();
>>>>>> bool lower_integer_multiplication();
>>>>>> + bool lower_simd_width();
>>>>>> bool opt_combine_constants();
>>>>>>
>>>>>> void emit_dummy_fs();
>>>>>> --
>>>>>> 2.4.3
>>>>>>
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