[Mesa-dev] [PATCH 13/25] i965/fs: Skip SIMD lowering destination zipping if possible.

[Francisco Jerez]

Jason Ekstrand <jason at jlekstrand.net> writes:

> On Fri, May 27, 2016 at 7:05 PM, Francisco Jerez <currojerez at riseup.net>
> wrote:
>
>> Skipping the temporary allocation and copy instructions is easy (just
>> return dst), but the conditions used to find out whether the copy can
>> be optimized out safely without breaking the program are rather
>> complex: The destination must be exactly one component of at most the
>> execution width of the lowered instruction, and all source regions of
>> the instruction must be either fully disjoint from the destination or
>> be aligned with it group by group.
>> ---
>>  src/mesa/drivers/dri/i965/brw_fs.cpp | 81
>> ++++++++++++++++++++++++++++++++++++
>>  1 file changed, 81 insertions(+)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp
>> b/src/mesa/drivers/dri/i965/brw_fs.cpp
>> index 8eff27e..5d26c68 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
>> @@ -5054,6 +5054,78 @@ emit_unzip(const fs_builder &lbld, bblock_t *block,
>> fs_inst *inst,
>>  }
>>
>>  /**
>> + * Return true if splitting out the group of channels of instruction \p
>> inst
>> + * given by lbld.group() requires allocating a temporary for the
>> destination
>> + * of the lowered instruction and copying the data back to the original
>> + * destination region.
>> + *
>> + * Note that the result of this function may be different for different
>> + * channel groups in cases where the overlap between a source and
>> destination
>> + * region is partial.
>> + */
>> +static inline bool
>> +needs_dst_copy(const fs_builder &lbld, const fs_inst *inst)
>> +{
>> +   /* If the instruction writes more than one component we'll have to
>> shuffle
>> +    * the results of multiple lowered instructions in order to make sure
>> that
>> +    * they end up arranged correctly in the original destination region.
>> +    */
>> +   if (inst->regs_written * REG_SIZE >
>> +       inst->dst.component_size(inst->exec_size))
>> +      return true;
>> +
>> +   /* If the lowered execution size is larger than the original the
>> result of
>> +    * the instruction won't fit in the original destination, so we'll
>> have to
>> +    * allocate a temporary in any case.
>> +    */
>> +   if (lbld.dispatch_width() > inst->exec_size)
>> +      return true;
>> +
>> +   for (unsigned i = 0; i < inst->sources; i++) {
>> +      /* Index of this lowered instruction and total number of lowered
>> +       * instructions.
>> +       */
>> +      const unsigned j = lbld.group() / lbld.dispatch_width();
>> +      const unsigned n = DIV_ROUND_UP(inst->exec_size,
>> lbld.dispatch_width());
>> +
>> +      /* Specified channels from the source and destination regions. */
>> +      const fs_reg src = horiz_offset(inst->src[i], lbld.group());
>> +      const fs_reg dst = horiz_offset(inst->dst, lbld.group());
>> +
>> +      /* Lowered component size of the source and destination regions. */
>> +      const unsigned dsrc =
>> inst->src[i].component_size(lbld.dispatch_width());
>> +      const unsigned ddst =
>> inst->dst.component_size(lbld.dispatch_width());
>> +
>> +      /* If we already made a copy of the source for other reasons there
>> won't
>> +       * be any overlap with the destination.
>> +       */
>> +      if (!is_periodic(inst->src[i], lbld.dispatch_width()) &&
>> +          inst->components_read(i) != 1)
>>
>
> might be good to have a needs_src_copy function to keep the logic in one
> place.
>
Sure, I'll fix that.

>
>> +         continue;
>> +
>> +      /* We need a copy if the specified group of channels of the
>> destination
>> +       * overlaps either the previous or subsequent groups of the
>> source.  We
>> +       * don't care whether the destination of a given lowered instruction
>> +       * overlaps its own source because there's arguably no
>> +       * source/destination hazard for this opcode if the source and
>> +       * destination of the unlowered instruction were already
>> overlapping.
>> +       *
>> +       * Note that it might be possible to avoid emitting a copy in more
>> cases
>> +       * by making assumptions about the ordering in which the lowered
>> +       * instructions will be emitted (ugh!), but this has the advantage
>> that
>> +       * the lowered instructions will be fully independent from each
>> other
>> +       * regardless of the form of overlap so they can be scheduled
>> freely.
>> +       */
>> +      if (regions_overlap(dst, ddst, inst->src[i], dsrc * j) ||
>> +          regions_overlap(dst, ddst, horiz_offset(src,
>> lbld.dispatch_width()),
>> +                           dsrc * (n - j - 1)))
>> +         return true;
>>
>
> Wow, this is annoyingly complicated.  It also appears to be correct.  Would
> it be sufficient to simply use the following?
>
> if (regions_overlap(inst->dst, inst->regs_written * REG_SIZE, inst->src[i],
> inst->regs_read() * REG_SIZE) &&
>     !inst->dst.equals(inst->src[i]))
>    return true;
>
> It's much simpler and covers the two cases we really care about.  If that's
> not sufficient, what you have above does look correct.
>
Yeah, that sounds like a reasonable approximation to me, but I've seen
cases of partial overlap in some shaders where the current logic would
emit less instructions [e.g. when FP64 code does something like 'mov
tmp:df, tmp:f' the logic above will emit a copy for the first half of
the instruction *only* :)], I need to make sure that they don't regress
From switching to the approximation you suggest.

> --Jason
>

Thanks.

>
>> +   }
>> +
>> +   return false;
>> +}
>> +
>> +/**
>>   * Insert data from a packed temporary into the channel group given by
>>   * lbld.group() of the destination region of instruction \p inst and
>> return
>>   * the temporary as result.  If any copy instructions are required they
>> will
>> @@ -5073,6 +5145,8 @@ emit_zip(const fs_builder &lbld, bblock_t *block,
>> fs_inst *inst)
>>     const fs_reg dst = horiz_offset(inst->dst, lbld.group());
>>     const unsigned dst_size = inst->regs_written * REG_SIZE /
>>              inst->dst.component_size(inst->exec_size);
>> +
>> +   if (needs_dst_copy(lbld, inst)) {
>>     const fs_reg tmp = lbld.vgrf(inst->dst.type, dst_size);
>>
>>     if (inst->predicate) {
>> @@ -5090,6 +5164,13 @@ emit_zip(const fs_builder &lbld, bblock_t *block,
>> fs_inst *inst)
>>            .MOV(offset(dst, inst->exec_size, k), offset(tmp, lbld, k));
>>
>>     return tmp;
>> +
>> +   } else {
>> +      /* No need to allocate a temporary for the lowered instruction, just
>> +       * take the right group of channels from the original region.
>> +       */
>> +      return dst;
>> +   }
>>  }
>>
>>  bool
>> --
>> 2.7.3
>>
>> _______________________________________________
>> mesa-dev mailing list
>> mesa-dev at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>>
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