[Mesa-dev] [PATCH 3/5] i965/fs: New peephole optimization to generate SEL.

[Paul Berry]

On 4 November 2013 10:31, Eric Anholt <eric at anholt.net> wrote:

> Matt Turner <mattst88 at gmail.com> writes:
> > diff --git a/src/mesa/drivers/dri/i965/brw_fs_sel_peephole.cpp
> b/src/mesa/drivers/dri/i965/brw_fs_sel_peephole.cpp
> > new file mode 100644
> > index 0000000..9626751
> > --- /dev/null
> > +++ b/src/mesa/drivers/dri/i965/brw_fs_sel_peephole.cpp
>
> > +bool
> > +fs_visitor::opt_peephole_sel()
> > +{
> > +   bool progress = false;
> > +
> > +   cfg_t cfg(this);
> > +
> > +   for (int b = 0; b < cfg.num_blocks; b++) {
> > +      bblock_t *block = cfg.blocks[b];
> > +
> > +      /* IF instructions, by definition, can only be found at the ends
> of
> > +       * basic blocks.
> > +       */
> > +      fs_inst *if_inst = (fs_inst *) block->end;
> > +      if (if_inst->opcode != BRW_OPCODE_IF)
> > +         continue;
> > +
> > +      if (!block->else_inst)
> > +         continue;
> > +
> > +      fs_inst *else_inst = (fs_inst *) block->else_inst;
> > +
> > +      fs_inst *else_mov[MAX_MOVS] = { NULL };
> > +      fs_inst *then_mov[MAX_MOVS] = { NULL };
> > +      bool malformed_mov_found = false;
> > +
> > +      int movs = count_movs_from_if(then_mov, else_mov, if_inst,
> else_inst);
> > +
> > +      if (movs == 0)
> > +         continue;
> > +
> > +      fs_inst *sel_inst[MAX_MOVS] = { NULL };
> > +      fs_inst *mov_imm_inst[MAX_MOVS] = { NULL };
> > +
> > +      /* Generate SEL instructions for pairs of MOVs to a common
> destination. */
> > +      for (int i = 0; i < movs; i++) {
> > +         if (!then_mov[i] || !else_mov[i])
> > +            break;
> > +
> > +         /* Check that the MOVs are the right form. */
> > +         if (!then_mov[i]->dst.equals(else_mov[i]->dst) ||
> > +             then_mov[i]->is_partial_write() ||
> > +             else_mov[i]->is_partial_write()) {
> > +            malformed_mov_found = true;
> > +            break;
> > +         }
> > +
> > +         /* Only the last source register can be a constant, so if the
> MOV in
> > +          * the "then" clause uses a constant, we need to put it in a
> > +          * temporary.
> > +          */
> > +         fs_reg src0(then_mov[i]->src[0]);
> > +         if (src0.file == IMM) {
> > +            src0 = fs_reg(this, glsl_type::float_type);
> > +            src0.type = then_mov[i]->src[0].type;
> > +            mov_imm_inst[i] = MOV(src0, then_mov[i]->src[0]);
> > +         }
> > +
> > +         sel_inst[i] = SEL(then_mov[i]->dst, src0, else_mov[i]->src[0]);
> > +
> > +         if (brw->gen == 6 && if_inst->conditional_mod) {
> > +            /* For Sandybridge with IF with embedded comparison */
> > +            sel_inst[i]->predicate = BRW_PREDICATE_NORMAL;
> > +         } else {
> > +            /* Separate CMP and IF instructions */
> > +            sel_inst[i]->predicate = if_inst->predicate;
> > +            sel_inst[i]->predicate_inverse = if_inst->predicate_inverse;
> > +         }
> > +      }
> > +
> > +      if (malformed_mov_found)
> > +         continue;
> > +
> > +      /* Emit a CMP if our IF used the embedded comparison */
> > +      if (brw->gen == 6 && if_inst->conditional_mod) {
> > +         fs_inst *cmp_inst = CMP(reg_null_d, if_inst->src[0],
> if_inst->src[1],
> > +                                 if_inst->conditional_mod);
> > +         if_inst->insert_before(cmp_inst);
> > +      }
>
> If the IF is using embedded comparison, then our MOVs that we've moved
> before it might be updating the registers used in the embedded
> comparsion.
>

I think this is ok, because we don't move any MOVs before we get to this
code.  We just set up the sel_inst and mov_imm_inst arrays.  The MOVs don't
get added to the instruction stream until after this code, so they appear
after the CMP, and stuff should still happen in the correct order.


>
> I'd be fine with either adding it to the malformed mov check, or just
> dropping support for embedded comparisons.
>
> _______________________________________________
> mesa-dev mailing list
> mesa-dev at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/mesa-dev/attachments/20131104/a8bbf014/attachment.html>