agx_optimizer.c

/*
 * Copyright (C) 2021 Alyssa Rosenzweig <[email protected]>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#include "agx_compiler.h"
#include "agx_minifloat.h"

/* AGX peephole optimizer responsible for instruction combining. It operates in
 * a forward direction and a backward direction, in each case traversing in
 * source order. SSA means the forward pass satisfies the invariant:
 *
 *    Every def is visited before any of its uses.
 *
 * Dually, the backend pass satisfies the invariant:
 *
 *    Every use of a def is visited before the def.
 *
 * This means the forward pass can propagate modifiers forward, whereas the
 * backwards pass propagates modifiers backward. Consider an example:
 *
 *    1 = fabs 0
 *    2 = fround 1
 *    3 = fsat 1
 *
 * The forwards pass would propagate the fabs to the fround (since we can
 * lookup the fabs from the fround source and do the replacement). By contrast
 * the backwards pass would propagate the fsat back to the fround (since when
 * we see the fround we know it has only a single user, fsat).  Propagatable
 * instruction have natural directions (like pushforwards and pullbacks).
 *
 * We are careful to update the tracked state whenever we modify an instruction
 * to ensure the passes are linear-time and converge in a single iteration.
 *
 * Size conversions are worth special discussion. Consider the snippet:
 *
 *    2 = fadd 0, 1
 *    3 = f2f16 2
 *    4 = fround 3
 *
 * A priori, we can move the f2f16 in either direction. But it's not equal --
 * if we move it up to the fadd, we get FP16 for two instructions, whereas if
 * we push it into the fround, we effectively get FP32 for two instructions. So
 * f2f16 is backwards. Likewise, consider
 *
 *    2 = fadd 0, 1
 *    3 = f2f32 1
 *    4 = fround 3
 *
 * This time if we move f2f32 up to the fadd, we get FP32 for two, but if we
 * move it down to the fround, we get FP16 to too. So f2f32 is backwards.
 */

static bool
agx_is_fmov(agx_instr *def)
{
   return (def->op == AGX_OPCODE_FADD)
      && agx_is_equiv(def->src[1], agx_negzero());
}

/* Compose floating-point modifiers with floating-point sources */

static agx_index
agx_compose_float_src(agx_index to, agx_index from)
{
   if (to.abs)
      from.neg = false;

   from.abs |= to.abs;
   from.neg |= to.neg;

   return from;
}

static void
agx_optimizer_fmov(agx_instr **defs, agx_instr *ins, unsigned srcs)
{
   for (unsigned s = 0; s < srcs; ++s) {
      agx_index src = ins->src[s];
      if (src.type != AGX_INDEX_NORMAL) continue;
      
      agx_instr *def = defs[src.value];
      if (!agx_is_fmov(def)) continue;
      if (def->saturate) continue;

      ins->src[s] = agx_compose_float_src(src, def->src[0]);
   }
}

static void
agx_optimizer_inline_imm(agx_instr **defs, agx_instr *I,
      unsigned srcs, bool is_float)
{
   for (unsigned s = 0; s < srcs; ++s) {
      agx_index src = I->src[s];
      if (src.type != AGX_INDEX_NORMAL) continue;

      agx_instr *def = defs[src.value];
      if (def->op != AGX_OPCODE_MOV_IMM) continue;

      uint8_t value = def->imm;
      bool float_src = is_float;

      /* cmpselsrc takes integer immediates only */
      if (s >= 2 && I->op == AGX_OPCODE_FCMPSEL) float_src = false;

      if (float_src) {
         bool fp16 = (def->dest[0].size == AGX_SIZE_16);
         assert(fp16 || (def->dest[0].size == AGX_SIZE_32));

         float f = fp16 ? _mesa_half_to_float(def->imm) : uif(def->imm);
         if (!agx_minifloat_exact(f)) continue;

         value = agx_minifloat_encode(f);
      } else if (value != def->imm) {
         continue;
      }

      I->src[s].type = AGX_INDEX_IMMEDIATE;
      I->src[s].value = value;
   }
}

static bool
agx_optimizer_fmov_rev(agx_instr *I, agx_instr *use)
{
   if (!agx_is_fmov(use)) return false;
   if (use->src[0].neg || use->src[0].abs) return false;

   /* saturate(saturate(x)) = saturate(x) */
   I->saturate |= use->saturate;
   I->dest[0] = use->dest[0];
   return true;
}

static void
agx_optimizer_forward(agx_context *ctx)
{
   agx_instr **defs = calloc(ctx->alloc, sizeof(*defs));

   agx_foreach_instr_global(ctx, I) {
      struct agx_opcode_info info = agx_opcodes_info[I->op];

      for (unsigned d = 0; d < info.nr_dests; ++d) {
         if (I->dest[d].type == AGX_INDEX_NORMAL)
            defs[I->dest[d].value] = I;
      }

      /* Propagate fmov down */
      if (info.is_float)
         agx_optimizer_fmov(defs, I, info.nr_srcs);

      /* Inline immediates if we can. TODO: systematic */
      if (I->op != AGX_OPCODE_ST_VARY && I->op != AGX_OPCODE_ST_TILE && I->op != AGX_OPCODE_P_EXTRACT && I->op != AGX_OPCODE_P_COMBINE)
         agx_optimizer_inline_imm(defs, I, info.nr_srcs, info.is_float);
   }

   free(defs);
}

static void
agx_optimizer_backward(agx_context *ctx)
{
   agx_instr **uses = calloc(ctx->alloc, sizeof(*uses));
   BITSET_WORD *multiple = calloc(BITSET_WORDS(ctx->alloc), sizeof(*multiple));

   agx_foreach_instr_global_rev(ctx, I) {
      struct agx_opcode_info info = agx_opcodes_info[I->op];

      for (unsigned s = 0; s < info.nr_srcs; ++s) {
         if (I->src[s].type == AGX_INDEX_NORMAL) {
            unsigned v = I->src[s].value;

            if (uses[v])
               BITSET_SET(multiple, v);
            else
               uses[v] = I;
         }
      }

      if (info.nr_dests != 1)
         continue;

      if (I->dest[0].type != AGX_INDEX_NORMAL)
         continue;

      agx_instr *use = uses[I->dest[0].value];

      if (!use || BITSET_TEST(multiple, I->dest[0].value))
         continue;

      /* Destination has a single use, try to propagate */
      if (info.is_float && agx_optimizer_fmov_rev(I, use)) {
         agx_remove_instruction(use);
         continue;
      }
   }

   free(uses);
   free(multiple);
}

void
agx_optimizer(agx_context *ctx)
{
   agx_optimizer_backward(ctx);
   agx_optimizer_forward(ctx);
}