freedreno/ir3: Add a NIR pass to select tex instructions eligible for pre-fetch

The pass should run once at the end of shader compilation, for a4xx
onwards. It iterates texture sampling instructions and mark those
eligibile for pre-dispatch by changing the tex op from 'tex' to
'tex_prefetch'. An instruction is eligibile if:

* The coordinate is a vector where all its components come from a
  shader input.
* The order of the components match exactly that of the input (no
  swizzles).
* The instruction is in the 'main' function, and in the outer
  most-block.

The first two restrictions were arrived to empirically, so more
testing could tighten or loosen it.

The 3rd restriction is there to allow moving the instructions
eligible for pre-dispatch to the beginning of the shader, so
that we don't block the registers holding the result for too
long.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>

src/freedreno/ir3/ir3_nir.h

@@ -40,6 +40,9 @@ bool ir3_nir_lower_io_offsets(nir_shader *shader);
 bool ir3_nir_lower_load_barycentric_at_sample(nir_shader *shader);
 bool ir3_nir_lower_load_barycentric_at_offset(nir_shader *shader);
 bool ir3_nir_move_varying_inputs(nir_shader *shader);
+int ir3_nir_coord_offset(nir_ssa_def *ssa);
+bool ir3_nir_lower_tex_prefetch(nir_shader *shader);
+
 
 void ir3_nir_lower_vs_to_explicit_io(nir_shader *shader, struct ir3_shader *s);
 void ir3_nir_lower_gs(nir_shader *shader, struct ir3_shader *s);

src/freedreno/ir3/ir3_nir_lower_tex_prefetch.c

@@ -0,0 +1,195 @@
+/*
+ * Copyright © 2019 Igalia S.L.
+ *
+ * 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 "ir3_nir.h"
+
+/**
+ * A pass which detects tex instructions which are candidate to be executed
+ * prior to FS shader start, and change them to nir_texop_tex_prefetch.
+ */
+
+static int
+coord_offset(nir_ssa_def *ssa)
+{
+	nir_instr *parent_instr = ssa->parent_instr;
+
+	/* The coordinate of a texture sampling instruction eligible for
+	 * pre-fetch is either going to be a load_interpolated_input/
+	 * load_input, or a vec2 assembling non-swizzled components of
+	 * a load_interpolated_input/load_input (due to varying packing)
+	 */
+
+	if (parent_instr->type == nir_instr_type_alu) {
+		nir_alu_instr *alu = nir_instr_as_alu(parent_instr);
+
+		if (alu->op != nir_op_vec2)
+			return -1;
+
+		for (int i = 0; i < 2; i++) {
+			if (!alu->src[i].src.is_ssa)
+				return -1;
+
+			if (alu->src[i].swizzle[0] != (alu->src[0].swizzle[0] + i))
+				return -1;
+
+			if (alu->src[i].src.ssa != alu->src[0].src.ssa)
+				return -1;
+		}
+
+		int off = coord_offset(alu->src[0].src.ssa);
+		if (off < 0)
+			return -1;
+
+		return off + alu->src[0].swizzle[0];
+	}
+
+	if (parent_instr->type != nir_instr_type_intrinsic)
+		return -1;
+
+	nir_intrinsic_instr *input = nir_instr_as_intrinsic(parent_instr);
+
+	if (input->intrinsic != nir_intrinsic_load_interpolated_input)
+		return -1;
+
+	/* limit to load_barycentric_pixel, other interpolation modes don't seem
+	 * to be supported:
+	 */
+	if (!input->src[0].is_ssa)
+		return -1;
+
+	nir_intrinsic_instr *interp =
+		nir_instr_as_intrinsic(input->src[0].ssa->parent_instr);
+
+	if (interp->intrinsic != nir_intrinsic_load_barycentric_pixel)
+		return -1;
+
+	/* we also need a const input offset: */
+	if (!nir_src_is_const(input->src[1]))
+		return -1;
+
+	unsigned base = nir_src_as_uint(input->src[1]) + nir_intrinsic_base(input);
+	unsigned comp = nir_intrinsic_component(input);
+
+	return (4 * base) + comp;
+}
+
+int
+ir3_nir_coord_offset(nir_ssa_def *ssa)
+{
+	/* only prefetch for simple 2d tex fetch case.  Note this check only
+	 * applies to the tex coord src itself, and not in the case where
+	 * we recursively chase a vecN's src.
+	 */
+	if (ssa->num_components != 2)
+		return -1;
+
+	return coord_offset(ssa);
+}
+
+static bool
+has_src(nir_tex_instr *tex, nir_tex_src_type type)
+{
+	return nir_tex_instr_src_index(tex, type) > 0;
+}
+
+static bool
+lower_tex_prefetch_block(nir_block *block)
+{
+	bool progress = false;
+
+	nir_foreach_instr_safe(instr, block) {
+		if (instr->type != nir_instr_type_tex)
+			continue;
+
+		nir_tex_instr *tex = nir_instr_as_tex(instr);
+		if (tex->op != nir_texop_tex)
+			continue;
+
+		if (has_src(tex, nir_tex_src_bias) ||
+				has_src(tex, nir_tex_src_lod) ||
+				has_src(tex, nir_tex_src_comparator) ||
+				has_src(tex, nir_tex_src_projector) ||
+				has_src(tex, nir_tex_src_offset) ||
+				has_src(tex, nir_tex_src_ddx) ||
+				has_src(tex, nir_tex_src_ddy) ||
+				has_src(tex, nir_tex_src_ms_index) ||
+				has_src(tex, nir_tex_src_texture_offset) ||
+				has_src(tex, nir_tex_src_sampler_offset))
+			continue;
+
+		int idx = nir_tex_instr_src_index(tex, nir_tex_src_coord);
+		/* First source should be the sampling coordinate. */
+		nir_tex_src *coord = &tex->src[idx];
+		debug_assert(coord->src.is_ssa);
+
+		if (ir3_nir_coord_offset(coord->src.ssa) >= 0) {
+			tex->op = nir_texop_tex_prefetch;
+
+			progress |= true;
+		}
+	}
+
+	return progress;
+}
+
+static bool
+lower_tex_prefetch_func(nir_function_impl *impl)
+{
+	/* Only instructions in the the outer-most block are considered
+	 * eligible for pre-dispatch, because they need to be move-able
+	 * to the beginning of the shader to avoid locking down the
+	 * register holding the pre-fetched result for too long.
+	 */
+	nir_block *block = nir_start_block(impl);
+	if (!block)
+		return false;
+
+	bool progress = lower_tex_prefetch_block(block);
+
+	if (progress) {
+		nir_metadata_preserve(impl, nir_metadata_block_index |
+				nir_metadata_dominance);
+	}
+
+	return progress;
+}
+
+bool
+ir3_nir_lower_tex_prefetch(nir_shader *shader)
+{
+	bool progress = false;
+
+	assert(shader->info.stage == MESA_SHADER_FRAGMENT);
+
+	nir_foreach_function(function, shader) {
+		/* Only texture sampling instructions inside the main function
+		 * are eligible for pre-dispatch.
+		 */
+		if (!function->impl || !function->is_entrypoint)
+			continue;
+
+		progress |= lower_tex_prefetch_func(function->impl);
+	}
+
+	return progress;
+}

src/freedreno/ir3/meson.build

@@ -67,6 +67,7 @@ libfreedreno_ir3_files = files(
   'ir3_nir_lower_load_barycentric_at_offset.c',
   'ir3_nir_lower_io_offsets.c',
   'ir3_nir_lower_tess.c',
+  'ir3_nir_lower_tex_prefetch.c',
   'ir3_nir_lower_tg4_to_tex.c',
   'ir3_nir_move_varying_inputs.c',
   'ir3_print.c',