nir: add a pass to optimize "gl_FragDepth = gl_FragCoord.z" away

gl_FragDepth default value is gl_FragCoord.z so if a shader does:

   gl_FragDepth = gl_FragCoord.z

we can drop this assignment.

v2: use nir_ssa_scalar_resolved and don't do this is gl_FragDepth
    is wrote multiple times (Jason)
v3: - move to its own pass (Jason)
    - handle var = NULL (Rhys)
v4: refactoring (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10697>

src/compiler/nir/meson.build

@@ -223,6 +223,7 @@ files_libnir = files(
   'nir_opt_dead_cf.c',
   'nir_opt_dead_write_vars.c',
   'nir_opt_find_array_copies.c',
+  'nir_opt_fragdepth.c',
   'nir_opt_gcm.c',
   'nir_opt_idiv_const.c',
   'nir_opt_if.c',

src/compiler/nir/nir.h

@@ -5444,6 +5444,8 @@ bool nir_opt_deref(nir_shader *shader);
 
 bool nir_opt_find_array_copies(nir_shader *shader);
 
+bool nir_opt_fragdepth(nir_shader *shader);
+
 bool nir_opt_gcm(nir_shader *shader, bool value_number);
 
 bool nir_opt_idiv_const(nir_shader *shader, unsigned min_bit_size);

src/compiler/nir/nir_opt_fragdepth.c

@@ -0,0 +1,111 @@
+/*
+ * Copyright © 2021 Advanced Micro Devices, Inc.
+ *
+ * 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 "nir.h"
+#include "nir_builder.h"
+
+/**
+ * This pass removes no-op assignment to gl_FragDepth.
+ *
+ * gl_FragDepth implicit value is gl_FragCoord.z, so if a shader only assign
+ * this value to gl_FragDepth, the store instruction is removed.
+ */
+
+static bool
+ssa_def_is_source_depth(nir_ssa_def *def)
+{
+   nir_ssa_scalar scalar = nir_ssa_scalar_resolved(def, 0);
+   nir_instr *instr = scalar.def->parent_instr;
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+   if (intrin->intrinsic != nir_intrinsic_load_frag_coord)
+      return false;
+
+   /* Depth is gl_FragCoord.z */
+   return scalar.comp == 2;
+}
+
+bool
+nir_opt_fragdepth(nir_shader *shader)
+{
+   bool progress = false;
+   nir_intrinsic_instr *store_intrin = NULL;
+
+   if (shader->info.stage != MESA_SHADER_FRAGMENT)
+      goto end;
+
+   nir_function_impl *impl = nir_shader_get_entrypoint(shader);
+   nir_foreach_block(block, impl) {
+      nir_foreach_instr(instr, block) {
+         if (instr->type != nir_instr_type_intrinsic)
+            continue;
+
+         nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+         if (intrin->intrinsic != nir_intrinsic_store_deref)
+            continue;
+
+         nir_deref_instr *deref = nir_src_as_deref(intrin->src[0]);
+         if (!nir_deref_mode_is(deref, nir_var_shader_out))
+            continue;
+
+         nir_variable *var = nir_deref_instr_get_variable(deref);
+         if (var->data.location != FRAG_RESULT_DEPTH)
+            continue;
+
+         /* We found a write to gl_FragDepth */
+         if (store_intrin) {
+            /* This isn't the only write: give up on this optimization */
+            goto end;
+         } else {
+            if (ssa_def_is_source_depth(intrin->src[1].ssa)) {
+               /* We're writing gl_FragCoord.z in gl_FragDepth: remember
+                * intrin so we can try to remove it later. */
+               store_intrin = intrin;
+            } else {
+               /* We're writing something else: give up. */
+               goto end;
+            }
+         }
+      }
+   }
+
+   if (store_intrin) {
+      /* Found a single store to gl_FragDepth, and it writes gl_FragCoord.z to it.
+       * Remove it since that's the implicit value of gl_FragDepth.
+       */
+      nir_instr_remove(&store_intrin->instr);
+
+      nir_metadata_preserve(impl, nir_metadata_block_index |
+                                  nir_metadata_dominance |
+                                  nir_metadata_loop_analysis |
+                                  nir_metadata_instr_index);
+      progress = true;
+   }
+
+end:
+   if (!progress)
+      nir_shader_preserve_all_metadata(shader);
+   return progress;
+}

src/mesa/state_tracker/st_glsl_to_nir.cpp

@@ -530,6 +530,7 @@ st_glsl_to_nir_post_opts(struct st_context *st, struct gl_program *prog,
       NIR_PASS_V(nir, gl_nir_lower_atomics, shader_program, true);
 
    NIR_PASS_V(nir, nir_opt_intrinsics);
+   NIR_PASS_V(nir, nir_opt_fragdepth);
 
    /* Lower 64-bit ops. */
    if (nir->options->lower_int64_options ||