nir: optimize nir_lower_discard_to_demote to lower discard/demote both ways

This is smarter and also lowers demote to discard if helper invocations are
not needed.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7586>

src/amd/vulkan/radv_shader.c

@@ -535,8 +535,8 @@ radv_shader_compile_to_nir(struct radv_device *device,
 
 		NIR_PASS_V(nir, nir_lower_clip_cull_distance_arrays);
 
-		if (device->instance->debug_flags & RADV_DEBUG_DISCARD_TO_DEMOTE)
-			NIR_PASS_V(nir, nir_lower_discard_to_demote);
+		NIR_PASS_V(nir, nir_lower_discard_or_demote,
+			   device->instance->debug_flags & RADV_DEBUG_DISCARD_TO_DEMOTE);
 
 		nir_lower_doubles_options lower_doubles =
 			nir->options->lower_doubles_options;

src/compiler/Makefile.sources

@@ -253,7 +253,7 @@ NIR_FILES = \
 	nir/nir_lower_clip_halfz.c \
 	nir/nir_lower_convert_alu_types.c \
 	nir/nir_lower_variable_initializers.c \
-	nir/nir_lower_discard_to_demote.c \
+	nir/nir_lower_discard_or_demote.c \
 	nir/nir_lower_double_ops.c \
 	nir/nir_lower_drawpixels.c \
 	nir/nir_lower_fb_read.c \

src/compiler/nir/meson.build

@@ -132,7 +132,7 @@ files_libnir = files(
   'nir_lower_clip_halfz.c',
   'nir_lower_convert_alu_types.c',
   'nir_lower_variable_initializers.c',
-  'nir_lower_discard_to_demote.c',
+  'nir_lower_discard_or_demote.c',
   'nir_lower_double_ops.c',
   'nir_lower_drawpixels.c',
   'nir_lower_fb_read.c',

src/compiler/nir/nir.h

@@ -4980,7 +4980,8 @@ typedef enum {
 bool nir_lower_interpolation(nir_shader *shader,
                              nir_lower_interpolation_options options);
 
-bool nir_lower_discard_to_demote(nir_shader *shader);
+bool nir_lower_discard_or_demote(nir_shader *shader,
+                                 bool force_correct_quad_ops_after_discard);
 
 bool nir_lower_memory_model(nir_shader *shader);
 

src/compiler/nir/nir_lower_discard_or_demote.c

@@ -0,0 +1,127 @@
+/*
+ * Copyright © 2020 Valve Corporation
+ *
+ * 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"
+
+static bool
+nir_lower_discard_to_demote_instr(nir_builder *b, nir_instr *instr, void *data)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+   switch (intrin->intrinsic) {
+   case nir_intrinsic_discard:
+      intrin->intrinsic = nir_intrinsic_demote;
+      return true;
+   case nir_intrinsic_discard_if:
+      intrin->intrinsic = nir_intrinsic_demote_if;
+      return true;
+   case nir_intrinsic_load_helper_invocation:
+      intrin->intrinsic = nir_intrinsic_is_helper_invocation;
+      return true;
+   default:
+      return false;
+   }
+}
+
+static bool
+nir_lower_demote_to_discard_instr(nir_builder *b, nir_instr *instr, void *data)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+   switch (intrin->intrinsic) {
+   case nir_intrinsic_demote:
+      intrin->intrinsic = nir_intrinsic_discard;
+      return true;
+   case nir_intrinsic_demote_if:
+      intrin->intrinsic = nir_intrinsic_discard_if;
+      return true;
+   case nir_intrinsic_is_helper_invocation:
+      intrin->intrinsic = nir_intrinsic_load_helper_invocation;
+      return true;
+   default:
+      return false;
+   }
+}
+
+/**
+ * Optimize discard and demote opcodes.
+ *
+ * If force_correct_quad_ops_after_discard is true and quad operations are
+ * used, discard() will be converted to demote() and gl_HelperInvocation will
+ * be lowered to helperInvocationEXT(). This is intended as workaround for
+ * game bugs to force correct derivatives after kill. This lowering is not
+ * valid in the general case as it might change the result of subgroup
+ * operations and loop behavior.
+ *
+ * Otherwise, if demote is used and no ops need helper invocations, demote()
+ * will be converted to discard() as an optimization.
+ */
+bool
+nir_lower_discard_or_demote(nir_shader *shader,
+                            bool force_correct_quad_ops_after_discard)
+{
+   if (shader->info.stage != MESA_SHADER_FRAGMENT)
+      return false;
+
+   /* We need uses_discard/demote and needs_*_helper_invocations. */
+   nir_shader_gather_info(shader, nir_shader_get_entrypoint(shader));
+   /* Validate that if uses_demote is set, uses_discard is also be set. */
+   assert(!shader->info.fs.uses_demote || shader->info.fs.uses_discard);
+
+   /* Quick skip. */
+   if (!shader->info.fs.uses_discard)
+      return false;
+
+   bool progress = false;
+
+   if (force_correct_quad_ops_after_discard &&
+       shader->info.fs.needs_quad_helper_invocations) {
+      /* If we need correct derivatives, convert discard to demote only when
+       * derivatives are actually used.
+       */
+      progress = nir_shader_instructions_pass(shader,
+                                              nir_lower_discard_to_demote_instr,
+                                              nir_metadata_all,
+                                              NULL);
+      shader->info.fs.uses_demote = true;
+   } else if (!shader->info.fs.needs_quad_helper_invocations &&
+              !shader->info.fs.needs_all_helper_invocations &&
+              shader->info.fs.uses_demote) {
+      /* If we don't need any helper invocations, convert demote to discard. */
+      progress = nir_shader_instructions_pass(shader,
+                                              nir_lower_demote_to_discard_instr,
+                                              nir_metadata_all,
+                                              NULL);
+      shader->info.fs.uses_demote = false;
+   }
+
+   /* Validate again that if uses_demote is set, uses_discard is also be set. */
+   assert(!shader->info.fs.uses_demote || shader->info.fs.uses_discard);
+   return progress;
+}

src/compiler/nir/nir_lower_discard_to_demote.c

@@ -1,70 +0,0 @@
-/*
- * Copyright © 2020 Valve Corporation
- *
- * 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"
-
-static bool
-nir_lower_discard_to_demote_instr(nir_builder *b, nir_instr *instr, void *data)
-{
-   if (instr->type != nir_instr_type_intrinsic)
-      return false;
-
-   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
-   switch (intrin->intrinsic) {
-   case nir_intrinsic_discard:
-      intrin->intrinsic = nir_intrinsic_demote;
-      b->shader->info.fs.uses_demote = true;
-      return true;
-   case nir_intrinsic_discard_if:
-      intrin->intrinsic = nir_intrinsic_demote_if;
-      b->shader->info.fs.uses_demote = true;
-      return true;
-   case nir_intrinsic_load_helper_invocation:
-      intrin->intrinsic = nir_intrinsic_is_helper_invocation;
-      return true;
-   default:
-      return false;
-   }
-}
-
-/**
- * This pass is intended as workaround for game bugs to force correct
- * derivatives after kill. This lowering is not valid in the general case
- * as it might change the result of subgroup operations and loop behavior.
- *
- * discard() will be lowered as demote() and gl_HelperInvocation
- * will be lowered as helperInvocationEXT().
- */
-bool
-nir_lower_discard_to_demote(nir_shader *shader)
-{
-   if (shader->info.stage != MESA_SHADER_FRAGMENT)
-      return false;
-
-   return nir_shader_instructions_pass(shader,
-                                       nir_lower_discard_to_demote_instr,
-                                       nir_metadata_all,
-                                       NULL);
-}

src/gallium/drivers/radeonsi/si_shader_nir.c

@@ -711,8 +711,8 @@ static void si_lower_nir(struct si_screen *sscreen, struct nir_shader *nir)
 
    NIR_PASS_V(nir, nir_remove_dead_variables, nir_var_function_temp, NULL);
 
-   if (sscreen->debug_flags & DBG(FS_CORRECT_DERIVS_AFTER_KILL))
-      NIR_PASS_V(nir, nir_lower_discard_to_demote);
+   NIR_PASS_V(nir, nir_lower_discard_or_demote,
+              sscreen->debug_flags & DBG(FS_CORRECT_DERIVS_AFTER_KILL));
 }
 
 void si_finalize_nir(struct pipe_screen *screen, void *nirptr, bool optimize)