agx: Add dead code eliminator

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10582>

src/asahi/compiler/agx_compile.c

@@ -714,6 +714,7 @@ agx_compile_shader_nir(nir_shader *nir,
       agx_print_shader(ctx, stdout);
 
    agx_optimizer(ctx);
+   agx_dce(ctx);
 
    if (agx_debug & AGX_DBG_SHADERS && !skip_internal)
       agx_print_shader(ctx, stdout);

src/asahi/compiler/agx_compiler.h

@@ -548,6 +548,7 @@ void agx_print_instr(agx_instr *I, FILE *fp);
 void agx_print_block(agx_block *block, FILE *fp);
 void agx_print_shader(agx_context *ctx, FILE *fp);
 void agx_optimizer(agx_context *ctx);
+void agx_dce(agx_context *ctx);
 void agx_ra(agx_context *ctx);
 void agx_pack(agx_context *ctx, struct util_dynarray *emission);
 

src/asahi/compiler/agx_dce.c

@@ -0,0 +1,55 @@
+/*
+ * Copyright (C) 2021 Alyssa Rosenzweig <alyssa@rosenzweig.io>
+ *
+ * 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"
+
+/* SSA-based scalar dead code elimination */
+
+void
+agx_dce(agx_context *ctx)
+{
+   BITSET_WORD *seen = calloc(BITSET_WORDS(ctx->alloc), sizeof(BITSET_WORD));
+
+   agx_foreach_instr_global_safe_rev(ctx, I) {
+      if (agx_opcodes_info[I->op].can_eliminate) {
+         bool needed = false;
+
+         agx_foreach_dest(I, d) {
+            if (I->dest[d].type == AGX_INDEX_NORMAL)
+               needed |= BITSET_TEST(seen, I->dest[d].value);
+         }
+
+         if (!needed) {
+            agx_remove_instruction(I);
+            continue;
+         }
+      }
+
+      agx_foreach_src(I, s) {
+         if (I->src[s].type == AGX_INDEX_NORMAL)
+            BITSET_SET(seen, I->src[s].value);
+      }
+   }
+
+   free(seen);
+}

src/asahi/compiler/meson.build

@@ -21,6 +21,7 @@
 
 libasahi_agx_files = files(
   'agx_compile.c',
+  'agx_dce.c',
   'agx_pack.c',
   'agx_print.c',
   'agx_optimizer.c',