pan/va: Add pass to insert flow control

To set flow control modifiers correctly and efficiently, we need a pass that
runs after register allocation and scheduling, but before packing. Add such a
pass.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16804>

src/panfrost/bifrost/compiler.h

@@ -684,6 +684,11 @@ typedef struct bi_block {
         /* Scoreboard state at the start/end of block */
         struct bi_scoreboard_state scoreboard_in, scoreboard_out;
 
+        /* On Valhall, indicates we need a terminal NOP to implement jumps to
+         * the end of the shader.
+         */
+        bool needs_nop;
+
         /* Flags available for pass-internal use */
         uint8_t pass_flags;
 } bi_block;

src/panfrost/bifrost/meson.build

@@ -46,6 +46,7 @@ libpanfrost_bifrost_files = files(
   'bi_validate.c',
   'bir.c',
   'bifrost_compile.c',
+  'valhall/va_insert_flow.c',
   'valhall/va_lower_constants.c',
   'valhall/va_lower_isel.c',
   'valhall/va_lower_split_64bit.c',

src/panfrost/bifrost/valhall/va_compiler.h

@@ -40,6 +40,7 @@ void va_repair_fau(bi_builder *b, bi_instr *I);
 void va_fuse_add_imm(bi_instr *I);
 void va_lower_constants(bi_context *ctx, bi_instr *I);
 void va_lower_isel(bi_instr *I);
+void va_insert_flow_control_nops(bi_context *ctx);
 uint64_t va_pack_instr(const bi_instr *I);
 
 static inline unsigned

src/panfrost/bifrost/valhall/va_insert_flow.c

@@ -0,0 +1,118 @@
+/*
+ * Copyright (C) 2022 Collabora Ltd.
+ *
+ * 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 "va_compiler.h"
+#include "valhall_enums.h"
+#include "bi_builder.h"
+
+/*
+ * Insert flow control into a scheduled and register allocated shader.  This
+ * pass runs after scheduling and register allocation. This pass only
+ * inserts NOPs with the appropriate flow control modifiers. It should be
+ * followed by a cleanup pass to merge flow control modifiers on adjacent
+ * instructions, eliminating the NOPs. This decouples optimization from
+ * correctness, simplifying both passes.
+ */
+
+/*
+ * Insert a NOP instruction with given flow control.
+ */
+static void
+bi_flow(bi_context *ctx, bi_cursor cursor, enum va_flow flow)
+{
+   bi_builder b = bi_init_builder(ctx, cursor);
+
+   bi_nop(&b)->flow = flow;
+}
+
+/*
+ * Determine if execution should terminate after a given block. Execution cannot
+ * terminate within a basic block.
+ */
+static bool
+va_should_end(bi_block *block)
+{
+   /* Don't return if we're succeeded by instructions */
+   for (unsigned i = 0; i < ARRAY_SIZE(block->successors); ++i) {
+      bi_block *succ = block->successors[i];
+
+      if (succ)
+         return false;
+   }
+
+   return true;
+}
+
+/*
+ * Given a program with no flow control modifiers, insert NOPs signaling the
+ * required flow control. Not much optimization happens here.
+ */
+void
+va_insert_flow_control_nops(bi_context *ctx)
+{
+   bi_foreach_block(ctx, block) {
+      bi_foreach_instr_in_block_safe(block, I) {
+         switch (I->op) {
+         /* Signal barriers immediately */
+         case BI_OPCODE_BARRIER:
+            bi_flow(ctx, bi_after_instr(I), VA_FLOW_WAIT);
+            break;
+
+         /* Insert waits for tilebuffer and depth/stencil instructions. These
+          * only happen in regular fragment shaders, as the required waits are
+          * assumed to already have happened in blend shaders.
+          */
+         case BI_OPCODE_BLEND:
+         case BI_OPCODE_LD_TILE:
+         case BI_OPCODE_ST_TILE:
+            if (!ctx->inputs->is_blend)
+               bi_flow(ctx, bi_before_instr(I), VA_FLOW_WAIT);
+            break;
+         case BI_OPCODE_ATEST:
+         case BI_OPCODE_ZS_EMIT:
+            if (!ctx->inputs->is_blend)
+               bi_flow(ctx, bi_before_instr(I), VA_FLOW_WAIT0126);
+            break;
+
+         /* TODO: Optimize waits for asynchronous instructions */
+         default:
+            if (bi_opcode_props[I->op].message)
+               bi_flow(ctx, bi_after_instr(I), VA_FLOW_WAIT0);
+            break;
+         }
+      }
+
+      /* End exeuction at the end of the block if needed, or reconverge if we
+       * continue but we don't need to end execution.
+       */
+      if (va_should_end(block) || block->needs_nop) {
+         /* Don't bother adding a NOP into an unreachable block */
+         if (block == bi_start_block(&ctx->blocks) || bi_num_predecessors(block))
+            bi_flow(ctx, bi_after_block(block), VA_FLOW_END);
+      } else if (bi_reconverge_branches(block)) {
+         /* TODO: Do we have ever need to reconverge from an empty block? */
+         if (!list_is_empty(&block->instructions))
+            bi_flow(ctx, bi_after_block(block), VA_FLOW_RECONVERGE);
+      }
+   }
+}