nir: Add new GS intrinsics that maintain a count of emitted vertices.

This patch also introduces a lowering pass to convert the simple GS
intrinsics to the new ones.  See the comments above that for the
rationale behind the new intrinsics.

This should be useful for i965; it's a generic enough mechanism that I
could see other drivers potentially using it as well, so I don't feel
too bad about putting it in the generic code.

v2:
- Use nir_after_block_before_jump for the cursor (caught by Jason
  Ekstrand - I'd mistakenly used nir_after_block when rebasing this
  code onto the new NIR control flow API).
- Remove the old emit_vertex intrinsic at the end, rather than in
  the middle (requested by Jason).
- Use state->... directly rather than locals (requested by Jason).
- Report progress from nir_lower_gs_intrinsics() (requested by me).
- Remove "Authors:" section from file comment (requested by
  Michael Schellenberger Costa).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/glsl/Makefile.sources

@@ -37,6 +37,7 @@ NIR_FILES = \
 	nir/nir_lower_atomics.c \
 	nir/nir_lower_clip.c \
 	nir/nir_lower_global_vars_to_local.c \
+	nir/nir_lower_gs_intrinsics.c \
 	nir/nir_lower_load_const_to_scalar.c \
 	nir/nir_lower_locals_to_regs.c \
 	nir/nir_lower_idiv.c \

src/glsl/nir/nir.h

@@ -1883,6 +1883,8 @@ void nir_lower_two_sided_color(nir_shader *shader);
 void nir_lower_atomics(nir_shader *shader);
 void nir_lower_to_source_mods(nir_shader *shader);
 
+bool nir_lower_gs_intrinsics(nir_shader *shader);
+
 bool nir_normalize_cubemap_coords(nir_shader *shader);
 
 void nir_live_variables_impl(nir_function_impl *impl);

src/glsl/nir/nir_intrinsics.h

@@ -79,9 +79,30 @@ BARRIER(memory_barrier)
 /** A conditional discard, with a single boolean source. */
 INTRINSIC(discard_if, 1, ARR(1), false, 0, 0, 0, 0)
 
+/**
+ * Basic Geometry Shader intrinsics.
+ *
+ * emit_vertex implements GLSL's EmitStreamVertex() built-in.  It takes a single
+ * index, which is the stream ID to write to.
+ *
+ * end_primitive implements GLSL's EndPrimitive() built-in.
+ */
 INTRINSIC(emit_vertex,   0, ARR(), false, 0, 0, 1, 0)
 INTRINSIC(end_primitive, 0, ARR(), false, 0, 0, 1, 0)
 
+/**
+ * Geometry Shader intrinsics with a vertex count.
+ *
+ * Alternatively, drivers may implement these intrinsics, and use
+ * nir_lower_gs_intrinsics() to convert from the basic intrinsics.
+ *
+ * These maintain a count of the number of vertices emitted, as an additional
+ * unsigned integer source.
+ */
+INTRINSIC(emit_vertex_with_counter, 1, ARR(1), false, 0, 0, 1, 0)
+INTRINSIC(end_primitive_with_counter, 1, ARR(1), false, 0, 0, 1, 0)
+INTRINSIC(set_vertex_count, 1, ARR(1), false, 0, 0, 0, 0)
+
 /*
  * Atomic counters
  *

src/glsl/nir/nir_lower_gs_intrinsics.c

@@ -0,0 +1,218 @@
+/*
+ * Copyright © 2015 Intel 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"
+
+/**
+ * \file nir_lower_gs_intrinsics.c
+ *
+ * Geometry Shaders can call EmitVertex()/EmitStreamVertex() to output an
+ * arbitrary number of vertices.  However, the shader must declare the maximum
+ * number of vertices that it will ever output - further attempts to emit
+ * vertices result in undefined behavior according to the GLSL specification.
+ *
+ * Drivers might use this maximum number of vertices to allocate enough space
+ * to hold the geometry shader's output.  Some drivers (such as i965) need to
+ * implement "safety checks" which ensure that the shader hasn't emitted too
+ * many vertices, to avoid overflowing that space and trashing other memory.
+ *
+ * The count of emitted vertices can also be useful in buffer offset
+ * calculations, so drivers know where to write the GS output.
+ *
+ * However, for simple geometry shaders that emit a statically determinable
+ * number of vertices, this extra bookkeeping is unnecessary and inefficient.
+ * By tracking the vertex count in NIR, we allow constant folding/propagation
+ * and dead control flow optimizations to eliminate most of it where possible.
+ *
+ * This pass introduces a new global variable which stores the current vertex
+ * count (initialized to 0), and converts emit_vertex/end_primitive intrinsics
+ * to their *_with_counter variants.  emit_vertex is also wrapped in a safety
+ * check to avoid buffer overflows.  Finally, it adds a set_vertex_count
+ * intrinsic at the end of the program, informing the driver of the final
+ * vertex count.
+ */
+
+struct state {
+   nir_builder *builder;
+   nir_variable *vertex_count_var;
+   bool progress;
+};
+
+/**
+ * Replace emit_vertex intrinsics with:
+ *
+ * if (vertex_count < max_vertices) {
+ *    emit_vertex_with_counter vertex_count ...
+ *    vertex_count += 1
+ * }
+ */
+static void
+rewrite_emit_vertex(nir_intrinsic_instr *intrin, struct state *state)
+{
+   nir_builder *b = state->builder;
+
+   /* Load the vertex count */
+   b->cursor = nir_before_instr(&intrin->instr);
+   nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);
+
+   nir_ssa_def *max_vertices = nir_imm_int(b, b->shader->gs.vertices_out);
+
+   /* Create: if (vertex_count < max_vertices) and insert it.
+    *
+    * The new if statement needs to be hooked up to the control flow graph
+    * before we start inserting instructions into it.
+    */
+   nir_if *if_stmt = nir_if_create(b->shader);
+   if_stmt->condition = nir_src_for_ssa(nir_ilt(b, count, max_vertices));
+   nir_builder_cf_insert(b, &if_stmt->cf_node);
+
+   /* Fill out the new then-block */
+   b->cursor = nir_after_cf_list(&if_stmt->then_list);
+
+   nir_intrinsic_instr *lowered =
+      nir_intrinsic_instr_create(b->shader,
+                                 nir_intrinsic_emit_vertex_with_counter);
+   lowered->const_index[0] = intrin->const_index[0];
+   lowered->src[0] = nir_src_for_ssa(count);
+   nir_builder_instr_insert(b, &lowered->instr);
+
+   /* Increment the vertex count by 1 */
+   nir_store_var(b, state->vertex_count_var,
+                 nir_iadd(b, count, nir_imm_int(b, 1)));
+
+   nir_instr_remove(&intrin->instr);
+
+   state->progress = true;
+}
+
+/**
+ * Replace end_primitive with end_primitive_with_counter.
+ */
+static void
+rewrite_end_primitive(nir_intrinsic_instr *intrin, struct state *state)
+{
+   nir_builder *b = state->builder;
+
+   b->cursor = nir_before_instr(&intrin->instr);
+   nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);
+
+   nir_intrinsic_instr *lowered =
+      nir_intrinsic_instr_create(b->shader,
+                                 nir_intrinsic_end_primitive_with_counter);
+   lowered->const_index[0] = intrin->const_index[0];
+   lowered->src[0] = nir_src_for_ssa(count);
+   nir_builder_instr_insert(b, &lowered->instr);
+
+   nir_instr_remove(&intrin->instr);
+
+   state->progress = true;
+}
+
+static bool
+rewrite_intrinsics(nir_block *block, void *closure)
+{
+   struct state *state = closure;
+
+   nir_foreach_instr_safe(block, instr) {
+      if (instr->type != nir_instr_type_intrinsic)
+         continue;
+
+      nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+      switch (intrin->intrinsic) {
+      case nir_intrinsic_emit_vertex:
+         rewrite_emit_vertex(intrin, state);
+         break;
+      case nir_intrinsic_end_primitive:
+         rewrite_end_primitive(intrin, state);
+         break;
+      default:
+         /* not interesting; skip this */
+         break;
+      }
+   }
+
+   return true;
+}
+
+/**
+ * Add a set_vertex_count intrinsic at the end of the program
+ * (representing the final vertex count).
+ */
+static void
+append_set_vertex_count(nir_block *end_block, struct state *state)
+{
+   nir_builder *b = state->builder;
+   nir_shader *shader = state->builder->shader;
+
+   /* Insert the new intrinsic in all of the predecessors of the end block,
+    * but before any jump instructions (return).
+    */
+   struct set_entry *entry;
+   set_foreach(end_block->predecessors, entry) {
+      nir_block *pred = (nir_block *) entry->key;
+      b->cursor = nir_after_block_before_jump(pred);
+
+      nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);
+
+      nir_intrinsic_instr *set_vertex_count =
+         nir_intrinsic_instr_create(shader, nir_intrinsic_set_vertex_count);
+      set_vertex_count->src[0] = nir_src_for_ssa(count);
+
+      nir_builder_instr_insert(b, &set_vertex_count->instr);
+   }
+}
+
+bool
+nir_lower_gs_intrinsics(nir_shader *shader)
+{
+   struct state state;
+   state.progress = false;
+
+   /* Create the counter variable */
+   nir_variable *var = rzalloc(shader, nir_variable);
+   var->data.mode = nir_var_global;
+   var->type = glsl_uint_type();
+   var->name = "vertex_count";
+   var->constant_initializer = rzalloc(shader, nir_constant); /* initialize to 0 */
+
+   exec_list_push_tail(&shader->globals, &var->node);
+   state.vertex_count_var = var;
+
+   nir_foreach_overload(shader, overload) {
+      if (overload->impl) {
+         nir_builder b;
+         nir_builder_init(&b, overload->impl);
+         state.builder = &b;
+
+         nir_foreach_block(overload->impl, rewrite_intrinsics, &state);
+
+         /* This only works because we have a single main() function. */
+         append_set_vertex_count(overload->impl->end_block, &state);
+
+         nir_metadata_preserve(overload->impl, 0);
+      }
+   }
+
+   return state.progress;
+}