nir: add conditional discard optimisation (v4)

This is ported from GLSL and converts

if (cond)
	discard;

into
discard_if(cond);

This removes a block, but also is needed by radv
to workaround a bug in the LLVM backend.

v2: handle if (a) discard_if(b) (nha)
cleanup and drop pointless loop (Matt)
make sure there are no dependent phis (Eric)
v3: make sure only one instruction in the then block.
v4: remove sneaky tabs, add cursor init (Eric)

Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: "13.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/compiler/Makefile.sources

@@ -228,6 +228,7 @@ NIR_FILES = \
 	nir/nir_metadata.c \
 	nir/nir_move_vec_src_uses_to_dest.c \
 	nir/nir_normalize_cubemap_coords.c \
+	nir/nir_opt_conditional_discard.c \
 	nir/nir_opt_constant_folding.c \
 	nir/nir_opt_copy_propagate.c \
 	nir/nir_opt_cse.c \

src/compiler/nir/nir.h

@@ -2531,6 +2531,8 @@ bool nir_opt_remove_phis(nir_shader *shader);
 
 bool nir_opt_undef(nir_shader *shader);
 
+bool nir_opt_conditional_discard(nir_shader *shader);
+
 void nir_sweep(nir_shader *shader);
 
 nir_intrinsic_op nir_intrinsic_from_system_value(gl_system_value val);

src/compiler/nir/nir_opt_conditional_discard.c

@@ -0,0 +1,125 @@
+/*
+ * Copyright © 2016 Red Hat
+ *
+ * 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_opt_conditional_discard.c
+ *
+ * Handles optimization of lowering if (cond) discard to discard_if(cond).
+ */
+
+static bool
+nir_opt_conditional_discard_block(nir_block *block, void *mem_ctx)
+{
+   nir_builder bld;
+
+   if (nir_cf_node_is_first(&block->cf_node))
+      return false;
+
+   nir_cf_node *prev_node = nir_cf_node_prev(&block->cf_node);
+   if (prev_node->type != nir_cf_node_if)
+      return false;
+
+   nir_if *if_stmt = nir_cf_node_as_if(prev_node);
+   nir_block *then_block = nir_if_first_then_block(if_stmt);
+   nir_block *else_block = nir_if_first_else_block(if_stmt);
+
+   /* check there is only one else block and it is empty */
+   if (nir_if_last_else_block(if_stmt) != else_block)
+      return false;
+   if (!exec_list_is_empty(&else_block->instr_list))
+      return false;
+
+   /* check there is only one then block and it has only one instruction in it */
+   if (nir_if_last_then_block(if_stmt) != then_block)
+      return false;
+   if (exec_list_is_empty(&then_block->instr_list))
+      return false;
+   if (exec_list_length(&then_block->instr_list) > 1)
+      return false;
+   /*
+    * make sure no subsequent phi nodes point at this if.
+    */
+   nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&if_stmt->cf_node));
+   nir_foreach_instr_safe(instr, after) {
+      if (instr->type != nir_instr_type_phi)
+         break;
+      nir_phi_instr *phi = nir_instr_as_phi(instr);
+
+      nir_foreach_phi_src(phi_src, phi) {
+         if (phi_src->pred == then_block ||
+             phi_src->pred == else_block)
+            return false;
+      }
+   }
+
+   /* Get the first instruction in the then block and confirm it is
+    * a discard or a discard_if
+    */
+   nir_instr *instr = nir_block_first_instr(then_block);
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+   if (intrin->intrinsic != nir_intrinsic_discard &&
+       intrin->intrinsic != nir_intrinsic_discard_if)
+      return false;
+
+   nir_src cond;
+
+   nir_builder_init(&bld, mem_ctx);
+   bld.cursor = nir_before_cf_node(prev_node);
+   if (intrin->intrinsic == nir_intrinsic_discard)
+      cond = if_stmt->condition;
+   else
+      cond = nir_src_for_ssa(nir_iand(&bld,
+                                      nir_ssa_for_src(&bld, if_stmt->condition, 1),
+                                      nir_ssa_for_src(&bld, intrin->src[0], 1)));
+
+   nir_intrinsic_instr *discard_if =
+      nir_intrinsic_instr_create(mem_ctx, nir_intrinsic_discard_if);
+   nir_src_copy(&discard_if->src[0], &cond, discard_if);
+
+   nir_instr_insert_before_cf(prev_node, &discard_if->instr);
+   nir_instr_remove(&intrin->instr);
+   nir_cf_node_remove(&if_stmt->cf_node);
+
+   return true;
+}
+
+bool
+nir_opt_conditional_discard(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_function(function, shader) {
+      if (function->impl) {
+         void *mem_ctx = ralloc_parent(function->impl);
+         nir_foreach_block_safe(block, function->impl) {
+            progress |= nir_opt_conditional_discard_block(block, mem_ctx);
+         }
+      }
+   }
+   return progress;
+}