[Mesa-dev] [PATCH 2/9] nir: insert ssa_undef instructions when cleanup up defs/uses

[Connor Abbott]

The point of cleanup_defs_uses() is to make an instruction safe to
remove by removing any references that the rest of the shader may have
to it. Previously, it was handling register use/def sets and removing
the instruction from the use sets of any SSA sources it had, but if the
instruction defined an SSA value that was used by other instructions it
wasn't doing anything. This was ok, since we were always careful to make
sure that no removed instruction ever had any uses, but now we want to
start removing unreachable instruction which might still be used in
reachable parts of the code. In that case, the value that any uses get
is undefined since the instruction never actually executes, so we can
just replace the instruction with an ssa_undef_instr.

Signed-off-by: Connor Abbott <cwabbott0 at gmail.com>
---
 src/glsl/nir/nir.c | 47 ++++++++++++++++++++++++++++++++++-------------
 1 file changed, 34 insertions(+), 13 deletions(-)

diff --git a/src/glsl/nir/nir.c b/src/glsl/nir/nir.c
index f03e80a..362ac30 100644
--- a/src/glsl/nir/nir.c
+++ b/src/glsl/nir/nir.c
@@ -1206,26 +1206,29 @@ stitch_blocks(nir_block *before, nir_block *after)
 }
 
 static void
-remove_defs_uses(nir_instr *instr);
+remove_defs_uses(nir_instr *instr, nir_function_impl *impl);
 
 static void
-cleanup_cf_node(nir_cf_node *node)
+cleanup_cf_node(nir_cf_node *node, nir_function_impl *impl)
 {
    switch (node->type) {
    case nir_cf_node_block: {
       nir_block *block = nir_cf_node_as_block(node);
       /* We need to walk the instructions and clean up defs/uses */
-      nir_foreach_instr(block, instr)
-         remove_defs_uses(instr);
+      nir_foreach_instr(block, instr) {
+         if (instr->type == nir_instr_type_jump)
+            handle_remove_jump(block, nir_instr_as_jump(instr)->type);
+         remove_defs_uses(instr, impl);
+      }
       break;
    }
 
    case nir_cf_node_if: {
       nir_if *if_stmt = nir_cf_node_as_if(node);
       foreach_list_typed(nir_cf_node, child, node, &if_stmt->then_list)
-         cleanup_cf_node(child);
+         cleanup_cf_node(child, impl);
       foreach_list_typed(nir_cf_node, child, node, &if_stmt->else_list)
-         cleanup_cf_node(child);
+         cleanup_cf_node(child, impl);
 
       list_del(&if_stmt->condition.use_link);
       break;
@@ -1234,13 +1237,12 @@ cleanup_cf_node(nir_cf_node *node)
    case nir_cf_node_loop: {
       nir_loop *loop = nir_cf_node_as_loop(node);
       foreach_list_typed(nir_cf_node, child, node, &loop->body)
-         cleanup_cf_node(child);
+         cleanup_cf_node(child, impl);
       break;
    }
    case nir_cf_node_function: {
-      nir_function_impl *impl = nir_cf_node_as_function(node);
       foreach_list_typed(nir_cf_node, child, node, &impl->body)
-         cleanup_cf_node(child);
+         cleanup_cf_node(child, impl);
       break;
    }
    default:
@@ -1254,6 +1256,8 @@ nir_cf_node_remove(nir_cf_node *node)
    nir_function_impl *impl = nir_cf_node_get_function(node);
    nir_metadata_preserve(impl, nir_metadata_none);
 
+   cleanup_cf_node(node, impl);
+
    if (node->type == nir_cf_node_block) {
       /*
        * Basic blocks can't really be removed by themselves, since they act as
@@ -1275,8 +1279,6 @@ nir_cf_node_remove(nir_cf_node *node)
       exec_node_remove(&node->node);
       stitch_blocks(before_block, after_block);
    }
-
-   cleanup_cf_node(node);
 }
 
 static bool
@@ -1443,16 +1445,35 @@ remove_def_cb(nir_dest *dest, void *state)
    return true;
 }
 
+static bool
+remove_ssa_def_cb(nir_ssa_def *def, void *state)
+{
+   nir_function_impl *impl = state;
+   nir_shader *shader = impl->overload->function->shader;
+
+   if (!list_empty(&def->uses) || !list_empty(&def->if_uses)) {
+      nir_ssa_undef_instr *undef =
+         nir_ssa_undef_instr_create(shader, def->num_components);
+      nir_instr_insert_before_cf_list(&impl->body, &undef->instr);
+      nir_ssa_def_rewrite_uses(def, nir_src_for_ssa(&undef->def), shader);
+   }
+
+   return true;
+}
+
+
 static void
-remove_defs_uses(nir_instr *instr)
+remove_defs_uses(nir_instr *instr, nir_function_impl *impl)
 {
    nir_foreach_dest(instr, remove_def_cb, instr);
    nir_foreach_src(instr, remove_use_cb, instr);
+   nir_foreach_ssa_def(instr, remove_ssa_def_cb, impl);
 }
 
 void nir_instr_remove(nir_instr *instr)
 {
-   remove_defs_uses(instr);
+   nir_function_impl *impl = nir_cf_node_get_function(&instr->block->cf_node);
+   remove_defs_uses(instr, impl);
    exec_node_remove(&instr->node);
 
    if (instr->type == nir_instr_type_jump) {
-- 
2.1.0