Module: Mesa
Branch: master
Commit: aec6744501be06e37dbcb620eba0deef54fcad31
URL:
http://cgit.freedesktop.org/mesa/mesa/commit/?id=aec6744501be06e37dbcb620eba0deef54fcad31
Author: Connor Abbott <cwabbo...@gmail.com>
Date: Fri May 8 14:42:14 2015 -0400
nir/dead_cf: add support for removing useless loops
v2: fix detecting if the loop has any phi nodes after it.
v2: use nir_foreach_ssa_def() instead of nir_foreach_dest() when
checking for values live after the loop to catch const_load
instructions.
v2: fix handling return instructions
v2: add some documentation to loop_is_dead()
Reviewed-by: Kenneth Graunke <kenn...@whitecape.org>
Reviewed-by: Matt Turner <matts...@gmail.com>
---
src/glsl/nir/nir_opt_dead_cf.c | 121 ++++++++++++++++++++++++++++++++++++----
1 file changed, 109 insertions(+), 12 deletions(-)
diff --git a/src/glsl/nir/nir_opt_dead_cf.c b/src/glsl/nir/nir_opt_dead_cf.c
index 66961ce..5c55104 100644
--- a/src/glsl/nir/nir_opt_dead_cf.c
+++ b/src/glsl/nir/nir_opt_dead_cf.c
@@ -29,9 +29,9 @@
#include "nir_control_flow.h"
/*
- * This file implements an optimization that deletes statically unreachable
- * code. In NIR, one way this can happen if if an if statement has a constant
- * condition:
+ * This file implements an optimization that deletes statically
+ * unreachable/dead code. In NIR, one way this can happen if if an if
+ * statement has a constant condition:
*
* if (true) {
* ...
@@ -41,7 +41,7 @@
* branch into the surrounding control flow, possibly removing more code if
* the branch had a jump at the end.
*
- * The other way is that control flow can end in a jump so that code after it
+ * Another way is that control flow can end in a jump so that code after it
* never gets executed. In particular, this can happen after optimizing
* something like:
*
@@ -60,6 +60,12 @@
* }
* ...
*
+ * Finally, we also handle removing useless loops, i.e. loops with no side
+ * effects and without any definitions that are used elsewhere. This case is a
+ * little different from the first two in that the code is actually run (it
+ * just never does anything), but there are similar issues with needing to
+ * be careful with restarting after deleting the cf_node (see dead_cf_list())
+ * so this is a convenient place to remove them.
*/
static void
@@ -131,19 +137,107 @@ opt_constant_if(nir_if *if_stmt, bool condition)
}
static bool
+block_has_no_side_effects(nir_block *block, void *state)
+{
+ (void) state;
+
+ nir_foreach_instr(block, instr) {
+ if (instr->type == nir_instr_type_call)
+ return false;
+
+ /* Return instructions can cause us to skip over other side-effecting
+ * instructions after the loop, so consider them to have side effects
+ * here.
+ */
+
+ if (instr->type == nir_instr_type_jump &&
+ nir_instr_as_jump(instr)->type == nir_jump_return)
+ return false;
+
+ if (instr->type != nir_instr_type_intrinsic)
+ continue;
+
+ nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+ if (!nir_intrinsic_infos[intrin->intrinsic].flags &
+ NIR_INTRINSIC_CAN_ELIMINATE)
+ return false;
+ }
+
+ return true;
+}
+
+static bool
+def_not_live_out(nir_ssa_def *def, void *state)
+{
+ nir_block *after = state;
+
+ return !BITSET_TEST(after->live_in, def->live_index);
+}
+
+/*
+ * Test if a loop is dead. A loop is dead if:
+ *
+ * 1) It has no side effects (i.e. intrinsics which could possibly affect the
+ * state of the program aside from producing an SSA value, indicated by a lack
+ * of NIR_INTRINSIC_CAN_ELIMINATE).
+ *
+ * 2) It has no phi nodes after it, since those indicate values inside the
+ * loop being used after the loop.
+ *
+ * 3) If there are no phi nodes after the loop, then the only way a value
+ * defined inside the loop can be used outside the loop is if its definition
+ * dominates the block after the loop. If none of the definitions that
+ * dominate the loop exit are used outside the loop, then the loop is dead
+ * and it can be deleted.
+ */
+
+static bool
+loop_is_dead(nir_loop *loop)
+{
+ nir_block *before = nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
+ nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&loop->cf_node));
+
+ if (!exec_list_is_empty(&after->instr_list) &&
+ nir_block_first_instr(after)->type == nir_instr_type_phi)
+ return false;
+
+ if (!nir_foreach_block_in_cf_node(&loop->cf_node, block_has_no_side_effects,
+ NULL))
+ return false;
+
+ for (nir_block *cur = after->imm_dom; cur != before; cur = cur->imm_dom) {
+ nir_foreach_instr(cur, instr) {
+ if (!nir_foreach_ssa_def(instr, def_not_live_out, after))
+ return false;
+ }
+ }
+
+ return true;
+}
+
+static bool
dead_cf_block(nir_block *block)
{
nir_if *following_if = nir_block_get_following_if(block);
- if (!following_if)
- return false;
+ if (following_if) {
+ nir_const_value *const_value =
+ nir_src_as_const_value(following_if->condition);
+
+ if (!const_value)
+ return false;
- nir_const_value *const_value =
- nir_src_as_const_value(following_if->condition);
+ opt_constant_if(following_if, const_value->u[0] != 0);
+ return true;
+ }
- if (!const_value)
- return false;
+ nir_loop *following_loop = nir_block_get_following_loop(block);
+ if (!following_loop)
+ return false;
- opt_constant_if(following_if, const_value->u[0] != 0);
+ if (!loop_is_dead(following_loop))
+ return false;
+
+ nir_cf_node_remove(&following_loop->cf_node);
return true;
}
@@ -170,7 +264,7 @@ dead_cf_list(struct exec_list *list, bool
*list_ends_in_jump)
case nir_cf_node_block: {
nir_block *block = nir_cf_node_as_block(cur);
if (dead_cf_block(block)) {
- /* We just deleted the if after this block, so we may have
+ /* We just deleted the if or loop after this block, so we may have
* deleted the block before or after it -- which one is an
* implementation detail. Therefore, to recover the place we were
* at, we have to use the previous cf_node.
@@ -240,6 +334,9 @@ dead_cf_list(struct exec_list *list, bool
*list_ends_in_jump)
static bool
opt_dead_cf_impl(nir_function_impl *impl)
{
+ nir_metadata_require(impl, nir_metadata_live_variables |
+ nir_metadata_dominance);
+
bool dummy;
bool progress = dead_cf_list(&impl->body, &dummy);
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