Mesa (main): nir: Better document the Boissinot algorithm in nir_from_ssa()
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Fri Jul 16 06:36:42 UTC 2021
Module: Mesa
Branch: main
Commit: 0ee322acdb633a0935f542494f7113d86df3f78a
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=0ee322acdb633a0935f542494f7113d86df3f78a
Author: Jason Ekstrand <jason.ekstrand at intel.com>
Date: Mon Feb 1 16:10:19 2021 -0600
nir: Better document the Boissinot algorithm in nir_from_ssa()
Reviewed-by: Yevhenii Kolesnikov <yevhenii.kolesnikov at globallogic.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8815>
---
src/compiler/nir/nir_from_ssa.c | 43 +++++++++++++++++++++++++++++++++++++----
1 file changed, 39 insertions(+), 4 deletions(-)
diff --git a/src/compiler/nir/nir_from_ssa.c b/src/compiler/nir/nir_from_ssa.c
index b624e457e4b..6bfeed23b39 100644
--- a/src/compiler/nir/nir_from_ssa.c
+++ b/src/compiler/nir/nir_from_ssa.c
@@ -105,9 +105,9 @@ ssa_def_dominates(nir_ssa_def *a, nir_ssa_def *b)
* Each SSA definition is associated with a merge_node and the association
* is represented by a combination of a hash table and the "def" parameter
* in the merge_node structure. The merge_set stores a linked list of
- * merge_nodes in dominance order of the ssa definitions. (Since the
- * liveness analysis pass indexes the SSA values in dominance order for us,
- * this is an easy thing to keep up.) It is assumed that no pair of the
+ * merge_nodes, ordered by a pre-order DFS walk of the dominance tree. (Since
+ * the liveness analysis pass indexes the SSA values in dominance order for
+ * us, this is an easy thing to keep up.) It is assumed that no pair of the
* nodes in a given set interfere. Merging two sets or checking for
* interference can be done in a single linear-time merge-sort walk of the
* two lists of nodes.
@@ -185,7 +185,11 @@ merge_nodes_interfere(merge_node *a, merge_node *b)
return nir_ssa_defs_interfere(a->def, b->def);
}
-/* Merges b into a */
+/* Merges b into a
+ *
+ * This algorithm uses def_after to ensure that the sets always stay in the
+ * same order as the pre-order DFS done by the liveness algorithm.
+ */
static merge_set *
merge_merge_sets(merge_set *a, merge_set *b)
{
@@ -223,6 +227,9 @@ merge_merge_sets(merge_set *a, merge_set *b)
static bool
merge_sets_interfere(merge_set *a, merge_set *b)
{
+ /* List of all the nodes which dominate the current node, in dominance
+ * order.
+ */
NIR_VLA(merge_node *, dom, a->size + b->size);
int dom_idx = -1;
@@ -231,6 +238,9 @@ merge_sets_interfere(merge_set *a, merge_set *b)
while (!exec_node_is_tail_sentinel(an) ||
!exec_node_is_tail_sentinel(bn)) {
+ /* We walk the union of the two sets in the same order as the pre-order
+ * DFS done by liveness analysis.
+ */
merge_node *current;
if (exec_node_is_tail_sentinel(an)) {
current = exec_node_data(merge_node, bn, node);
@@ -251,10 +261,35 @@ merge_sets_interfere(merge_set *a, merge_set *b)
}
}
+ /* Because our walk is a pre-order DFS, we can maintain the list of
+ * dominating nodes as a simple stack, pushing every node onto the list
+ * after we visit it and popping any non-dominating nodes off before we
+ * visit the current node.
+ */
while (dom_idx >= 0 &&
!ssa_def_dominates(dom[dom_idx]->def, current->def))
dom_idx--;
+ /* There are three invariants of this algorithm that are important here:
+ *
+ * 1. There is no interference within either set a or set b.
+ * 2. None of the nodes processed up until this point interfere.
+ * 3. All the dominators of `current` have been processed
+ *
+ * Because of these invariants, we only need to check the current node
+ * against its minimal dominator. If any other node N in the union
+ * interferes with current, then N must dominate current because we are
+ * in SSA form. If N dominates current then it must also dominate our
+ * minimal dominator dom[dom_idx]. Since N is live at current it must
+ * also be live at the minimal dominator which means N interferes with
+ * the minimal dominator dom[dom_idx] and, by invariants 2 and 3 above,
+ * the algorithm would have already terminated. Therefore, if we got
+ * here, the only node that can possibly interfere with current is the
+ * minimal dominator dom[dom_idx].
+ *
+ * This is what allows us to do a interference check of the union of the
+ * two sets with a single linear-time walk.
+ */
if (dom_idx >= 0 && merge_nodes_interfere(current, dom[dom_idx]))
return true;
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