Consider the following NIR:
block block_0;
/* succs: block_1 block_2 */
if (...) {
block block_1;
...
} else {
block block_2;
}
Calling split_block_beginning() on block_1 would break block_0's
successors: link_block() sets both successors of a block, so calling
link_block(block_0, new_block, NULL) would throw away the second
successor, leaving only /* succ: new_block */. This is invalid: the
block before an if statement must have two successors.
Changing the call to link_block(pred, new_block, pred->successors[0])
would correctly leave both successors in place, but because unlink_block
may shift successor[1] to successor[0], it may not preserve the original
order. NIR maintains a convention that successor[0] must point to the
"then" block, while successor[1] points to the "else" block, so we need
to take care to preserve this ordering.
This patch creates a new function that swaps out one successor for
another, preserving the ordering. It then uses this to fix the issue.
Signed-off-by: Kenneth Graunke <kenn...@whitecape.org>
---
src/glsl/nir/nir_control_flow.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/src/glsl/nir/nir_control_flow.c b/src/glsl/nir/nir_control_flow.c
index c0b54ac..1623667 100644
--- a/src/glsl/nir/nir_control_flow.c
+++ b/src/glsl/nir/nir_control_flow.c
@@ -200,6 +200,23 @@ link_block_to_non_block(nir_block *block, nir_cf_node
*node)
}
/**
+ * Replace a block's successor with a different one.
+ */
+static void
+replace_successor(nir_block *block, nir_block *old_succ, nir_block *new_succ)
+{
+ if (block->successors[0] == old_succ) {
+ block->successors[0] = new_succ;
+ } else {
+ assert(block->successors[1] == old_succ);
+ block->successors[1] = new_succ;
+ }
+
+ block_remove_pred(old_succ, block);
+ block_add_pred(new_succ, block);
+}
+
+/**
* Takes a basic block and inserts a new empty basic block before it, making
its
* predecessors point to the new block. This essentially splits the block into
* an empty header and a body so that another non-block CF node can be inserted
@@ -217,9 +234,7 @@ split_block_beginning(nir_block *block)
struct set_entry *entry;
set_foreach(block->predecessors, entry) {
nir_block *pred = (nir_block *) entry->key;
-
- unlink_blocks(pred, block);
- link_blocks(pred, new_block, NULL);
+ replace_successor(pred, block, new_block);
}
/* Any phi nodes must stay part of the new block, or else their
--
2.5.0
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