cmd/compile: place combined loads at the location of the last byte load
We need to make sure all the bounds checks pass before issuing
a load which combines several others. We do this by issuing the
combined load at the last load's block, where "last" = closest to
the leaf of the dominator tree.
Fixes #15002
Change-Id: I7358116db1e039a072c12c0a73d861f3815d72af
Reviewed-on: https://go-review.googlesource.com/21246
Reviewed-by: Todd Neal <todd@tneal.org>
diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index 3ee6b00..e0cb7f5 100644
--- a/src/cmd/compile/internal/ssa/rewrite.go
+++ b/src/cmd/compile/internal/ssa/rewrite.go
@@ -261,3 +261,53 @@
}
return off, adj
}
+
+// mergePoint finds a block among a's blocks which dominates b and is itself
+// dominated by all of a's blocks. Returns nil if it can't find one.
+// Might return nil even if one does exist.
+func mergePoint(b *Block, a ...*Value) *Block {
+ // Walk backward from b looking for one of the a's blocks.
+
+ // Max distance
+ d := 100
+
+ for d > 0 {
+ for _, x := range a {
+ if b == x.Block {
+ goto found
+ }
+ }
+ if len(b.Preds) > 1 {
+ // Don't know which way to go back. Abort.
+ return nil
+ }
+ b = b.Preds[0]
+ d--
+ }
+ return nil // too far away
+found:
+ // At this point, r is the first value in a that we find by walking backwards.
+ // if we return anything, r will be it.
+ r := b
+
+ // Keep going, counting the other a's that we find. They must all dominate r.
+ na := 0
+ for d > 0 {
+ for _, x := range a {
+ if b == x.Block {
+ na++
+ }
+ }
+ if na == len(a) {
+ // Found all of a in a backwards walk. We can return r.
+ return r
+ }
+ if len(b.Preds) > 1 {
+ return nil
+ }
+ b = b.Preds[0]
+ d--
+
+ }
+ return nil // too far away
+}
