cmd/compile: fix liveness for open-coded defer args for infinite loops

Once defined, a stack slot holding an open-coded defer arg should always be marked
live, since it may be used at any time if there is a panic. These stack slots are
typically kept live naturally by the open-defer code inlined at each return/exit point.
However, we need to do extra work to make sure that they are kept live if a
function has an infinite loop or a panic exit.

For this fix, only in the case of a function that is using open-coded defers, we
compute the set of blocks (most often empty) that cannot reach a return or a
BlockExit (panic) because of an infinite loop. Then, for each block b which
cannot reach a return or BlockExit or is a BlockExit block, we mark each defer arg
slot as live, as long as the definition of the defer arg slot dominates block b.

For this change, had to export (*Func).sdom (-> Sdom) and SparseTree.isAncestorEq
(-> IsAncestorEq)

Updates #35277

Change-Id: I7b53c9bd38ba384a3794386dd0eb94e4cbde4eb1
Reviewed-on: https://go-review.googlesource.com/c/go/+/204802
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/ssa/cse.go b/src/cmd/compile/internal/ssa/cse.go
index 1fdcffcae8..15dfe6d795 100644
--- a/src/cmd/compile/internal/ssa/cse.go
+++ b/src/cmd/compile/internal/ssa/cse.go
@@ -155,7 +155,7 @@ func cse(f *Func) {
 		}
 	}
 
-	sdom := f.sdom()
+	sdom := f.Sdom()
 
 	// Compute substitutions we would like to do. We substitute v for w
 	// if v and w are in the same equivalence class and v dominates w.
@@ -179,7 +179,7 @@ func cse(f *Func) {
 				if w == nil {
 					continue
 				}
-				if sdom.isAncestorEq(v.Block, w.Block) {
+				if sdom.IsAncestorEq(v.Block, w.Block) {
 					rewrite[w.ID] = v
 					e[j] = nil
 				} else {