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/loopbce.go b/src/cmd/compile/internal/ssa/loopbce.go
index d53014943d..d99b15b2b6 100644
--- a/src/cmd/compile/internal/ssa/loopbce.go
+++ b/src/cmd/compile/internal/ssa/loopbce.go
@@ -83,7 +83,7 @@ func parseIndVar(ind *Value) (min, inc, nxt *Value) {
 // TODO: handle 32 bit operations
 func findIndVar(f *Func) []indVar {
 	var iv []indVar
-	sdom := f.sdom()
+	sdom := f.Sdom()
 
 	for _, b := range f.Blocks {
 		if b.Kind != BlockIf || len(b.Preds) != 2 {
@@ -187,7 +187,7 @@ func findIndVar(f *Func) []indVar {
 
 		// Second condition: b.Succs[0] dominates nxt so that
 		// nxt is computed when inc < max, meaning nxt <= max.
-		if !sdom.isAncestorEq(b.Succs[0].b, nxt.Block) {
+		if !sdom.IsAncestorEq(b.Succs[0].b, nxt.Block) {
 			// inc+ind can only be reached through the branch that enters the loop.
 			continue
 		}