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, 27 insertions, 0 deletions

diff --git a/src/runtime/defer_test.go b/src/runtime/defer_test.go
index 51cd4bb9cc..3d8f81277f 100644
--- a/src/runtime/defer_test.go
+++ b/src/runtime/defer_test.go
@@ -254,3 +254,30 @@ func TestNonSSAableArgs(t *testing.T) {
 		save4 = element.z
 	}(sideeffect2(foo).element)
 }
+
+//go:noinline
+func doPanic() {
+	panic("Test panic")
+}
+
+func TestDeferForFuncWithNoExit(t *testing.T) {
+	cond := 1
+	defer func() {
+		if cond != 2 {
+			t.Fatal(fmt.Sprintf("cond: wanted 2, got %v", cond))
+		}
+		if recover() != "Test panic" {
+			t.Fatal("Didn't find expected panic")
+		}
+	}()
+	x := 0
+	// Force a stack copy, to make sure that the &cond pointer passed to defer
+	// function is properly updated.
+	growStackIter(&x, 1000)
+	cond = 2
+	doPanic()
+
+	// This function has no exit/return, since it ends with an infinite loop
+	for {
+	}
+}