cmd/compile: fix defer/deferreturn

Make sure we do any just-before-return cleanup on all paths out of a
function, including when recovering.  Each exit path should include
deferreturn (if there are any defers) and then the exit
code (e.g. copying heap-escaping return values back to the stack).

Introduce a Defer SSA block type which has two outgoing edges - one the
fallthrough edge (the defer was queued successfully) and one which
immediately returns (the defer had a successful recover() call and
normal execution should resume at the return point).

Fixes #14725

Change-Id: Iad035c9fd25ef8b7a74dafbd7461cf04833d981f
Reviewed-on: https://go-review.googlesource.com/20486
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/check.go b/src/cmd/compile/internal/ssa/check.go
index 7243cdc310..83aae3af33 100644
--- a/src/cmd/compile/internal/ssa/check.go
+++ b/src/cmd/compile/internal/ssa/check.go
@@ -125,6 +125,16 @@ func checkFunc(f *Func) {
 			if !b.Control.Type.IsMemory() {
 				f.Fatalf("call block %s has non-memory control value %s", b, b.Control.LongString())
 			}
+		case BlockDefer:
+			if len(b.Succs) != 2 {
+				f.Fatalf("defer block %s len(Succs)==%d, want 2", b, len(b.Succs))
+			}
+			if b.Control == nil {
+				f.Fatalf("defer block %s has no control value", b)
+			}
+			if !b.Control.Type.IsMemory() {
+				f.Fatalf("defer block %s has non-memory control value %s", b, b.Control.LongString())
+			}
 		case BlockCheck:
 			if len(b.Succs) != 1 {
 				f.Fatalf("check block %s len(Succs)==%d, want 1", b, len(b.Succs))