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/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go
index d7e931d0b8..e125ae4239 100644
--- a/src/cmd/compile/internal/ssa/regalloc.go
+++ b/src/cmd/compile/internal/ssa/regalloc.go
@@ -683,7 +683,7 @@ func (s *regAllocState) init(f *Func) {
 	s.endRegs = make([][]endReg, f.NumBlocks())
 	s.startRegs = make([][]startReg, f.NumBlocks())
 	s.spillLive = make([][]ID, f.NumBlocks())
-	s.sdom = f.sdom()
+	s.sdom = f.Sdom()
 
 	// wasm: Mark instructions that can be optimized to have their values only on the WebAssembly stack.
 	if f.Config.ctxt.Arch.Arch == sys.ArchWasm {
@@ -1916,7 +1916,7 @@ func (e *edgeState) setup(idx int, srcReg []endReg, dstReg []startReg, stacklive
 	for _, spillID := range stacklive {
 		v := e.s.orig[spillID]
 		spill := e.s.values[v.ID].spill
-		if !e.s.sdom.isAncestorEq(spill.Block, e.p) {
+		if !e.s.sdom.IsAncestorEq(spill.Block, e.p) {
 			// Spills were placed that only dominate the uses found
 			// during the first regalloc pass. The edge fixup code
 			// can't use a spill location if the spill doesn't dominate