cmd/compile: Avoid and filter out zero-length location-lists.

This change avoids creating zero length location lists by
repairing an overly aggressive change in CL146718
and by explicitly checking for and filtering out any
zero-length lists that are detected (building
compiler+runtime creates a single one).

Updates #28486.

Change-Id: I01c571fee2376474c7f3038e801bd58fd9e0b820
Reviewed-on: https://go-review.googlesource.com/c/150097
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/compile/internal/ssa/debug.go

@@ -881,12 +881,11 @@ func (state *debugState) buildLocationLists(blockLocs []*BlockDebug) {
 	for _, b := range state.f.Blocks {
 		state.mergePredecessors(b, blockLocs, prevBlock)
 
-		// Handle any differences among predecessor blocks and previous block (perhaps not a predecessor)
-		for _, varID := range state.changedVars.contents() {
-			state.updateVar(VarID(varID), b, BlockStart)
-		}
-
 		if !blockLocs[b.ID].relevant {
+			// Handle any differences among predecessor blocks and previous block (perhaps not a predecessor)
+			for _, varID := range state.changedVars.contents() {
+				state.updateVar(VarID(varID), b, BlockStart)
+			}
 			continue
 		}
 
@@ -1019,6 +1018,14 @@ func (state *debugState) writePendingEntry(varID VarID, endBlock, endValue ID) {
 		// they get incomplete debug info on 32-bit platforms.
 		return
 	}
+	if start == end {
+		if state.loggingEnabled {
+			// Printf not logf so not gated by GOSSAFUNC; this should fire very rarely.
+			fmt.Printf("Skipping empty location list for %v in %s\n", state.vars[varID], state.f.Name)
+		}
+		return
+	}
+
 	list := state.lists[varID]
 	list = appendPtr(state.ctxt, list, start)
 	list = appendPtr(state.ctxt, list, end)