ssa: clarify semantics of ID-to-PC translation function

Make it clear that the `block` argument is not always used, and stop
passing it in a case where it didn't really make sense - at the end of a
function.

Change-Id: I2fa86bed6ceb53a1b1cbfda5c3392e7e9da9579d
Reviewed-on: https://go-review.googlesource.com/c/go/+/502115
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: David Chase <drchase@google.com>

Author: andreimatei

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

@@ -42,7 +42,10 @@ type FuncDebug struct {
 	OptDcl []*ir.Name
 
 	// Filled in by the user. Translates Block and Value ID to PC.
-	GetPC func(ID, ID) int64
+	//
+	// NOTE: block is only used if value is BlockStart.ID or BlockEnd.ID.
+	// Otherwise, it is ignored.
+	GetPC func(block, value ID) int64
 }
 
 type BlockDebug struct {
@@ -1368,7 +1371,7 @@ func (state *debugState) buildLocationLists(blockLocs []*BlockDebug) {
 
 	// Flush any leftover entries live at the end of the last block.
 	for varID := range state.lists {
-		state.writePendingEntry(VarID(varID), state.f.Blocks[len(state.f.Blocks)-1].ID, FuncEnd.ID)
+		state.writePendingEntry(VarID(varID), -1, FuncEnd.ID)
 		list := state.lists[varID]
 		if state.loggingLevel > 0 {
 			if len(list) == 0 {

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

@@ -7578,9 +7578,9 @@ func genssa(f *ssa.Func, pp *objw.Progs) {
 		for i, b := range f.Blocks {
 			idToIdx[b.ID] = i
 		}
-		// Note that at this moment, Prog.Pc is a sequence number; it's
-		// not a real PC until after assembly, so this mapping has to
-		// be done later.
+		// Register a callback that will be used later to fill in PCs into location
+		// lists. At the moment, Prog.Pc is a sequence number; it's not a real PC
+		// until after assembly, so the translation needs to be deferred.
 		debugInfo.GetPC = func(b, v ssa.ID) int64 {
 			switch v {
 			case ssa.BlockStart.ID: