cmd/compile: avoid truncating fieldname var locations

Don't include package path when creating LSyms for auto and param
variables during Prog generation, and update the DWARF emit routine
accordingly (remove the code that chops off package path from names in
DWARF var location expressions). Implementation suggested by mdempsky@.

The intent of this change is to have saner location expressions in cases
where the variable corresponds to a structure field. For example, the
SSA compiler's "decompose" phase can take a slice value and break it
apart into three scalar variables corresponding to the fields (slice "X"
gets split into "X.len", "X.cap", "X.ptr"). In such cases we want the
name in the location expression to omit the package path but preserve
the original variable name (e.g. "X").

Fixes #16338

Change-Id: Ibc444e7f3454b70fc500a33f0397e669d127daa1
Reviewed-on: https://go-review.googlesource.com/31819
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/pgen.go

@@ -423,6 +423,7 @@ func compile(fn *Node) {
 			fallthrough
 		case PPARAM, PPARAMOUT:
 			p := Gins(obj.ATYPE, n, nil)
+			p.From.Sym = obj.Linklookup(Ctxt, n.Sym.Name, 0)
 			p.To.Type = obj.TYPE_MEM
 			p.To.Name = obj.NAME_EXTERN
 			p.To.Sym = Linksym(ngotype(n))

src/cmd/internal/dwarf/dwarf.go

@@ -588,11 +588,6 @@ func PutFunc(ctxt Context, s Sym, name string, external bool, startPC Sym, size
 		}
 		names[n] = true
 
-		// Drop the package prefix from locals and arguments.
-		if i := strings.LastIndex(n, "."); i >= 0 {
-			n = n[i+1:]
-		}
-
 		Uleb128put(ctxt, s, int64(v.Abbrev))
 		putattr(ctxt, s, v.Abbrev, DW_FORM_string, DW_CLS_STRING, int64(len(n)), n)
 		loc := append(encbuf[:0], DW_OP_call_frame_cfa)

src/runtime/runtime-gdb_test.go

@@ -68,14 +68,18 @@ func checkGdbPython(t *testing.T) {
 const helloSource = `
 package main
 import "fmt"
+var gslice []string
 func main() {
 	mapvar := make(map[string]string,5)
 	mapvar["abc"] = "def"
 	mapvar["ghi"] = "jkl"
 	strvar := "abc"
 	ptrvar := &strvar
-	fmt.Println("hi") // line 10
+	slicevar := make([]string, 0, 16)
+	slicevar = append(slicevar, mapvar["abc"])
+	fmt.Println("hi") // line 12
 	_ = ptrvar
+	gslice = slicevar
 }
 `
 
@@ -120,6 +124,9 @@ func TestGdbPython(t *testing.T) {
 		"-ex", "echo BEGIN print strvar\n",
 		"-ex", "print strvar",
 		"-ex", "echo END\n",
+		"-ex", "echo BEGIN info locals\n",
+		"-ex", "info locals",
+		"-ex", "echo END\n",
 		"-ex", "down", // back to fmt.Println (goroutine 2 below only works at bottom of stack.  TODO: fix that)
 		"-ex", "echo BEGIN goroutine 2 bt\n",
 		"-ex", "goroutine 2 bt",
@@ -168,6 +175,15 @@ func TestGdbPython(t *testing.T) {
 		t.Fatalf("print strvar failed: %s", bl)
 	}
 
+	// Issue 16338: ssa decompose phase can split a structure into
+	// a collection of scalar vars holding the fields. In such cases
+	// the DWARF variable location expression should be of the
+	// form "var.field" and not just "field".
+	infoLocalsRe := regexp.MustCompile(`^slicevar.len = `)
+	if bl := blocks["info locals"]; !infoLocalsRe.MatchString(bl) {
+		t.Fatalf("info locals failed: %s", bl)
+	}
+
 	btGoroutineRe := regexp.MustCompile(`^#0\s+runtime.+at`)
 	if bl := blocks["goroutine 2 bt"]; !btGoroutineRe.MatchString(bl) {
 		t.Fatalf("goroutine 2 bt failed: %s", bl)