cmd/compile: emit DIEs for zero sized variables

Fixes the compiler to emit DIEs for zero sized variables.

Fixes #54615

Change-Id: I1e0c86a97f1abcc7edae516b6a7fe35bcb65ed0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/433479
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/dwarfgen/dwarf.go

@@ -151,6 +151,21 @@ func createDwarfVars(fnsym *obj.LSym, complexOK bool, fn *ir.Func, apDecls []*ir
 	} else {
 		decls, vars, selected = createSimpleVars(fnsym, apDecls)
 	}
+	if fn.DebugInfo != nil {
+		// Recover zero sized variables eliminated by the stackframe pass
+		for _, n := range fn.DebugInfo.(*ssa.FuncDebug).OptDcl {
+			if n.Class != ir.PAUTO {
+				continue
+			}
+			types.CalcSize(n.Type())
+			if n.Type().Size() == 0 {
+				decls = append(decls, n)
+				vars = append(vars, createSimpleVar(fnsym, n))
+				vars[len(vars)-1].StackOffset = 0
+				fnsym.Func().RecordAutoType(reflectdata.TypeLinksym(n.Type()))
+			}
+		}
+	}
 
 	dcl := apDecls
 	if fnsym.WasInlined() {

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

@@ -38,6 +38,8 @@ type FuncDebug struct {
 	// Register-resident output parameters for the function. This is filled in at
 	// SSA generation time.
 	RegOutputParams []*ir.Name
+	// Variable declarations that were removed during optimization
+	OptDcl []*ir.Name
 
 	// Filled in by the user. Translates Block and Value ID to PC.
 	GetPC func(ID, ID) int64

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

@@ -140,6 +140,7 @@ func (s *ssafn) AllocFrame(f *ssa.Func) {
 			continue
 		}
 		if !n.Used() {
+			fn.DebugInfo.(*ssa.FuncDebug).OptDcl = fn.Dcl[i:]
 			fn.Dcl = fn.Dcl[:i]
 			break
 		}

src/cmd/link/internal/ld/dwarf_test.go

@@ -1917,3 +1917,56 @@ func main() {
 		t.Errorf("no LPT entries for test.go")
 	}
 }
+
+func TestZeroSizedVariable(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+	t.Parallel()
+
+	// This test verifies that the compiler emits DIEs for zero sized variables
+	// (for example variables of type 'struct {}').
+	// See go.dev/issues/54615.
+
+	const prog = `
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	zeroSizedVariable := struct{}{}
+	fmt.Println(zeroSizedVariable)
+}
+`
+
+	for _, opt := range []string{NoOpt, DefaultOpt} {
+		dir := t.TempDir()
+		f := gobuild(t, dir, prog, opt)
+		defer f.Close()
+		defer os.RemoveAll(dir)
+
+		d, err := f.DWARF()
+		if err != nil {
+			t.Fatalf("error reading DWARF: %v", err)
+		}
+
+		rdr := d.Reader()
+		ex := dwtest.Examiner{}
+		if err := ex.Populate(rdr); err != nil {
+			t.Fatalf("error reading DWARF: %v", err)
+		}
+
+		// Locate the main.zeroSizedVariable DIE
+		abcs := ex.Named("zeroSizedVariable")
+		if len(abcs) == 0 {
+			t.Fatalf("unable to locate DIE for zeroSizedVariable")
+		}
+		if len(abcs) != 1 {
+			t.Fatalf("more than one zeroSizedVariable DIE")
+		}
+	}
+}