cmd/compile: when CSEing two values, prefer the statement marked one

Fixes #75249

Change-Id: Ifd61bf5341f23ce2c9735e607e00d987489caacf
Reviewed-on: https://go-review.googlesource.com/c/go/+/701295
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Alexander Musman <alexander.musman@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -0,0 +1,105 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dwarfgen
+
+import (
+	"debug/dwarf"
+	"internal/platform"
+	"internal/testenv"
+	"io"
+	"runtime"
+	"testing"
+)
+
+func TestIssue75249(t *testing.T) {
+	testenv.MustHaveGoRun(t)
+	t.Parallel()
+
+	if !platform.ExecutableHasDWARF(runtime.GOOS, runtime.GOARCH) {
+		t.Skipf("skipping on %s/%s: no DWARF symbol table in executables", runtime.GOOS, runtime.GOARCH)
+	}
+
+	code := `
+package main
+
+type Data struct {
+        Field1 int
+        Field2 *int
+        Field3 int
+        Field4 *int
+        Field5 int
+        Field6 *int
+        Field7 int
+        Field8 *int
+}
+
+//go:noinline
+func InitializeData(d *Data) {
+        d.Field1++            // line 16
+        d.Field2 = d.Field4
+        d.Field3++
+        d.Field4 = d.Field6
+        d.Field5++
+        d.Field6 = d.Field8
+        d.Field7++
+        d.Field8 = d.Field2   // line 23
+}
+
+func main() {
+        var data Data
+        InitializeData(&data)
+}
+`
+
+	_, f := gobuild(t, t.TempDir(), true, []testline{{line: code}})
+	defer f.Close()
+
+	dwarfData, err := f.DWARF()
+	if err != nil {
+		t.Fatal(err)
+	}
+	dwarfReader := dwarfData.Reader()
+
+	for {
+		entry, err := dwarfReader.Next()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if entry == nil {
+			break
+		}
+		if entry.Tag != dwarf.TagCompileUnit {
+			continue
+		}
+		name := entry.AttrField(dwarf.AttrName)
+		if name == nil || name.Class != dwarf.ClassString || name.Val != "main" {
+			continue
+		}
+		lr, err := dwarfData.LineReader(entry)
+		if err != nil {
+			t.Fatal(err)
+		}
+		stmts := map[int]bool{}
+		for {
+			var le dwarf.LineEntry
+			err := lr.Next(&le)
+			if err == io.EOF {
+				break
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !le.IsStmt {
+				continue
+			}
+			stmts[le.Line] = true
+		}
+		for i := 16; i <= 23; i++ {
+			if !stmts[i] {
+				t.Errorf("missing statement at line %d", i)
+			}
+		}
+	}
+}

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

@@ -181,9 +181,10 @@ func cse(f *Func) {
 	for _, e := range partition {
 		slices.SortFunc(e, func(v, w *Value) int {
 			c := cmp.Compare(sdom.domorder(v.Block), sdom.domorder(w.Block))
-			if v.Op != OpLocalAddr || c != 0 {
+			if c != 0 {
 				return c
 			}
+			if v.Op == OpLocalAddr {
 				// compare the memory args for OpLocalAddrs in the same block
 				vm := v.Args[1]
 				wm := w.Args[1]
@@ -209,6 +210,16 @@ func cse(f *Func) {
 					f.Fatalf("unable to determine the order of %s", wm.LongString())
 				}
 				return cmp.Compare(vs, ws)
+			}
+			vStmt := v.Pos.IsStmt() == src.PosIsStmt
+			wStmt := w.Pos.IsStmt() == src.PosIsStmt
+			if vStmt != wStmt {
+				if vStmt {
+					return -1
+				}
+				return +1
+			}
+			return 0
 		})
 
 		for i := 0; i < len(e)-1; i++ {