cmd/compile/internal/escape: evaluate any side effects when rewriting with literals

CL 649035 and CL 649079 updated escape analysis to rewrite
certain operands in OMAKE and OCONVIFACE nodes from non-constant
expressions to basic literals that evaluate to the same value.

However, when doing that rewriting, we need to evaluate any
side effects prior to replacing the expression, which is what
this CL now does.

Issue #74379 reported a problem with OCONVIFACE nodes due to CL 649079.
CL 649035 has essentially the same issue with OMAKE nodes. To illustrate
that, we add a test for the OMAKE case in fixedbugs/issue74379b.go,
which fails without this change. To avoid introducing an unnecessary
temporary for OMAKE nodes, we also conditionalize the main work of
CL 649035 on whether the OMAKE operand is already an OLITERAL.

CL 649555 and CL 649078 were related changes that created read-only
global storage for composite literals used in an interface conversion.
This CL adds a test in fixedbugs/issue74379c.go to illustrate
that they do not have the same problem.

Updates #71359
Fixes #74379

Change-Id: I6645575ef34f1fe2b0241a22dc205875d66b7ada
Reviewed-on: https://go-review.googlesource.com/c/go/+/684116
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/compile/internal/escape/escape.go

@@ -545,6 +545,14 @@ func (b *batch) rewriteWithLiterals(n ir.Node, fn *ir.Func) {
 		base.Fatalf("no ReassignOracle for function %v with closure parent %v", fn, fn.ClosureParent)
 	}
 
+	assignTemp := func(n ir.Node, init *ir.Nodes) {
+		// Preserve any side effects of n by assigning it to an otherwise unused temp.
+		pos := n.Pos()
+		tmp := typecheck.TempAt(pos, fn, n.Type())
+		init.Append(typecheck.Stmt(ir.NewDecl(pos, ir.ODCL, tmp)))
+		init.Append(typecheck.Stmt(ir.NewAssignStmt(pos, tmp, n)))
+	}
+
 	switch n.Op() {
 	case ir.OMAKESLICE:
 		// Check if we can replace a non-constant argument to make with
@@ -556,13 +564,17 @@ func (b *batch) rewriteWithLiterals(n ir.Node, fn *ir.Func) {
 			r = &n.Len
 		}
 
-		if s := ro.StaticValue(*r); s.Op() == ir.OLITERAL {
-			lit, ok := s.(*ir.BasicLit)
-			if !ok || lit.Val().Kind() != constant.Int {
-				base.Fatalf("unexpected BasicLit Kind")
-			}
-			if constant.Compare(lit.Val(), token.GEQ, constant.MakeInt64(0)) {
-				*r = lit
+		if (*r).Op() != ir.OLITERAL {
+			if s := ro.StaticValue(*r); s.Op() == ir.OLITERAL {
+				lit, ok := s.(*ir.BasicLit)
+				if !ok || lit.Val().Kind() != constant.Int {
+					base.Fatalf("unexpected BasicLit Kind")
+				}
+				if constant.Compare(lit.Val(), token.GEQ, constant.MakeInt64(0)) {
+					// Preserve any side effects of the original expression, then replace it.
+					assignTemp(*r, n.PtrInit())
+					*r = lit
+				}
 			}
 		}
 	case ir.OCONVIFACE:
@@ -575,6 +587,8 @@ func (b *batch) rewriteWithLiterals(n ir.Node, fn *ir.Func) {
 				if base.Debug.EscapeDebug >= 3 {
 					base.WarnfAt(n.Pos(), "rewriting OCONVIFACE value from %v (%v) to %v (%v)", conv.X, conv.X.Type(), v, v.Type())
 				}
+				// Preserve any side effects of the original expression, then replace it.
+				assignTemp(conv.X, conv.PtrInit())
 				v := v.(*ir.BasicLit)
 				conv.X = ir.NewBasicLit(conv.X.Pos(), conv.X.Type(), v.Val())
 				typecheck.Expr(conv)

test/fixedbugs/issue74379.go

@@ -0,0 +1,30 @@
+// run
+
+// 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 main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+)
+
+func crashOnErr(err error) bool {
+	if err != nil {
+		panic(err)
+	}
+	return false
+}
+
+func main() {
+	defer func() {
+		if recover() == nil {
+			fmt.Println("failed to have expected panic")
+			os.Exit(1)
+		}
+	}()
+	fmt.Println(crashOnErr(errors.New("test error")))
+}

test/fixedbugs/issue74379b.go

@@ -0,0 +1,32 @@
+// run
+
+// 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 main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+)
+
+func crashOnErr(err error) int {
+	if err != nil {
+		panic(err)
+	}
+	return 10
+}
+
+func main() {
+	defer func() {
+		if recover() == nil {
+			fmt.Println("failed to have expected panic")
+			os.Exit(1)
+		}
+	}()
+
+	s := make([]int, crashOnErr(errors.New("test error")))
+	println("unreachable: len(s) =", len(s))
+}

test/fixedbugs/issue74379c.go

@@ -0,0 +1,54 @@
+// run
+
+// 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 main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+)
+
+type S struct{ a, b int }
+
+func crashOnErr1(err error) S {
+	if err != nil {
+		panic(err)
+	}
+	return S{} // zero value struct
+}
+
+func f1() {
+	defer func() {
+		if recover() == nil {
+			fmt.Println("failed to have expected panic")
+			os.Exit(1)
+		}
+	}()
+	fmt.Println(crashOnErr1(errors.New("test error")))
+}
+
+func crashOnErr2(err error) S {
+	if err != nil {
+		panic(err)
+	}
+	return S{1, 2} // not zero value struct
+}
+
+func f2() {
+	defer func() {
+		if recover() == nil {
+			fmt.Println("failed to have expected panic")
+			os.Exit(1)
+		}
+	}()
+	fmt.Println(crashOnErr2(errors.New("test error")))
+}
+
+func main() {
+	f1()
+	f2()
+}