internal/bytealg: extend memchr result correctly on wasm

The mem address should be regarded as uint32.

Fixes #65571

Change-Id: Icee38d11f2d93eeca7d50b2e133159e321daeb90
GitHub-Last-Rev: c2568b1
GitHub-Pull-Request: #68400
Reviewed-on: https://go-review.googlesource.com/c/go/+/597955
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/bytes/bytes_js_wasm_test.go

@@ -0,0 +1,21 @@
+// Copyright 2024 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.
+
+//go:build js && wasm
+
+package bytes_test
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestIssue65571(t *testing.T) {
+	b := make([]byte, 1<<31+1)
+	b[1<<31] = 1
+	i := bytes.IndexByte(b, 1)
+	if i != 1<<31 {
+		t.Errorf("IndexByte(b, 1) = %d; want %d", i, 1<<31)
+	}
+}

src/internal/bytealg/indexbyte_wasm.s

@@ -12,7 +12,7 @@ TEXT ·IndexByte(SB), NOSPLIT, $0-40
 	I64Load b_len+8(FP)
 	I32WrapI64
 	Call memchr<>(SB)
-	I64ExtendI32S
+	I64ExtendI32U
 	Set R0
 
 	Get SP
@@ -35,7 +35,7 @@ TEXT ·IndexByteString(SB), NOSPLIT, $0-32
 	I64Load s_len+8(FP)
 	I32WrapI64
 	Call memchr<>(SB)
-	I64ExtendI32S
+	I64ExtendI32U
 	Set R0
 
 	I64Const $-1