purego: add wincallback.go (#337)

zcallback_amd64.s and zcallback_arm64.s were generated by wincallback.go
in the Go repository. To keep the reproducible repository states,
add wincallback.go with some modifications.

Author: hajimehoshi

README.md

@@ -84,6 +84,7 @@ License that can be found [in the Go Source](https://github.com/golang/go/blob/m
 This is a list of the copied files:
 
 * `abi_*.h` from package `runtime/cgo`
+* `wincallback.go` from package `runtime`
 * `zcallback_darwin_*.s` from package `runtime`
 * `internal/fakecgo/abi_*.h` from package `runtime/cgo`
 * `internal/fakecgo/asm_GOARCH.s` from package `runtime/cgo`

gen.go

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: 2025 The Ebitengine Authors
+
+package purego
+
+//go:generate go run wincallback.go

wincallback.go

@@ -0,0 +1,78 @@
+// Copyright 2014 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 ignore
+
+// Generate Windows callback assembly file.
+// This was copied from the Go repository and modified to generate non-Windows assembly files.
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+)
+
+const maxCallback = 2000
+
+func genasmAmd64() {
+	var buf bytes.Buffer
+
+	buf.WriteString(`// Code generated by wincallback.go using 'go generate'. DO NOT EDIT.
+
+//go:build darwin || freebsd || linux || netbsd
+
+// runtime·callbackasm is called by external code to
+// execute Go implemented callback function. It is not
+// called from the start, instead runtime·compilecallback
+// always returns address into runtime·callbackasm offset
+// appropriately so different callbacks start with different
+// CALL instruction in runtime·callbackasm. This determines
+// which Go callback function is executed later on.
+#include "textflag.h"
+
+TEXT callbackasm(SB),NOSPLIT|NOFRAME,$0
+`)
+	for i := 0; i < maxCallback; i++ {
+		buf.WriteString("\tCALL\tcallbackasm1(SB)\n")
+	}
+	if err := os.WriteFile("zcallback_amd64.s", buf.Bytes(), 0644); err != nil {
+		fmt.Fprintf(os.Stderr, "wincallback: %s\n", err)
+		os.Exit(2)
+	}
+}
+
+func genasmArm64() {
+	var buf bytes.Buffer
+
+	buf.WriteString(`// Code generated by wincallback.go using 'go generate'. DO NOT EDIT.
+
+//go:build darwin || freebsd || linux || netbsd
+
+// External code calls into callbackasm at an offset corresponding
+// to the callback index. Callbackasm is a table of MOV and B instructions.
+// The MOV instruction loads R12 with the callback index, and the
+// B instruction branches to callbackasm1.
+// callbackasm1 takes the callback index from R12 and
+// indexes into an array that stores information about each callback.
+// It then calls the Go implementation for that callback.
+#include "textflag.h"
+
+TEXT callbackasm(SB),NOSPLIT|NOFRAME,$0
+`)
+	for i := 0; i < maxCallback; i++ {
+		fmt.Fprintf(&buf, "\tMOVD\t$%d, R12\n", i)
+		buf.WriteString("\tB\tcallbackasm1(SB)\n")
+	}
+	if err := os.WriteFile("zcallback_arm64.s", buf.Bytes(), 0644); err != nil {
+		fmt.Fprintf(os.Stderr, "wincallback: %s\n", err)
+		os.Exit(2)
+	}
+}
+
+func main() {
+	genasmAmd64()
+	genasmArm64()
+}