runtime: generate windows/arm64 callback asm

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.

Change-Id: I5e2b589797808626bcca771cdf860d5cb85586cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/288826
Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/runtime/syscall_windows.go

@@ -71,8 +71,8 @@ func callbackasmAddr(i int) uintptr {
 		panic("unsupported architecture")
 	case "386", "amd64":
 		entrySize = 5
-	case "arm":
+	case "arm", "arm64":
-		// On ARM, each entry is a MOV instruction
+		// On ARM and ARM64, each entry is a MOV instruction
 		// followed by a branch instruction
 		entrySize = 8
 	}

src/runtime/wincallback.go

@@ -72,6 +72,34 @@ TEXT runtime·callbackasm(SB),NOSPLIT|NOFRAME,$0
 	}
 }
 
+func genasmArm64() {
+	var buf bytes.Buffer
+
+	buf.WriteString(`// Code generated by wincallback.go using 'go generate'. DO NOT EDIT.
+
+// 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 runtime·callbackasm(SB),NOSPLIT|NOFRAME,$0
+`)
+	for i := 0; i < maxCallback; i++ {
+		buf.WriteString(fmt.Sprintf("\tMOVD\t$%d, R12\n", i))
+		buf.WriteString("\tB\truntime·callbackasm1(SB)\n")
+	}
+
+	err := os.WriteFile("zcallback_windows_arm64.s", buf.Bytes(), 0666)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "wincallback: %s\n", err)
+		os.Exit(2)
+	}
+}
+
 func gengo() {
 	var buf bytes.Buffer
 
@@ -91,5 +119,6 @@ const cb_max = %d // maximum number of windows callbacks allowed
 func main() {
 	genasm386Amd64()
 	genasmArm()
+	genasmArm64()
 	gengo()
 }