runtime: support windows/arm

Updates #26148

Change-Id: I8f68b2c926c7b11dc86c9664ed7ff2d2f78b64b4
Reviewed-on: https://go-review.googlesource.com/128715
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 19 insertions, 5 deletions

diff --git a/src/runtime/syscall_windows.go b/src/runtime/syscall_windows.go
index 8264070569..0858efaf61 100644
--- a/src/runtime/syscall_windows.go
+++ b/src/runtime/syscall_windows.go
@@ -25,18 +25,32 @@ func (c *wincallbackcontext) setCleanstack(cleanstack bool) {
 var (
 	cbs     callbacks
 	cbctxts **wincallbackcontext = &cbs.ctxt[0] // to simplify access to cbs.ctxt in sys_windows_*.s
-
-	callbackasm byte // type isn't really byte, it's code in runtime
 )
 
+func callbackasm()
+
 // callbackasmAddr returns address of runtime.callbackasm
 // function adjusted by i.
-// runtime.callbackasm is just a series of CALL instructions
-// (each is 5 bytes long), and we want callback to arrive at
+// On x86 and amd64, runtime.callbackasm is a series of CALL instructions,
+// and we want callback to arrive at
 // correspondent call instruction instead of start of
 // runtime.callbackasm.
+// On ARM, runtime.callbackasm is a series of mov and branch instructions.
+// R12 is loaded with the callback index. Each entry is two instructions,
+// hence 8 bytes.
 func callbackasmAddr(i int) uintptr {
-	return uintptr(add(unsafe.Pointer(&callbackasm), uintptr(i*5)))
+	var entrySize int
+	switch GOARCH {
+	default:
+		panic("unsupported architecture")
+	case "386", "amd64":
+		entrySize = 5
+	case "arm":
+		// On ARM, each entry is a MOV instruction
+		// followed by a branch instruction
+		entrySize = 8
+	}
+	return funcPC(callbackasm) + uintptr(i*entrySize)
 }
 
 //go:linkname compileCallback syscall.compileCallback