runtime: support nil libcall.args when calling stdcall on Windows

Having to pass a dummy pointer to the libcall.args field is a bit
annoying. This change allows nil to be passed instead.

windows/arm and windows/arm64 already support nil libcall.args.

Change-Id: I07a2bdb7d1f76b13d125397ff5177337c43536a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/526016
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/runtime/os_windows.go

@@ -985,7 +985,7 @@ func stdcall(fn stdFunction) uintptr {
 func stdcall0(fn stdFunction) uintptr {
 	mp := getg().m
 	mp.libcall.n = 0
-	mp.libcall.args = uintptr(noescape(unsafe.Pointer(&fn))) // it's unused but must be non-nil, otherwise crashes
+	mp.libcall.args = 0
 	return stdcall(fn)
 }
 

src/runtime/sys_windows_386.s

@@ -14,13 +14,18 @@
 // void runtime·asmstdcall(void *c);
 TEXT runtime·asmstdcall(SB),NOSPLIT,$0
 	MOVL	fn+0(FP), BX
+	MOVL	SP, BP	// save stack pointer
 
 	// SetLastError(0).
 	MOVL	$0, 0x34(FS)
 
+	MOVL	libcall_n(BX), CX
+
+	// Fast version, do not store args on the stack.
+	CMPL	CX, $0
+	JE	docall
+
 	// Copy args to the stack.
-	MOVL	SP, BP
-	MOVL	libcall_n(BX), CX	// words
 	MOVL	CX, AX
 	SALL	$2, AX
 	SUBL	AX, SP			// room for args
@@ -29,6 +34,7 @@ TEXT runtime·asmstdcall(SB),NOSPLIT,$0
 	CLD
 	REP; MOVSL
 
+docall:
 	// Call stdcall or cdecl function.
 	// DI SI BP BX are preserved, SP is not
 	CALL	libcall_fn(BX)

src/runtime/sys_windows_amd64.s

@@ -29,6 +29,11 @@ TEXT runtime·asmstdcall(SB),NOSPLIT,$16
 
 	SUBQ	$(const_maxArgs*8), SP	// room for args
 
+	// Fast version, do not store args on the stack nor
+	// load them into registers.
+	CMPL	CX, $0
+	JE	docall
+
 	// Fast version, do not store args on the stack.
 	CMPL	CX, $4
 	JLE	loadregs
@@ -59,6 +64,7 @@ loadregs:
 	MOVQ	R8, X2
 	MOVQ	R9, X3
 
+docall:
 	// Call stdcall function.
 	CALL	AX