runtime: use the right number of parameters in syscall_SyscallX on Windows

The syscall_SyscallX functions currently discard the nargs parameter
when calling syscall_SyscallN. This precludes some optimizations
down the line. For example, on amd64, a syscall that takes 0 arguments
don't need to set any of the params passing registers (CX, DX, R8, and
R9).

This CL updates all syscall_SyscallX functions so they call
syscall_SyscallN with an argument slice of the right length.

While here, remove the hack in syscall_SyscallN to support less than 4
arguments, and update instead asmstdcall on amd64 to properly handle
this case.

Change-Id: I0328e14f34c2b000fde06cc6a579b09e8c32f2b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/563315
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/runtime/sys_windows_amd64.s

@@ -33,14 +33,12 @@ 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
+	CMPL	CX, $0;	JE	_0args
-	JLE	loadregs
+	CMPL	CX, $1;	JE	_1args
+	CMPL	CX, $2;	JE	_2args
+	CMPL	CX, $3;	JE	_3args
+	CMPL	CX, $4;	JE	_4args
 
 	// Check we have enough room for args.
 	CMPL	CX, $const_maxArgs
@@ -53,22 +51,25 @@ TEXT runtime·asmstdcall(SB),NOSPLIT,$16
 	REP; MOVSQ
 	MOVQ	SP, SI
 
-loadregs:
 	// Load first 4 args into correspondent registers.
-	MOVQ	0(SI), CX
-	MOVQ	8(SI), DX
-	MOVQ	16(SI), R8
-	MOVQ	24(SI), R9
 	// Floating point arguments are passed in the XMM
 	// registers. Set them here in case any of the arguments
 	// are floating point values. For details see
 	//	https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170
-	MOVQ	CX, X0
+_4args:
-	MOVQ	DX, X1
+	MOVQ	24(SI), R9
-	MOVQ	R8, X2
 	MOVQ	R9, X3
+_3args:
+	MOVQ	16(SI), R8
+	MOVQ	R8, X2
+_2args:
+	MOVQ	8(SI), DX
+	MOVQ	DX, X1
+_1args:
+	MOVQ	0(SI), CX
+	MOVQ	CX, X0
+_0args:
 
-docall:
 	// Call stdcall function.
 	CALL	AX
 

src/runtime/syscall_windows.go

@@ -415,63 +415,36 @@ const _LOAD_LIBRARY_SEARCH_SYSTEM32 = 0x00000800
 
 //go:linkname syscall_loadsystemlibrary syscall.loadsystemlibrary
 //go:nosplit
-//go:cgo_unsafe_args
 func syscall_loadsystemlibrary(filename *uint16) (handle, err uintptr) {
-	lockOSThread()
+	fn := getLoadLibraryEx()
-	c := &getg().m.syscall
+	handle, _, err = syscall_SyscallN(fn, uintptr(unsafe.Pointer(filename)), 0, _LOAD_LIBRARY_SEARCH_SYSTEM32)
-	c.fn = getLoadLibraryEx()
-	c.n = 3
-	args := struct {
-		lpFileName *uint16
-		hFile      uintptr // always 0
-		flags      uint32
-	}{filename, 0, _LOAD_LIBRARY_SEARCH_SYSTEM32}
-	c.args = uintptr(noescape(unsafe.Pointer(&args)))
-
-	cgocall(asmstdcallAddr, unsafe.Pointer(c))
 	KeepAlive(filename)
-	handle = c.r1
+	if handle != 0 {
-	if handle == 0 {
+		err = 0
-		err = c.err
 	}
-	unlockOSThread() // not defer'd after the lockOSThread above to save stack frame size.
 	return
 }
 
 //go:linkname syscall_loadlibrary syscall.loadlibrary
 //go:nosplit
-//go:cgo_unsafe_args
 func syscall_loadlibrary(filename *uint16) (handle, err uintptr) {
-	lockOSThread()
+	fn := getLoadLibrary()
-	defer unlockOSThread()
+	handle, _, err = syscall_SyscallN(fn, uintptr(unsafe.Pointer(filename)))
-	c := &getg().m.syscall
-	c.fn = getLoadLibrary()
-	c.n = 1
-	c.args = uintptr(noescape(unsafe.Pointer(&filename)))
-	cgocall(asmstdcallAddr, unsafe.Pointer(c))
 	KeepAlive(filename)
-	handle = c.r1
+	if handle != 0 {
-	if handle == 0 {
+		err = 0
-		err = c.err
 	}
 	return
 }
 
 //go:linkname syscall_getprocaddress syscall.getprocaddress
 //go:nosplit
-//go:cgo_unsafe_args
 func syscall_getprocaddress(handle uintptr, procname *byte) (outhandle, err uintptr) {
-	lockOSThread()
+	fn := getGetProcAddress()
-	defer unlockOSThread()
+	outhandle, _, err = syscall_SyscallN(fn, handle, uintptr(unsafe.Pointer(procname)))
-	c := &getg().m.syscall
-	c.fn = getGetProcAddress()
-	c.n = 2
-	c.args = uintptr(noescape(unsafe.Pointer(&handle)))
-	cgocall(asmstdcallAddr, unsafe.Pointer(c))
 	KeepAlive(procname)
-	outhandle = c.r1
+	if outhandle != 0 {
-	if outhandle == 0 {
+		err = 0
-		err = c.err
 	}
 	return
 }
@@ -479,37 +452,43 @@ func syscall_getprocaddress(handle uintptr, procname *byte) (outhandle, err uint
 //go:linkname syscall_Syscall syscall.Syscall
 //go:nosplit
 func syscall_Syscall(fn, nargs, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
-	return syscall_SyscallN(fn, a1, a2, a3)
+	args := [...]uintptr{a1, a2, a3}
+	return syscall_SyscallN(fn, args[:nargs]...)
 }
 
 //go:linkname syscall_Syscall6 syscall.Syscall6
 //go:nosplit
 func syscall_Syscall6(fn, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr) {
-	return syscall_SyscallN(fn, a1, a2, a3, a4, a5, a6)
+	args := [...]uintptr{a1, a2, a3, a4, a5, a6}
+	return syscall_SyscallN(fn, args[:nargs]...)
 }
 
 //go:linkname syscall_Syscall9 syscall.Syscall9
 //go:nosplit
 func syscall_Syscall9(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2, err uintptr) {
-	return syscall_SyscallN(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9)
+	args := [...]uintptr{a1, a2, a3, a4, a5, a6, a7, a8, a9}
+	return syscall_SyscallN(fn, args[:nargs]...)
 }
 
 //go:linkname syscall_Syscall12 syscall.Syscall12
 //go:nosplit
 func syscall_Syscall12(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12 uintptr) (r1, r2, err uintptr) {
-	return syscall_SyscallN(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12)
+	args := [...]uintptr{a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12}
+	return syscall_SyscallN(fn, args[:nargs]...)
 }
 
 //go:linkname syscall_Syscall15 syscall.Syscall15
 //go:nosplit
 func syscall_Syscall15(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15 uintptr) (r1, r2, err uintptr) {
-	return syscall_SyscallN(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15)
+	args := [...]uintptr{a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15}
+	return syscall_SyscallN(fn, args[:nargs]...)
 }
 
 //go:linkname syscall_Syscall18 syscall.Syscall18
 //go:nosplit
 func syscall_Syscall18(fn, nargs, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18 uintptr) (r1, r2, err uintptr) {
-	return syscall_SyscallN(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18)
+	args := [...]uintptr{a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18}
+	return syscall_SyscallN(fn, args[:nargs]...)
 }
 
 // maxArgs should be divisible by 2, as Windows stack
@@ -521,26 +500,22 @@ const maxArgs = 42
 
 //go:linkname syscall_SyscallN syscall.SyscallN
 //go:nosplit
-func syscall_SyscallN(trap uintptr, args ...uintptr) (r1, r2, err uintptr) {
+func syscall_SyscallN(fn uintptr, args ...uintptr) (r1, r2, err uintptr) {
-	nargs := len(args)
+	if len(args) > maxArgs {
-
-	// asmstdcall expects it can access the first 4 arguments
-	// to load them into registers.
-	var tmp [4]uintptr
-	switch {
-	case nargs < 4:
-		copy(tmp[:], args)
-		args = tmp[:]
-	case nargs > maxArgs:
 		panic("runtime: SyscallN has too many arguments")
 	}
 
+	// The cgocall parameters are stored in m instead of in
+	// the stack because the stack can move during if fn
+	// calls back into Go.
 	lockOSThread()
 	defer unlockOSThread()
 	c := &getg().m.syscall
-	c.fn = trap
+	c.fn = fn
-	c.n = uintptr(nargs)
+	c.n = uintptr(len(args))
-	c.args = uintptr(noescape(unsafe.Pointer(&args[0])))
+	if c.n != 0 {
+		c.args = uintptr(noescape(unsafe.Pointer(&args[0])))
+	}
 	cgocall(asmstdcallAddr, unsafe.Pointer(c))
 	return c.r1, c.r2, c.err
 }