runtime: use dispatch semaphores on Darwin

Changes Darwin semaphore support from using pthread mutexes and
condition variables to using dispatch semaphores. Signaling a dispatch
semaphore is async-signal-safe.

Fixes #31264

Change-Id: If0ce47623501db13e3804b14ace5f4d8eaef461e
Reviewed-on: https://go-review.googlesource.com/c/go/+/182258Reviewed-by: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/os_darwin.go

@@ -7,10 +7,7 @@ package runtime
 import "unsafe"
 
 type mOS struct {
-	initialized bool
-	mutex       pthreadmutex
-	cond        pthreadcond
-	count       int
+	sema uintptr
 }
 
 func unimplemented(name string) {
@@ -20,59 +17,32 @@ func unimplemented(name string) {
 
 //go:nosplit
 func semacreate(mp *m) {
-	if mp.initialized {
-		return
-	}
-	mp.initialized = true
-	if err := pthread_mutex_init(&mp.mutex, nil); err != 0 {
-		throw("pthread_mutex_init")
-	}
-	if err := pthread_cond_init(&mp.cond, nil); err != 0 {
-		throw("pthread_cond_init")
+	if mp.sema == 0 {
+		mp.sema = dispatch_semaphore_create(0)
 	}
 }
 
+const (
+	_DISPATCH_TIME_NOW     = uint64(0)
+	_DISPATCH_TIME_FOREVER = ^uint64(0)
+)
+
 //go:nosplit
 func semasleep(ns int64) int32 {
-	var start int64
+	mp := getg().m
+	t := _DISPATCH_TIME_FOREVER
 	if ns >= 0 {
-		start = nanotime()
+		t = dispatch_time(_DISPATCH_TIME_NOW, ns)
 	}
-	mp := getg().m
-	pthread_mutex_lock(&mp.mutex)
-	for {
-		if mp.count > 0 {
-			mp.count--
-			pthread_mutex_unlock(&mp.mutex)
-			return 0
-		}
-		if ns >= 0 {
-			spent := nanotime() - start
-			if spent >= ns {
-				pthread_mutex_unlock(&mp.mutex)
-				return -1
-			}
-			var t timespec
-			t.setNsec(ns - spent)
-			err := pthread_cond_timedwait_relative_np(&mp.cond, &mp.mutex, &t)
-			if err == _ETIMEDOUT {
-				pthread_mutex_unlock(&mp.mutex)
-				return -1
-			}
-		} else {
-			pthread_cond_wait(&mp.cond, &mp.mutex)
-		}
+	if dispatch_semaphore_wait(mp.sema, t) != 0 {
+		return -1
 	}
+	return 0
 }
 
 //go:nosplit
 func semawakeup(mp *m) {
-	pthread_mutex_lock(&mp.mutex)
-	mp.count++
-	if mp.count > 0 {
-		pthread_cond_signal(&mp.cond)
-	}
-	pthread_mutex_unlock(&mp.mutex)
+	dispatch_semaphore_signal(mp.sema)
 }
 
 // BSD interface for threading.

src/runtime/sys_darwin.go

@@ -339,52 +339,33 @@ func kevent_trampoline()
 
 //go:nosplit
 //go:cgo_unsafe_args
-func pthread_mutex_init(m *pthreadmutex, attr *pthreadmutexattr) int32 {
-	return libcCall(unsafe.Pointer(funcPC(pthread_mutex_init_trampoline)), unsafe.Pointer(&m))
-}
-func pthread_mutex_init_trampoline()
-
-//go:nosplit
-//go:cgo_unsafe_args
-func pthread_mutex_lock(m *pthreadmutex) int32 {
-	return libcCall(unsafe.Pointer(funcPC(pthread_mutex_lock_trampoline)), unsafe.Pointer(&m))
-}
-func pthread_mutex_lock_trampoline()
-
-//go:nosplit
-//go:cgo_unsafe_args
-func pthread_mutex_unlock(m *pthreadmutex) int32 {
-	return libcCall(unsafe.Pointer(funcPC(pthread_mutex_unlock_trampoline)), unsafe.Pointer(&m))
-}
-func pthread_mutex_unlock_trampoline()
-
-//go:nosplit
-//go:cgo_unsafe_args
-func pthread_cond_init(c *pthreadcond, attr *pthreadcondattr) int32 {
-	return libcCall(unsafe.Pointer(funcPC(pthread_cond_init_trampoline)), unsafe.Pointer(&c))
+func dispatch_semaphore_create(val int) (sema uintptr) {
+	libcCall(unsafe.Pointer(funcPC(dispatch_semaphore_create_trampoline)), unsafe.Pointer(&val))
+	return
 }
-func pthread_cond_init_trampoline()
+func dispatch_semaphore_create_trampoline()
 
 //go:nosplit
 //go:cgo_unsafe_args
-func pthread_cond_wait(c *pthreadcond, m *pthreadmutex) int32 {
-	return libcCall(unsafe.Pointer(funcPC(pthread_cond_wait_trampoline)), unsafe.Pointer(&c))
+func dispatch_semaphore_wait(sema uintptr, t uint64) int32 {
+	return libcCall(unsafe.Pointer(funcPC(dispatch_semaphore_wait_trampoline)), unsafe.Pointer(&sema))
 }
-func pthread_cond_wait_trampoline()
+func dispatch_semaphore_wait_trampoline()
 
 //go:nosplit
 //go:cgo_unsafe_args
-func pthread_cond_timedwait_relative_np(c *pthreadcond, m *pthreadmutex, t *timespec) int32 {
-	return libcCall(unsafe.Pointer(funcPC(pthread_cond_timedwait_relative_np_trampoline)), unsafe.Pointer(&c))
+func dispatch_semaphore_signal(sema uintptr) {
+	libcCall(unsafe.Pointer(funcPC(dispatch_semaphore_signal_trampoline)), unsafe.Pointer(&sema))
 }
-func pthread_cond_timedwait_relative_np_trampoline()
+func dispatch_semaphore_signal_trampoline()
 
 //go:nosplit
 //go:cgo_unsafe_args
-func pthread_cond_signal(c *pthreadcond) int32 {
-	return libcCall(unsafe.Pointer(funcPC(pthread_cond_signal_trampoline)), unsafe.Pointer(&c))
+func dispatch_time(base uint64, delta int64) (result uint64) {
+	libcCall(unsafe.Pointer(funcPC(dispatch_time_trampoline)), unsafe.Pointer(&base))
+	return
 }
-func pthread_cond_signal_trampoline()
+func dispatch_time_trampoline()
 
 // Not used on Darwin, but must be defined.
 func exitThread(wait *uint32) {
@@ -430,13 +411,10 @@ func closeonexec(fd int32) {
 //go:cgo_import_dynamic libc_kqueue kqueue "/usr/lib/libSystem.B.dylib"
 //go:cgo_import_dynamic libc_kevent kevent "/usr/lib/libSystem.B.dylib"
 
-//go:cgo_import_dynamic libc_pthread_mutex_init pthread_mutex_init "/usr/lib/libSystem.B.dylib"
-//go:cgo_import_dynamic libc_pthread_mutex_lock pthread_mutex_lock "/usr/lib/libSystem.B.dylib"
-//go:cgo_import_dynamic libc_pthread_mutex_unlock pthread_mutex_unlock "/usr/lib/libSystem.B.dylib"
-//go:cgo_import_dynamic libc_pthread_cond_init pthread_cond_init "/usr/lib/libSystem.B.dylib"
-//go:cgo_import_dynamic libc_pthread_cond_wait pthread_cond_wait "/usr/lib/libSystem.B.dylib"
-//go:cgo_import_dynamic libc_pthread_cond_timedwait_relative_np pthread_cond_timedwait_relative_np "/usr/lib/libSystem.B.dylib"
-//go:cgo_import_dynamic libc_pthread_cond_signal pthread_cond_signal "/usr/lib/libSystem.B.dylib"
+//go:cgo_import_dynamic libc_dispatch_semaphore_create dispatch_semaphore_create "/usr/lib/libSystem.B.dylib"
+//go:cgo_import_dynamic libc_dispatch_semaphore_wait dispatch_semaphore_wait "/usr/lib/libSystem.B.dylib"
+//go:cgo_import_dynamic libc_dispatch_semaphore_signal dispatch_semaphore_signal "/usr/lib/libSystem.B.dylib"
+//go:cgo_import_dynamic libc_dispatch_time dispatch_time "/usr/lib/libSystem.B.dylib"
 
 // Magic incantation to get libSystem actually dynamically linked.
 // TODO: Why does the code require this?  See cmd/link/internal/ld/go.go

src/runtime/sys_darwin_386.s

@@ -532,96 +532,63 @@ TEXT runtime·raise_trampoline(SB),NOSPLIT,$0
 	POPL    BP
 	RET
 
-TEXT runtime·pthread_mutex_init_trampoline(SB),NOSPLIT,$0
+TEXT runtime·dispatch_semaphore_create_trampoline(SB),NOSPLIT,$0
 	PUSHL	BP
 	MOVL	SP, BP
 	SUBL	$8, SP
-	MOVL	16(SP), CX
-	MOVL	0(CX), AX	// arg 1 mutex
-	MOVL	AX, 0(SP)
-	MOVL	4(CX), AX	// arg 2 attr
-	MOVL	AX, 4(SP)
-	CALL	libc_pthread_mutex_init(SB)
-	MOVL	BP, SP
-	POPL	BP
-	RET
-
-TEXT runtime·pthread_mutex_lock_trampoline(SB),NOSPLIT,$0
-	PUSHL	BP
-	MOVL	SP, BP
-	SUBL	$8, SP
-	MOVL	16(SP), CX
-	MOVL	0(CX), AX	// arg 1 mutex
-	MOVL	AX, 0(SP)
-	CALL	libc_pthread_mutex_lock(SB)
-	MOVL	BP, SP
-	POPL	BP
-	RET
-
-TEXT runtime·pthread_mutex_unlock_trampoline(SB),NOSPLIT,$0
-	PUSHL	BP
-	MOVL	SP, BP
-	SUBL	$8, SP
-	MOVL	16(SP), CX
-	MOVL	0(CX), AX	// arg 1 mutex
+	MOVL	16(SP), BX
+	MOVL	0(BX), AX	// arg 1 value
 	MOVL	AX, 0(SP)
-	CALL	libc_pthread_mutex_unlock(SB)
+	CALL	libc_dispatch_semaphore_create(SB)
+	MOVL	AX, 4(BX)	// result sema
 	MOVL	BP, SP
 	POPL	BP
 	RET
 
-TEXT runtime·pthread_cond_init_trampoline(SB),NOSPLIT,$0
+TEXT runtime·dispatch_semaphore_wait_trampoline(SB),NOSPLIT,$0
 	PUSHL	BP
 	MOVL	SP, BP
-	SUBL	$8, SP
-	MOVL	16(SP), CX
-	MOVL	0(CX), AX	// arg 1 cond
+	SUBL	$24, SP
+	MOVL	32(SP), CX
+	MOVL	0(CX), AX	// arg 1 sema
 	MOVL	AX, 0(SP)
-	MOVL	4(CX), AX	// arg 2 attr
+	MOVL	4(CX), AX	// arg 2 timeout/0
 	MOVL	AX, 4(SP)
-	CALL	libc_pthread_cond_init(SB)
+	MOVL	8(CX), AX	// arg 2 timeout/1
+	MOVL	AX, 8(SP)
+	CALL	libc_dispatch_semaphore_wait(SB)
 	MOVL	BP, SP
 	POPL	BP
 	RET
 
-TEXT runtime·pthread_cond_wait_trampoline(SB),NOSPLIT,$0
+TEXT runtime·dispatch_semaphore_signal_trampoline(SB),NOSPLIT,$0
 	PUSHL	BP
 	MOVL	SP, BP
 	SUBL	$8, SP
 	MOVL	16(SP), CX
-	MOVL	0(CX), AX	// arg 1 cond
+	MOVL	0(CX), AX	// arg 1 sema
 	MOVL	AX, 0(SP)
-	MOVL	4(CX), AX	// arg 2 mutex
-	MOVL	AX, 4(SP)
-	CALL	libc_pthread_cond_wait(SB)
+	CALL	libc_dispatch_semaphore_signal(SB)
 	MOVL	BP, SP
 	POPL	BP
 	RET
 
-TEXT runtime·pthread_cond_timedwait_relative_np_trampoline(SB),NOSPLIT,$0
+TEXT runtime·dispatch_time_trampoline(SB),NOSPLIT,$0
 	PUSHL	BP
 	MOVL	SP, BP
 	SUBL	$24, SP
-	MOVL	32(SP), CX
-	MOVL	0(CX), AX	// arg 1 cond
+	MOVL	32(SP), BX
+	MOVL	0(BX), AX	// arg 1 base/0
 	MOVL	AX, 0(SP)
-	MOVL	4(CX), AX	// arg 2 mutex
+	MOVL	4(BX), AX	// arg 1 base/1
 	MOVL	AX, 4(SP)
-	MOVL	8(CX), AX	// arg 3 timeout
+	MOVL	8(BX), AX	// arg 2 delta/0
 	MOVL	AX, 8(SP)
-	CALL	libc_pthread_cond_timedwait_relative_np(SB)
-	MOVL	BP, SP
-	POPL	BP
-	RET
-
-TEXT runtime·pthread_cond_signal_trampoline(SB),NOSPLIT,$0
-	PUSHL	BP
-	MOVL	SP, BP
-	SUBL	$8, SP
-	MOVL	16(SP), CX
-	MOVL	0(CX), AX	// arg 1 cond
-	MOVL	AX, 0(SP)
-	CALL	libc_pthread_cond_signal(SB)
+	MOVL	12(BX), AX	// arg 2 delta/1
+	MOVL	AX, 12(SP)
+	CALL	libc_dispatch_time(SB)
+	MOVL	AX, 16(BX)	// result/0
+	MOVL	DX, 20(BX)	// result/1
 	MOVL	BP, SP
 	POPL	BP
 	RET

src/runtime/sys_darwin_amd64.s

@@ -482,64 +482,44 @@ TEXT runtime·raise_trampoline(SB),NOSPLIT,$0
 	POPQ	BP
 	RET
 
-TEXT runtime·pthread_mutex_init_trampoline(SB),NOSPLIT,$0
+TEXT runtime·dispatch_semaphore_create_trampoline(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP
-	MOVQ	8(DI), SI	// arg 2 attr
-	MOVQ	0(DI), DI	// arg 1 mutex
-	CALL	libc_pthread_mutex_init(SB)
-	POPQ	BP
-	RET
-
-TEXT runtime·pthread_mutex_lock_trampoline(SB),NOSPLIT,$0
-	PUSHQ	BP
-	MOVQ	SP, BP
-	MOVQ	0(DI), DI	// arg 1 mutex
-	CALL	libc_pthread_mutex_lock(SB)
-	POPQ	BP
-	RET
-
-TEXT runtime·pthread_mutex_unlock_trampoline(SB),NOSPLIT,$0
-	PUSHQ	BP
-	MOVQ	SP, BP
-	MOVQ	0(DI), DI	// arg 1 mutex
-	CALL	libc_pthread_mutex_unlock(SB)
-	POPQ	BP
-	RET
-
-TEXT runtime·pthread_cond_init_trampoline(SB),NOSPLIT,$0
-	PUSHQ	BP
-	MOVQ	SP, BP
-	MOVQ	8(DI), SI	// arg 2 attr
-	MOVQ	0(DI), DI	// arg 1 cond
-	CALL	libc_pthread_cond_init(SB)
+	MOVQ	DI, BX
+	MOVQ	0(BX), DI	// arg 1 value
+	CALL	libc_dispatch_semaphore_create(SB)
+	MOVQ	AX, 8(BX)	// result sema
 	POPQ	BP
 	RET
 
-TEXT runtime·pthread_cond_wait_trampoline(SB),NOSPLIT,$0
+TEXT runtime·dispatch_semaphore_wait_trampoline(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP
-	MOVQ	8(DI), SI	// arg 2 mutex
-	MOVQ	0(DI), DI	// arg 1 cond
-	CALL	libc_pthread_cond_wait(SB)
+	MOVQ	8(DI), SI	// arg 2 timeout
+	MOVQ	0(DI), DI	// arg 1 sema
+	CALL	libc_dispatch_semaphore_wait(SB)
+	TESTQ	AX, AX	// For safety convert 64-bit result to int32 0 or 1.
+	JEQ	2(PC)
+	MOVL	$1, AX
 	POPQ	BP
 	RET
 
-TEXT runtime·pthread_cond_timedwait_relative_np_trampoline(SB),NOSPLIT,$0
+TEXT runtime·dispatch_semaphore_signal_trampoline(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP
-	MOVQ	8(DI), SI	// arg 2 mutex
-	MOVQ	16(DI), DX	// arg 3 timeout
-	MOVQ	0(DI), DI	// arg 1 cond
-	CALL	libc_pthread_cond_timedwait_relative_np(SB)
+	MOVQ	0(DI), DI	// arg 1 sema
+	CALL	libc_dispatch_semaphore_signal(SB)
 	POPQ	BP
 	RET
 
-TEXT runtime·pthread_cond_signal_trampoline(SB),NOSPLIT,$0
+TEXT runtime·dispatch_time_trampoline(SB),NOSPLIT,$0
 	PUSHQ	BP
 	MOVQ	SP, BP
-	MOVQ	0(DI), DI	// arg 1 cond
-	CALL	libc_pthread_cond_signal(SB)
+	MOVQ	DI, BX
+	MOVQ	0(BX), DI	// arg 1 base
+	MOVQ	8(BX), SI	// arg 2 delta
+	CALL	libc_dispatch_time(SB)
+	MOVQ	AX, 16(BX)
 	POPQ	BP
 	RET
 

src/runtime/sys_darwin_arm.s

@@ -343,44 +343,34 @@ TEXT runtime·raise_trampoline(SB),NOSPLIT,$0
 	BL	libc_raise(SB)
 	RET
 
-TEXT runtime·pthread_mutex_init_trampoline(SB),NOSPLIT,$0
-	MOVW	4(R0), R1	// arg 2 attr
-	MOVW	0(R0), R0	// arg 1 mutex
-	BL	libc_pthread_mutex_init(SB)
-	RET
-
-TEXT runtime·pthread_mutex_lock_trampoline(SB),NOSPLIT,$0
-	MOVW	0(R0), R0	// arg 1 mutex
-	BL	libc_pthread_mutex_lock(SB)
-	RET
-
-TEXT runtime·pthread_mutex_unlock_trampoline(SB),NOSPLIT,$0
-	MOVW	0(R0), R0	// arg 1 mutex
-	BL	libc_pthread_mutex_unlock(SB)
-	RET
-
-TEXT runtime·pthread_cond_init_trampoline(SB),NOSPLIT,$0
-	MOVW	4(R0), R1	// arg 2 attr
-	MOVW	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_init(SB)
+TEXT runtime·dispatch_semaphore_create_trampoline(SB),NOSPLIT,$0
+	MOVW	R0, R8
+	MOVW	0(R8), R0	// arg 1 value
+	BL	libc_dispatch_semaphore_create(SB)
+	MOVW	R0, 4(R8)	// result sema
 	RET
 
-TEXT runtime·pthread_cond_wait_trampoline(SB),NOSPLIT,$0
-	MOVW	4(R0), R1	// arg 2 mutex
-	MOVW	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_wait(SB)
+TEXT runtime·dispatch_semaphore_wait_trampoline(SB),NOSPLIT,$0
+	MOVW	4(R0), R1	// arg 2 timeout/0
+	MOVW	8(R0), R2	// arg 2 timeout/1
+	MOVW	0(R0), R0	// arg 1 sema
+	BL	libc_dispatch_semaphore_wait(SB)
 	RET
 
-TEXT runtime·pthread_cond_timedwait_relative_np_trampoline(SB),NOSPLIT,$0
-	MOVW	4(R0), R1	// arg 2 mutex
-	MOVW	8(R0), R2	// arg 3 timeout
-	MOVW	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_timedwait_relative_np(SB)
+TEXT runtime·dispatch_semaphore_signal_trampoline(SB),NOSPLIT,$0
+	MOVW	0(R0), R0	// arg 1 sema
+	BL	libc_dispatch_semaphore_signal(SB)
 	RET
 
-TEXT runtime·pthread_cond_signal_trampoline(SB),NOSPLIT,$0
-	MOVW	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_signal(SB)
+TEXT runtime·dispatch_time_trampoline(SB),NOSPLIT,$0
+	MOVW	R0, R8
+	MOVW	0(R8), R0	// arg 1 base/0
+	MOVW	4(R8), R1	// arg 1 base/1
+	MOVW	8(R8), R2	// arg 2 delta/0
+	MOVW	12(R8), R3	// arg 2 delta/1
+	BL	libc_dispatch_time(SB)
+	MOVW	R0, 16(R8)	// result/0
+	MOVW	R1, 20(R8)	// result/1
 	RET
 
 // syscall calls a function in libc on behalf of the syscall package.

src/runtime/sys_darwin_arm64.s

@@ -409,44 +409,33 @@ TEXT runtime·raise_trampoline(SB),NOSPLIT,$0
 	BL	libc_raise(SB)
 	RET
 
-TEXT runtime·pthread_mutex_init_trampoline(SB),NOSPLIT,$0
-	MOVD	8(R0), R1	// arg 2 attr
-	MOVD	0(R0), R0	// arg 1 mutex
-	BL	libc_pthread_mutex_init(SB)
-	RET
-
-TEXT runtime·pthread_mutex_lock_trampoline(SB),NOSPLIT,$0
-	MOVD	0(R0), R0	// arg 1 mutex
-	BL	libc_pthread_mutex_lock(SB)
-	RET
-
-TEXT runtime·pthread_mutex_unlock_trampoline(SB),NOSPLIT,$0
-	MOVD	0(R0), R0	// arg 1 mutex
-	BL	libc_pthread_mutex_unlock(SB)
-	RET
-
-TEXT runtime·pthread_cond_init_trampoline(SB),NOSPLIT,$0
-	MOVD	8(R0), R1	// arg 2 attr
-	MOVD	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_init(SB)
+TEXT runtime·dispatch_semaphore_create_trampoline(SB),NOSPLIT,$0
+	MOVD	R0, R19
+	MOVD	0(R19), R0	// arg 1 value
+	BL	libc_dispatch_semaphore_create(SB)
+	MOVD	R0, 8(R19)	// result sema
 	RET
 
-TEXT runtime·pthread_cond_wait_trampoline(SB),NOSPLIT,$0
-	MOVD	8(R0), R1	// arg 2 mutex
-	MOVD	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_wait(SB)
+TEXT runtime·dispatch_semaphore_wait_trampoline(SB),NOSPLIT,$0
+	MOVD	8(R0), R1	// arg 2 timeout
+	MOVD	0(R0), R0	// arg 1 sema
+	BL	libc_dispatch_semaphore_wait(SB)
+	CMP	$0, R0	// For safety convert 64-bit result to int32 0 or 1.
+	BEQ	2(PC)
+	MOVW	$1, R0
 	RET
 
-TEXT runtime·pthread_cond_timedwait_relative_np_trampoline(SB),NOSPLIT,$0
-	MOVD	8(R0), R1	// arg 2 mutex
-	MOVD	16(R0), R2	// arg 3 timeout
-	MOVD	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_timedwait_relative_np(SB)
+TEXT runtime·dispatch_semaphore_signal_trampoline(SB),NOSPLIT,$0
+	MOVD	0(R0), R0	// arg 1 sema
+	BL	libc_dispatch_semaphore_signal(SB)
 	RET
 
-TEXT runtime·pthread_cond_signal_trampoline(SB),NOSPLIT,$0
-	MOVD	0(R0), R0	// arg 1 cond
-	BL	libc_pthread_cond_signal(SB)
+TEXT runtime·dispatch_time_trampoline(SB),NOSPLIT,$0
+	MOVD	R0, R19
+	MOVD	0(R19), R0	// arg 1 base
+	MOVD	8(R19), R1	// arg 2 delta
+	BL	libc_dispatch_time(SB)
+	MOVD	R0, 16(R19)	// result
 	RET
 
 // syscall calls a function in libc on behalf of the syscall package.