runtime: use a single definition of time_now for faketime

Build other definitions with the !faketime build tag.

This makes it easy for us to add new assembly implementations of time.now.

Change-Id: I4e48e41a4a04ab001030e6d1cdd9cebfa0161b0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/314274
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/time_fake.go

@@ -5,17 +5,10 @@
 //go:build faketime && !windows
 // +build faketime,!windows
 
-// Faketime isn't currently supported on Windows. This would require:
-//
-// 1. Shadowing time_now, which is implemented in assembly on Windows.
-//    Since that's exported directly to the time package from runtime
-//    assembly, this would involve moving it from sys_windows_*.s into
-//    its own assembly files build-tagged with !faketime and using the
-//    implementation of time_now from timestub.go in faketime mode.
-//
-// 2. Modifying syscall.Write to call syscall.faketimeWrite,
-//    translating the Stdout and Stderr handles into FDs 1 and 2.
-//    (See CL 192739 PS 3.)
+// Faketime isn't currently supported on Windows. This would require
+// modifying syscall.Write to call syscall.faketimeWrite,
+// translating the Stdout and Stderr handles into FDs 1 and 2.
+// (See CL 192739 PS 3.)
 
 package runtime
 
@@ -48,6 +41,12 @@ func walltime() (sec int64, nsec int32) {
 	return faketime / 1000000000, int32(faketime % 1000000000)
 }
 
+//go:linkname time_now time.now
+func time_now() (sec int64, nsec int32, mono int64) {
+	sec, nsec = walltime()
+	return sec, nsec, nanotime()
+}
+
 func write(fd uintptr, p unsafe.Pointer, n int32) int32 {
 	if !(fd == 1 || fd == 2) {
 		// Do an ordinary write.

src/runtime/time_windows_386.s

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !faketime
+// +build !faketime
+
 #include "go_asm.h"
 #include "textflag.h"
 #include "time_windows.h"

src/runtime/time_windows_amd64.s

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !faketime
+// +build !faketime
+
 #include "go_asm.h"
 #include "textflag.h"
 #include "time_windows.h"

src/runtime/time_windows_arm.s

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !faketime
+// +build !faketime
+
 #include "go_asm.h"
 #include "textflag.h"
 #include "time_windows.h"

src/runtime/time_windows_arm64.s

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !faketime
+// +build !faketime
+
 #include "go_asm.h"
 #include "textflag.h"
 #include "time_windows.h"

src/runtime/timeasm.go

@@ -4,8 +4,8 @@
 
 // Declarations for operating systems implementing time.now directly in assembly.
 
-//go:build windows
-// +build windows
+//go:build !faketime && windows
+// +build !faketime,windows
 
 package runtime
 

src/runtime/timestub.go

@@ -5,8 +5,8 @@
 // Declarations for operating systems implementing time.now
 // indirectly, in terms of walltime and nanotime assembly.
 
-//go:build !windows
-// +build !windows
+//go:build !faketime && !windows
+// +build !faketime,!windows
 
 package runtime