runtime: avoid loading winmm.dll on newer Windows versions

winmm.dll is only used for timeBeginPeriod and timeEndPeriod, which are
not needed on Windows versions supporting high resolution timers, that
is Windows 10 version 1803, and later.

Updates #56745.

Change-Id: Ie9576638fb8d2b4e648283bec3170aefa76f9f82
Reviewed-on: https://go-review.googlesource.com/c/go/+/556935
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

diff --git a/src/runtime/os_windows.go b/src/runtime/os_windows.go
index 6533b6400490d140b9b419d7d0e750e682e9fdfa..cd0a3c260ec7f91303785743db0eeef436251d2b 100644 (file)
--- a/src/runtime/os_windows.go
+++ b/src/runtime/os_windows.go
@@ -257,16 +257,6 @@ func loadOptionalSyscalls() {
        _RtlGetCurrentPeb = windowsFindfunc(n32, []byte("RtlGetCurrentPeb\000"))
        _RtlGetNtVersionNumbers = windowsFindfunc(n32, []byte("RtlGetNtVersionNumbers\000"))
 
-       m32 := windowsLoadSystemLib(winmmdll[:])
-       if m32 == 0 {
-               throw("winmm.dll not found")
-       }
-       _timeBeginPeriod = windowsFindfunc(m32, []byte("timeBeginPeriod\000"))
-       _timeEndPeriod = windowsFindfunc(m32, []byte("timeEndPeriod\000"))
-       if _timeBeginPeriod == nil || _timeEndPeriod == nil {
-               throw("timeBegin/EndPeriod not found")
-       }
-
        ws232 := windowsLoadSystemLib(ws2_32dll[:])
        if ws232 == 0 {
                throw("ws2_32.dll not found")
@@ -421,6 +411,21 @@ func initHighResTimer() {
        if h != 0 {
                haveHighResTimer = true
                stdcall1(_CloseHandle, h)
+       } else {
+               // Only load winmm.dll if we need it.
+               // This avoids a dependency on winmm.dll for Go programs
+               // that run on new Windows versions.
+               m32 := windowsLoadSystemLib(winmmdll[:])
+               if m32 == 0 {
+                       print("runtime: LoadLibraryExW failed; errno=", getlasterror(), "\n")
+                       throw("winmm.dll not found")
+               }
+               _timeBeginPeriod = windowsFindfunc(m32, []byte("timeBeginPeriod\000"))
+               _timeEndPeriod = windowsFindfunc(m32, []byte("timeEndPeriod\000"))
+               if _timeBeginPeriod == nil || _timeEndPeriod == nil {
+                       print("runtime: GetProcAddress failed; errno=", getlasterror(), "\n")
+                       throw("timeBegin/EndPeriod not found")
+               }
        }
 }