The internal function GetTimerImpl has a rather complex expression for
its return type. Prevously this was derived using declspec, but one of
the versions of Visual Studio barfed on that for some reason. So now
declare the return type explicitly.
It should be possible to query a vtkm::cont::Timer without modifying it.
As such, its query functions (such as Stopped and GetElapsedTime) should
be const.
Previously, in order to specify a device with the timer, it had to be
specified in the timer's construction or had to be specified every time
GetElapsedTime was called. The first method was inconvienient in the
case where there are multiple code paths to define the device and the
latter method was inconvienient because you would have to pass around a
device id.
Both these techniques still exist, but we have also added a new form of
Reset that allows you to change the device the timer is used on.
Previously, a vtkm::cont::Timer had to be initialized with either no
device or with a device adapter tag. However, this precluded
initializing the timer with a DeviceAdapterId, which made it difficult
to create a timer at runtime. Instead, just accept a DeviceAdapterId
(which all device adapter tags inherit from) and do runtime checks.
The timer class now is asynchronous and device independent. it's using an
similiar API as vtkOpenGLRenderTimer with Start(), Stop(), Reset(), Ready(),
and GetElapsedTime() function. For convenience and backward compability, Each
Start() function call will call Reset() internally and each GetElapsedTime()
function call will call Stop() function if it hasn't been called yet for keeping
backward compatibility purpose.
Bascially it can be used in two modes:
* Create a Timer without any device info. vtkm::cont::Timer time;
* It would enable timers for all enabled devices on the machine. Users can get a
specific elapsed time by passing a device id into the GetElapsedtime function.
If no device is provided, it would pick the maximum of all timer results - the
logic behind this decision is that if cuda is disabled, openmp, serial and tbb
roughly give the same results; if cuda is enabled it's safe to return the
maximum elapsed time since users are more interested in the device execution
time rather than the kernal launch time. The Ready function can be handy here
to query the status of the timer.
* Create a Timer with a device id. vtkm::cont::Timer time((vtkm::cont::DeviceAdapterTagCuda()));
* It works as the old timer that times for a specific device id.
