Thanks. That’s true, but there are cases (like making BLAS calls) where I have
to nest more than 4 `withUnsafeMutable…` closures. It’s safe by really clumsy.
I just wish there were a cleaner way that looks like the do-notation in Haskell
or for-notation in Scala.
-Richard
> On Dec 16, 2016, at 14:16, Joe Groff <jgr...@apple.com> wrote:
>
>>
>> On Dec 16, 2016, at 12:10 PM, Richard Wei <rxr...@gmail.com> wrote:
>>
>> `sync` is not escaping. Shadow copy causes the device memory to make a copy,
>> which can’t be a solution either. I’ll file a radar.
>>
>>> Note that, independently, this part looks fishy:
>>>
>>>> try! fill<<<(blockSize, blockCount)>>>[
>>>> .pointer(to: &self)
>>>
>>> UnsafePointers formed by passing an argument inout are not valid after the
>>> called function returns, so if this function is forming and returning a
>>> pointer, it will likely result in undefined behavior. You should use
>>> `withUnsafePointer(&self) { p in ... }` to get a usable pointer.
>>
>> This part is a semi-"type safe” wrapper to CUDA kernel launcher. The purpose
>> is to make explicit which argument gets passed to the kernel as a pointer;
>> in this case, self is a `DeviceArray`. `.pointer` is a factory method under
>> `KernelArgument`. Since most arguments to the kernel are passed in by
>> pointers, IMO using a bunch of `withUnsafe...` clauses would only make it
>> look unnecessarily clumsy.
>
> Unfortunately, that's the only defined way to write this code. The pointer
> you get as an argument from `&self` is only good until pointer(to:) returns,
> so it won't be guaranteed to be valid by the time you use it, and the
> compiler will assume that `self` doesn't change afterward, so any mutation
> done to `self` through that pointer will lead to undefined behavior.
> Rewriting this code to use withUnsafePointer should also work around the
> capture bug without requiring a shadow copy:
>
> let blockSize = min(512, count)
> let blockCount = (count+blockSize-1)/blockSize
> withUnsafePointer(to: &self) { selfPtr in
> device.sync { // Launch CUDA kernel
> try! fill<<<(blockSize, blockCount)>>>[
> .pointer(to: selfPtr), .value(value), .value(Int64(count))
> ]
> }
> }
>
> -Joe
_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users
