[vm/ffi] Investigate copy-free Strings

[dcharkes]

However, we might need something more performant (which avoids copying), if this is not performant enough.

Originally posted by @dcharkes in #35762 (comment)

Our null-terminated Utf8 and Utf16 string helpers in package:ffi require copying bytes from Dart to C.

We should investigate whether we can have pass strings from C to Dart without copying, and whether we can pass Utf16 strings from Dart to C without copying. The latter is unlikely though, as the Dart Garbage Collector might relocate the String.

[timsneath]

This is particularly painful for a Win32 function like SysAllocString(), since creating a BSTR (a string type which includes a length prefix) requires two copies:

• The original Dart string
• Copy 1: null-terminated C string
• Copy 2: the BSTR

[mkustermann]

Generally speaking we cannot hand out pointers to memory inside the VM's managed / garbage collected heap, because while C code runs the GC can move objects.

In addition to that, the VM uses different string representations which might be incompatible with what the C side wants.

@timsneath Why does it require two copies? Can you not examine the Dart string to determine what the encoded length would be, then allocate a correctly sized buffer, write length + encoded string?

[timsneath]

SysAllocString takes an unmanaged null-terminated string and copies it to a BSTR-formatted string. While the documentation describes the format, it also notes that they are allocated using COM memory allocation functions. While I could speculate how they are created and try and replicate that from Dart code, I can't guarantee that my implementation would be compatible with SysAllocString, since the source code isn't available. So I have to do something like:

const MAX_STRING = 256;
final rawString = Pointer<Uint16>.allocate(count: MAX_STRING).cast<Utf16>();
rawString = Utf16.fromString('Aarhus is a beautiful city.'); // copy from Dart to unmanaged
final bstrString = SysAllocString(rawString); // Win32 makes a second copy here
... // do stuff
SysFreeString(bstrString);
free(rawString);

[mkustermann]

@timsneath If the only goal is to avoid the second memory allocation and copy, would something like this do the trick:

foo(String string) {
  // Allocate BSTR without initializing it (i.e. no copy of bytes)
  final bstr = SysAllocStringByteLen(nullptr, 2 * string.length).cast<Uint16>();

  // Initialize the BSTR (remember "bstr" points to the actual 16-bit character buffer, not at the length prefix)
  for (int i = 0; i < string.length; ++i) {
    bstr[i] = string.codeUnitAt(i);
  }

  // <do something with "bstr">

  // Free the BSTR
  SysFreeString(bstr);
}

?

[timsneath]

Yes, this works. But I think I'll need to wind up wrapping BSTR as a whole so that I can embed this kind of logic rather than expecting the package user to be aware of these subtleties. Thanks, Martin.

…

On Tue, Dec 22, 2020 at 7:21 AM Martin Kustermann ***@***.***> wrote: @timsneath <https://github.com/timsneath> If the only goal is to avoid the second memory allocation and copy, would something like this do the trick: foo(String string) { // Allocate BSTR without initializing it (i.e. no copy of bytes) final bstr = SysAllocStringByteLen(nullptr, 2 * string.length).cast<Uint16>(); // Initialize the BSTR (remember "bstr" points to the actual 16-bit character buffer, not at the length prefix) for (int i = 0; i < string.length; ++i) { bstr[i] = string.codeUnitAt(i); } // <do something with "bstr"> // Free the BSTR SysFreeString(bstr); } ? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#39787 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AARWL6YMZSYWMFHLNQWWHNDSWC2OTANCNFSM4J2O2CQQ> .

[dcharkes]

In leaf-calls we could consider allowing conversion between String and Pointer. In a fashion similar to unwrapping TypedData #44589.

The type argument of pointer in the signature should then specify what encoding to use.