[swift-dev] Understanding runtime entry points
Austin Zheng
austinzheng at gmail.com
Thu Jan 7 01:38:03 CST 2016
Hi Slava,
I took a closer look at how reflectAny() and MagicMirror's C++ constructors work. It appears that instances of value types are being boxed at the time the mirror is created, which would explain the difference in semantics between MagicMirrorData's 'ptr' property and the raw value I'm passing into my function. (In retrospect, this should have been obvious: the 'ptr' property is of type Builtin.RawPointer.)
To step back and provide some more context: I'm working on SR-88 (https://bugs.swift.org/browse/SR-88 <https://bugs.swift.org/browse/SR-88>). This task involves moving the standard library APIs off the old reflection API (using _reflect(), _Reflectable, _MirrorType, etc) and onto the CustomReflectable API. This work is going to precede Joe Groff designing a new runtime reflection API, like you mentioned.
Part of this involves rewriting the _adHocPrint() function in OutputStream.swift. (This is the function that the standard library uses to print out any type that doesn't conform to the various string convertible or output stream protocols.) Currently, that function uses _reflect() to create a mirror of the object to print out, and then matches on its type to determine how to print it. For enums (which generate an _EnumMirror), the case name is printed out by using the _EnumMirror instance's 'caseName' property, which in turn calls into the "swift_EnumMirror_caseName" function in reflection.mm.
Since we don't want to rely upon _reflect(), and want to delete as many of the uses of the underscored mirror APIs as possible, my goal is to figure out how to call into the runtime to get the enum case name without needing to create an _EnumMirror. This is sort of transitory work, admittedly, because once the new runtime reflection API is in place it should provide that functionality instead. It's possible that it might just be easier to create an _EnumMirror and use it to get the case name in that one specific situation, without resorting to calling into the runtime.
I got something extremely rudimentary working. It needs to be cleaned up (if it goes in at all), it probably leaks memory, but the following seems to produce the proper enum cases if the variable passed into the Swift function isn't of type Any:
SWIFT:
@_silgen_name("swift_EnumCaseName")
func _getEnumCaseName<T>(value: T, _ type: Any.Type) -> UnsafePointer<CChar>
C++:
extern "C"
const char *swift_EnumCaseName(OpaqueValue *value, const Metadata *type) {
if (!isEnumReflectable(type))
return nullptr;
// Put the enum in a box so we can get a pointer to it
BoxPair box = swift_allocBox(type);
type->vw_initializeWithCopy(box.second, value);
const auto Enum = static_cast<const EnumMetadata *>(type);
const auto &Description = Enum->Description->Enum;
const auto tag = type->vw_getEnumTag(box.second);
//type->vw_destroy(value);
return getFieldName(Description.CaseNames, tag);
}
Thanks for your help, and if you have any questions let me know.
Best,
Austin
> On Jan 6, 2016, at 1:11 PM, Slava Pestov <spestov at apple.com> wrote:
>
>
>> On Jan 6, 2016, at 12:57 PM, Austin Zheng <austinzheng at gmail.com <mailto:austinzheng at gmail.com>> wrote:
>>
>> Hey Slava,
>>
>> Thanks a lot for your detailed responses; it definitely helps to understand how structs are passed to the C++ function.
>>
>> In a separate email, Joe Groff mentioned that there was a difference between passing the enum value and passing a pointer to it. I think that might be the root of my problem. I'll try a few things and send over a better code sample tonight, if there are still issues.
>
> Yeah, that sounds like it could be the problem.
>
> In general, expecting the Swift calling convention to line up with C++ in this manner is really not something we want to officially support, and I believe JoeG is currently working on refactoring the runtime reflection interface to be a bit more minimal and orthogonal on the runtime side.
>
> It would be great to see what you're working on, because I suspect we should be able to fold your use-case into the general reflection API.
>
> Slava
>
>>
>> Best,
>> Austin
>>
>> On Wed, Jan 6, 2016 at 11:37 AM, Slava Pestov <spestov at apple.com <mailto:spestov at apple.com>> wrote:
>> Hi Austin,
>>
>> > On Jan 1, 2016, at 10:58 PM, Austin Zheng via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>> >
>> > Hello,
>> >
>> > I'm trying to better understand how calls are made between the Swift standard library code and the runtime entry points. I've read through most of the documentation in the repo but still have some questions.
>> >
>> > More specifically, here's an example: the '_EnumMirror' struct below represents a mirror reflecting a value whose type is an enum.
>> >
>> > ------
>> >
>> > struct _EnumMirror : _MirrorType {
>> > let data: _MagicMirrorData
>> > var value: Any { return data.value }
>> > var valueType: Any.Type { return data.valueType }
>> > // ... more stuff
>> >
>> > var caseName: UnsafePointer<CChar> {
>> > @_silgen_name("swift_EnumMirror_caseName")get
>> > }
>> > // ... (more stuff)
>> > }
>> >
>> > ------
>> >
>> > The 'caseName' property represents the name of the enum value's case (e.g. "FirstCase" in Foo.FirstCase) as a C string. This property's getter calls into a C++ runtime function named "swift_EnumMirror_caseName", which is reproduced below (from Reflection.mm):
>> >
>> > extern "C"
>> > const char *swift_EnumMirror_caseName(HeapObject *owner,
>> > const OpaqueValue *value,
>> > const Metadata *type) {
>> > if (!isEnumReflectable(type))
>> > return nullptr;
>> >
>> > const auto Enum = static_cast<const EnumMetadata *>(type);
>> > const auto &Description = Enum->Description->Enum;
>> >
>> > unsigned tag;
>> > getEnumMirrorInfo(value, type, &tag, nullptr, nullptr); // effectively, same as "tag = type->vw_getEnumTag(value);"
>> > return getFieldName(Description.CaseNames, tag);
>> > }
>> >
>> > Now, I had a few questions about exactly how this interoperation works, because I'd like to be able to get the name of an enum case using this entry point from a different context (not from within an _EnumMirror property).
>> >
>> > * swift_EnumMirror_caseName takes three arguments, but the Swift call site doesn't seem to specify what gets passed into the function.
>>
>> The three arguments together form the 'self' value of the call. That is, an EnumMirror is a struct containing a pointer to the owner object, a pointer to the value being reflected, and runtime type information for the value. You can see this if you look at how the _MagicMirrorData struct is defined on the swift side.
>>
>> > Is there a convention that is implicitly passing properties on _EnumMirror as arguments into the C++ function when it's being called? I did note that there were other runtime entry points (like "swift_MagicMirrorData_summary") where the number of arguments in the Swift function matched the number of arguments in the C++ function, but in those cases the Swift function was a free function and not a method.
>>
>> Right.
>>
>> >
>> > * What I really want to do is to get the tag of an enum. I wrote a different entry point that omits the unused "owner" property and simply calls swift_EnumMirror_caseName with nullptr as the first argument. This other C++ function takes 'value' (an OpaqueValue*) and 'type' (a Metadata*). I've surmised that 'type' should be the Swift metatype of the enum instance (e.g. myEnum.dynamicType), and I do get the case names table. However, if I pass in the enum instance itself as 'value', my tag is always retrieved as 0.
>>
>> The value should indeed be a pointer to the enum value itself. Not sure why it's not working for you, maybe you can share more code?
>>
>> > I noticed that there's some sort of indirection in the form of "vw_getEnumTag", which goes through something called the "value witness". Is there somewhere I can read up about the value witness concept? I assume the reason the 'original' code worked was because it was passing in a different object as 'value', maybe one that could serve as a value witness for the reflected-upon instance's type.
>>
>> The value witness table is a member of the type metadata. It contains entry points for runtime manipulation of values of that type. The value witness table is used for runtime generics (when I have a generic parameter 'T' and a value of type 'T', the value witness functions are used for copying/moving/destroying/etc values of type 'T'). They are also used for reflection.
>>
>> They're not really documented anywhere except for in the source code itself -- see here:
>>
>> include/swift/Runtime/Metadata.h
>> lib/IRGen/ValueWitness.h
>>
>> Slava
>>
>> >
>> > Thanks a lot for your time.
>> >
>> > Best,
>> > Austin
>> >
>> > _______________________________________________
>> > swift-dev mailing list
>> > swift-dev at swift.org <mailto:swift-dev at swift.org>
>> > https://lists.swift.org/mailman/listinfo/swift-dev <https://lists.swift.org/mailman/listinfo/swift-dev>
>>
>>
>
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