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Previous article: Friday Q&A 2013-03-08: Let's Build NSInvocation, Part I
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Friday Q&A 2013-03-22: Let's Build NSInvocation, Part II
by Mike Ash  

Last time on Friday Q&A, I began the reimplementation of parts of NSInvocation as MAInvocation. In that article, I discussed the basic theory, the architecture calling conventions, and presented the assembly language glue code needed for the implementation. Today, I present the Objective-C part of MAInvocation.

MAInvocation is my reimplementation of a large chunk of NSInvocation. For simplicity, it doesn't support floating point arguments or return values, and it also doesn't support struct arguments. It only supports the x86-64 architecture. The code is on GitHub here:


The first six parameters to a function are passed in six registers: rdi, rsi, rdx, rcx, r8, and r9. Subsequent parameters, if any, are passed on the stack. Return values are returned in rax. For the special case of a two-element struct, the second element is returned in rdx. Larger structs are returned by having the caller allocate memory, and then a pointer to that memory is implicitly passed in as the first parameter in rdi, with all explicit parameters bumped down. These are called stret calls in the Objective-C world.

Assembly language glue is used to translate between values held in a struct and actual function calls. The struct holds all of the registers in question, plus a pointer to stack arguments, plus some additional data:

    struct RawArguments
        void *fptr;

        uint64_t rdi;
        uint64_t rsi;
        uint64_t rdx;
        uint64_t rcx;
        uint64_t r8;
        uint64_t r9;

        uint64_t stackArgsCount;
        uint64_t *stackArgs;

        uint64_t rax_ret;
        uint64_t rdx_ret;

        uint64_t isStretCall;

The MAInvocationCall function is written in assembly, and was explored in the previous article. It has this prototype:

    void MAInvocationCall(struct RawArguments *);

Objective-C code can fill out a struct RawArguments with a function pointer and the appropriate register values, then call this function. It will make the function call, and on return, the two return value register fields in the struct will be filled out with whatever the function returned.

There are also two forwarding handlers:

    void MAInvocationForward(void);
    void MAInvocationForwardStret(void);

These are designed to be invoked by an arbitrary Objective-C method call. They both create a new struct RawArguments, fill out the argument registers and stack arguments pointer, and then invoke a C function called MAInvocationForwardC. When that returns, the handlers pass the rax_ret and rdx_ret values back to the caller. The only difference between these two handlers is whether they set the isStretCall to 0 or 1.

The stage is now set for the Objective-C implementation of MAInvocation.

The interface to MAInvocation is the same as NSInvocation:

    @interface MAInvocation : NSObject

    + (MAInvocation *)invocationWithMethodSignature:(NSMethodSignature *)sig;

    - (NSMethodSignature *)methodSignature;

    - (void)retainArguments;
    - (BOOL)argumentsRetained;

    - (id)target;
    - (void)setTarget:(id)target;

    - (SEL)selector;
    - (void)setSelector:(SEL)selector;

    - (void)getReturnValue:(void *)retLoc;
    - (void)setReturnValue:(void *)retLoc;

    - (void)getArgument:(void *)argumentLocation atIndex:(NSInteger)idx;
    - (void)setArgument:(void *)argumentLocation atIndex:(NSInteger)idx;

    - (void)invoke;
    - (void)invokeWithTarget:(id)target;


Instance Variables
The method signature is central to an invocation object. The method signature describes how many parameters the method takes, as well as what the types are. This is critical information to be able to figure out how to deal with method parameters and return types. This is why the only way to create an MAInvocation is with an NSMethodSignature, and that signature is stored in an instance variable:

    @implementation MAInvocation {
        NSMethodSignature *_sig;

The invocation keeps a local struct RawArguments. This struct is manipulated directly when setting or getting arguments and return values. When the invocation is invoked, a pointer to the instance variable can be passed directly to the assembly language glue:

        struct RawArguments _raw;

Invocations can retain their arguments. This sends retain to all object arguments, and it also copies C string arguments. Whether arguments are currently retained needs to be tracked, so that they can be properly freed, and so that newly-set arguments can be retained, so there's a flag for that:

        BOOL _argumentsRetained;

Finally, there needs to be a buffer to store the return value for stret calls:

        void *_stretBuffer;

The factory method just calls an init method:

    + (NSInvocation *)invocationWithMethodSignature: (NSMethodSignature *)sig
        return [[[self alloc] initWithMethodSignature: sig] autorelease];

The init method saves the method signature:

    - (id)initWithMethodSignature: (NSMethodSignature *)sig
        if((self = [super init]))
            _sig = [sig retain];

It then populates the isStretCall of the struct RawArguments by examining the method signature to determine whether it fits the requirements for a stret call. This is done by calling another method. The code for that method is rather involved, and will come later:

            _raw.isStretCall = [self isStretReturn];

Next, the stack arguments are set up. The first thing to do here is to get the total number of arguments from the method signature:

            NSUInteger argsCount = [sig numberOfArguments];

Note that this count includes the two implicit arguments, self and _cmd, so this number is exactly equal to the number of function arguments being passed.

If it's a stret call, then there's effectively one more argument, because of the implicit pointer for the return value passed in rdi:


If there are more than six arguments (potentially including the implicit stret parameter), then there are stack arguments. stackArgsCount is set to the number of remaining arguments, and memory is allocated so that stackArgs can hold them:

            if(argsCount > 6)
                _raw.stackArgsCount = argsCount - 6;
                _raw.stackArgs = calloc(argsCount - 6, sizeof(*_raw.stackArgs));
        return self;

Wrapper Methods
There are a few methods in the API that are simply small wrappers around other methods. I'll cover them here before we get to the real meat.

The target method just gets the value of the first argument in a slightly more convenient way. The method is just a small wrapper around the getArgument:atIndex: method:

    - (id)target
        id target;
        [self getArgument: &target atIndex: 0];
        return target;

The setTarget: method is an even simpler wrapper around the setArgument:atIndex: method:

    - (void)setTarget: (id)target
        [self setArgument: &target atIndex: 0];

The selector and setSelector: methods are virtually identical, but manipulate the second argument:

    - (SEL)selector
        SEL sel;
        [self getArgument: &sel atIndex: 1];
        return sel;

    - (void)setSelector: (SEL)selector
        [self setArgument: &selector atIndex: 1];

Finally, the invoke method calls invokeWithTarget:, passing [self target]:

    - (void)invoke
        [self invokeWithTarget: [self target]];

Getting Arguments
In order to get an argument out of the invocation, the code first needs to know where an argument is stored. This small wrapper method handles that:

    - (uint64_t *)argumentPointerAtIndex: (NSInteger)idx
        uint64_t *ptr = NULL;
        if(idx == 0)
            ptr = &_raw.rdi;
        if(idx == 1)
            ptr = &_raw.rsi;
        if(idx == 2)
            ptr = &_raw.rdx;
        if(idx == 3)
            ptr = &_raw.rcx;
        if(idx == 4)
            ptr = &_raw.r8;
        if(idx == 5)
            ptr = &_raw.r9;
        if(idx >= 6)
            ptr = _raw.stackArgs + idx - 6;
        return ptr;

This method takes a raw argument index, which is to say that it's already been adjusted to take into account whether or not this is a stret call. It then maps that index onto the appropriate register or stack slot.

It's also handy to be able to get the size of a particular argument, so it can copy the right number of bytes for arguments that are smaller than 8 bytes. This method wraps the Foundation function NSGetSizeAndAlignment, which takes an Objective-C type string and returns the size (and alignment!) of the type in question:

    - (NSUInteger)sizeOfType: (const char *)type
        NSUInteger size;
        NSGetSizeAndAlignment(type, &size, NULL);
        return size;

Another small wrapper around this method provides the size of a given argument:

    - (NSUInteger)sizeAtIndex: (NSInteger)idx
        return [self sizeOfType: [_sig getArgumentTypeAtIndex: idx]];

To actually fetch an argument, the method first adjusts the requested index to take into account the possible stret return:

    - (void)getArgument: (void *)argumentLocation atIndex: (NSInteger)idx
        NSInteger rawArgumentIndex = idx;

Next, it grabs the pointer from the above method, and checks it for sanity:

        uint64_t *src = [self argumentPointerAtIndex: rawArgumentIndex];

Then it grabs the argument size and copies the appropriate number of bytes out of the argument location:

        NSUInteger size = [self sizeAtIndex: idx];
        memcpy(argumentLocation, src, size);

Getting and Setting Return Values
To get and set the return value, the value's size is needed. This is easy to obtain by just getting the size of the return type obtained from the method signature:

    - (NSUInteger)returnValueSize
        return [self sizeOfType: [_sig methodReturnType]];

It's also necessary to get a pointer to the location where the return value is stored. If the invocation is for a stret call, then it returns _stretBuffer. If the buffer isn't allocated yet, it allocates it:

    - (void *)returnValuePtr
            if(_stretBuffer == NULL)
                _stretBuffer = calloc(1, [self returnValueSize]);
            return _stretBuffer;

For regular calls, it just returns the address of the rax_ret field in the raw arguments struct:

            return &_raw.rax_ret;

This takes care of the case where the return value uses both return registers. Since they're contiguous in the struct, copying a sufficiently large value into this address will write to both register fields in the struct.

With these methods available, writing the methods to get and set the return value is easy. All they have to do is call memcpy with the computed size and pointer:

    - (void)getReturnValue: (void *)retLoc
        NSUInteger size = [self returnValueSize];
        memcpy(retLoc, [self returnValuePtr], size);

    - (void)setReturnValue: (void *)retLoc
        NSUInteger size = [self returnValueSize];
        memcpy([self returnValuePtr], retLoc, size);

Type Classification
Determining whether a method's return type requires a stret call requires classifying that type according to the x86-64 calling conventions. The NSInvocation API allows for retaining the arguments to the invocation, which also requires classifying the argument types, so that all of the object types can be found.

The different classifications get put into an enum which combines the relevant parts of the x86-64 ABI with the distinctions necessary for retaining arguments. This boils down to objects, blocks, C strings, other integer types (including non-object pointers), a struct containing two integers, an empty struct, any other struct, and any other type not already covered:

    enum TypeClassification

The classification process itself consists of two mutually-recursive methods: one that classifies arbitrary types, and one specialized to classify struct types.

The general method starts by creating the type strings for 'id', blocks, and C strings by using the @encode directive:

    - (enum TypeClassification)classifyType: (const char *)type
        const char *idType = @encode(id);
        const char *blockType = @encode(void (^)(void));
        const char *charPtrType = @encode(char *);

Note that all blocks have the same type string when it comes to @encode, so the choice of block type here is completely arbitrary.

With these in hand, it compares type against them, and returns the appropriate enum value if there's a match:

        if(strcmp(type, idType) == 0)
            return TypeObject;
        if(strcmp(type, blockType) == 0)
            return TypeBlock;
        if(strcmp(type, charPtrType) == 0)
            return TypeCString;

Next, it checks integer types. This crazy bit of code constructs a C string that contains every the character for every integer type, plus function pointers (which is just ?), plus any other pointer (which all start with ^):

        char intTypes[] = { @encode(signed char)[0], @encode(unsigned char)[0], @encode(short)[0], @encode(unsigned short)[0], @encode(int)[0], @encode(unsigned int)[0], @encode(long)[0], @encode(unsigned long)[0], @encode(long long)[0], @encode(unsigned long long)[0], '?', '^', 0 };

With that C string in hand, the strchr function can be used to check the first character in type aganist all of these characters. If there's a match, then the type is an integer type:

        if(strchr(intTypes, type[0]))
            return TypeInteger;

struct types begin with the { character. If the type string starts with that character, then call into the struct classifier:

        if(type[0] == '{')
            return [self classifyStructType: type];

If nothing matches, then return the "other" type:

        return TypeOther;

The struct classifier uses a helper method that takes the type string for the struct and enumerates over all of its contents. It tracks the struct's classification at each point, and updates it with each new element in the struct. It starts out with an empty struct:

    - (enum TypeClassification)classifyStructType: (const char *)type
        __block enum TypeClassification structClassification = TypeEmptyStruct;

Then it enumerates and classifies each type within:

        [self enumerateStructElementTypes: type block: ^(const char *type) {
            enum TypeClassification elementClassification = [self classifyType: type];

If the current classification is an empty struct, then the new classification is the same as the element classification. A struct with one element is classified the same as the element it contains:

            if(structClassification == TypeEmptyStruct)
                structClassification = elementClassification;

If the current classification is an integer type and the element classification is also an integer type, then the struct gets the special classification of a struct containing two integers:

            else if([self isIntegerClass: structClassification] && [self isIntegerClass: elementClassification])
                structClassification = TypeTwoIntegers;

In any other circumstance (struct contains more than two elements, struct contains floating-point elements, etc.) the classification is just a generic struct:

                structClassification = TypeStruct;
        return structClassification;

The method to enumerate over a struct type string's elements is short. A struct type string consists of the struct's name, the = symbol, and then each element's type concatenated together, all contained within a pair of {}. For example, NSRange would look like:


The first thing the method does is find the = and start scanning just beyond it:

    - (void)enumerateStructElementTypes: (const char *)type block: (void (^)(const char *type))block
        const char *equals = strchr(type, '=');
        const char *cursor = equals + 1;

Then it enumerates over each type contained within, taking advantage of NSGetSizeAndAlignment to move the cursor to the end of each type encountered, even if the type contains more than one character. It does this until it encounters a closing brace:

        while(*cursor != '}')
            cursor = NSGetSizeAndAlignment(cursor, NULL, NULL);

There's also a short helper method that determines whether a particular type classification is considered an integer. This just checks to see if the classification is an object, block, C string, or an actual integer or other pointer:

    - (BOOL)isIntegerClass: (enum TypeClassification)classification
        return classification == TypeObject || classification == TypeBlock || classification == TypeCString || classification == TypeInteger;

This finishes the type classification system. This is somewhat rudimentary compared to the full complexity of the x86-64 spec, but it's enough for MAInvocation's needs. With type classification available, we can finally implement the isStretReturn method used in the initializer:

    - (BOOL)isStretReturn
        return [self classifyType: [_sig methodReturnType]] == TypeStruct;

Setting Arguments
With type classification in place, it's finally time to implement setting arguments. The basic form of setArgument:atIndex: is nearly identical to getArgument:atIndex:, but the need to support retained arguments makes everything far more complicated.

It's possible to create an NSInvocation, set it up, and then keep it around for a while. In order for the NSInvocation to remain valid, it needs to be able to do proper memory management on the arguments it contains. In a nod to flexibility, this is optional. A freshly-made NSInvocation doesn't do any memory management on its arguments, but it can be enabled by sending it a retainArguments message.

MAInvocation mimics this functionality. When it receives retainArguments it performs the following operations on its arguments:

    1. Block arguments are copied.
    2. Non-block object arguments are retained.
    3. C string arguments are copied.
    4. All others are left alone.

In addition to doing this for retainArguments, the setArgument:atIndex: method needs to do this for each newly set argument as well. This is what makes it so much more complex than getArgument:atIndex:.

The method starts off by computing the raw argument index:

    - (void)setArgument: (void *)argumentLocation atIndex: (NSInteger)idx
        NSInteger rawArgumentIndex = idx;

Next, it gets the argument pointer at that index:

        uint64_t *dest = [self argumentPointerAtIndex: rawArgumentIndex];

Then it classifies the argument at this index:

        enum TypeClassification c = [self classifyArgumentAtIndex: idx];

If arguments are retained, it will then check the classification of the arguments to see if it's a block, a non-block object, or a C string. If it is, then it directly sets dest to the value found at argumentLocation using the appropriate retain or copy semantics. If argument is of a different type, or if arguments aren't retained, it does a simple memcpy.

The first case is for plain objects. This just does a fairly standard retain/release combination, with a bunch of casting to treat both pointers as object pointers. The release is done at the end using the old variable to avoid problems where releasing the old value invalidates the new one:

        if(_argumentsRetained && c == TypeObject)
            id old = *(id *)dest;
            *(id *)dest = [*(id *)argumentLocation retain];
            [old release];

Blocks get the same treatment, except they get a copy rather than a retain:

        else if(_argumentsRetained && c == TypeBlock)
            id old = *(id *)dest;
            *(id *)dest = [*(id *)argumentLocation copy];
            [old release];

C strings are similar, but use strdup and free:

        else if(_argumentsRetained && c == TypeCString)
            char *old = *(char **)dest;

            char *cstr = *(char **)argumentLocation;
            if(cstr != NULL)
                cstr = strdup(cstr);
            *(char **)dest = cstr;


In all other cases, the appropriate number of bytes is copied over using memcpy:

            NSUInteger size = [self sizeAtIndex: idx];
            memcpy(dest, argumentLocation, size);

The classifyArgumentAtIndex: is a small wrapper around classifyType: that retrieves the argument type from the method signature and classifies it:

    - (enum TypeClassification)classifyArgumentAtIndex: (NSUInteger)idx
        return [self classifyType: [_sig getArgumentTypeAtIndex: idx]];

Retaining Arguments
In addition to retaining each argument as it arrives in setArgument:atIndex:, MAInvocation also needs to retain all existing arguments in the retainArguments method. Only the first call does anything, so the first thing that method does is check to see if arguments are already retained, and bail out if so:

    - (void)retainArguments

Next, it iterates over all retainable arguments, using a helper method. This method invokes a block for each retainable argument that passes in the argument index as well as the argument's value. There are three value arguments in the block, and only one is set for any given call.

        [self iterateRetainableArguments: ^(NSUInteger idx, id obj, id block, char *cstr) {

If it's an object argument, then that argument is retained:

                [obj retain];

If it's a block argument, the block is copied, and the new value set as the argument value. Note that _argumentsRetained has not yet been set to YES, so setArgument:atIndex: won't try to do its own memory management, avoiding any conflict between the two:

            else if(block)
                block = [block copy];
                [self setArgument: &block atIndex: idx];

If it's a C string argument, it uses strdup:

            else if(cstr)
                if(cstr != NULL)
                    cstr = strdup(cstr);
                [self setArgument: &cstr atIndex: idx];

Finally, it sets _argumentsRetained:

        _argumentsRetained = YES;

The iterateRetainableArguments: method uses the type classification system to figure out what each argument is, then calls getArgument:atIndex: to fetch the value. It first iterates over each argument and classifies it:

    - (void)iterateRetainableArguments: (void (^)(NSUInteger idx, id obj, id block, char *cstr))block
        for(NSUInteger i = 0; i < [_sig numberOfArguments]; i++)
            enum TypeClassification c = [self classifyArgumentAtIndex: i];

Objects and blocks are both handled by the same branch. It first retrieves the argument into a local id variable:

            if(c == TypeObject || c == TypeBlock)
                id arg;
                [self getArgument: &arg atIndex: i];

It then moves arg into one of two other local variables depending on whether the type is a block or a plain object:

                id o = c == TypeObject ? arg : nil;
                id b = c == TypeBlock ? arg : nil;

At this point, o contains the argument value if it's a plain argument, and b contains the argument value if it's a block. The iteration block can then be called with these values:

                block(i, o, b, NULL);

C strings are similar, but less complex, because there's only one possible type here:

            else if(c == TypeCString)
                char *arg;
                [self getArgument: &arg atIndex: i];

                block(i, nil, nil, arg);

While we're at it, here's a quick getter method for argumentsRetained:

    - (BOOL)argumentsRetained
        return _argumentsRetained;

The hardest part of dealloc is freeing the retained arguments. The iterateRetainableArguments: method takes care of most of the work:

    - (void)dealloc
            [self iterateRetainableArguments: ^(NSUInteger idx, id obj, id block, char *cstr) {
                [obj release];
                [block release];

With that taken care of, all that remains is freeing the method signature, the stackArgs pointer, and calling super:

        [_sig release];

        [super dealloc];

The code so far has kept the struct RawArguments almost completely up to date. Implementing invokeWithTarget: is simply a matter of filling out the last details, then making a call to the MAInvocationCall assembly glue function. The method starts out by setting the target value:

    - (void)invokeWithTarget: (id)target
        [self setTarget: target];

It then uses methodForSelector: to get the function pointer for the invocation's selector and places that into the fptr field. This is what the glue code will call:

        _raw.fptr = [target methodForSelector: [self selector]];

If this is a stret call, then rdi needs to be set up to point to space that can hold the return value:

            _raw.rdi = (uint64_t)[self returnValuePtr];

Finally, call the assembly glue:


With all of the register fields and the stack arguments pointer set up, and the function pointer field set to the target IMP, the assembly glue is able to make the call. Upon return, the assembly glue copies rax and rdx into the return value fields of the struct RawArguments. This means that the return value is already set when the assembly glue returns, and will be available from getReturnValue: without any additional action in the Objective-C code.

The last major piece of MAInvocation is the MAInvocationForwardC function. The assembly language forwarding glue intercepts unknown message calls. It then constructs a struct RawArguments on the stack from the function call, and then calls through to MAInvocationForwardC, passing it a pointer to the struct RawArguments. The remainder of the logic is implemented in Objective-C:

    void MAInvocationForwardC(struct RawArguments *r)

The first order of business is to get the object that the message was sent to, and the selector being sent. For a stret call, the object is in rsi and the selector is in rdx. For a normal call, the object is in rdi and the selector is in rsi:

        id obj;
        SEL sel;

            obj = (id)r->rsi;
            sel = (SEL)r->rdx;
            obj = (id)r->rdi;
            sel = (SEL)r->rsi;

A method signature is critical to creating an invocation object. With the object and selector available, a simple call to methodSignatureForSelector: obtains that:

        NSMethodSignature *sig = [obj methodSignatureForSelector: sel];

With the method signature in hand, the forwarding function can now create an MAInvocation:

        MAInvocation *inv = [[MAInvocation alloc] initWithMethodSignature: sig];

The next order of business is to copy all of the pertinent information from r into the invocation's_raw` instance variable. First come the registers:

        inv->_raw.rdi = r->rdi;
        inv->_raw.rsi = r->rsi;
        inv->_raw.rdx = r->rdx;
        inv->_raw.rcx = r->rcx;
        inv->_raw.r8 = r->r8;
        inv->_raw.r9 = r->r9;

After that, stack arguments are copied. Although r always contains 0 for stackArgsCount, the invocation has now computed the number of actual stack arguments, so its _raw variable can be consulted to get the count:

        memcpy(inv->_raw.stackArgs, r->stackArgs, inv->_raw.stackArgsCount * sizeof(uint64_t));

The invocation is now fully constructed and filled out. The object is sent forwardInvocation: with the newly constructed invocation.

        [obj forwardInvocation: (id)inv];

After that call returns, the return value from the invocation needs to be copied back into r. The assembly glue will then pass the value back to the caller. It first copies the two return value registers over:

        r->rax_ret = inv->_raw.rax_ret;
        r->rdx_ret = inv->_raw.rdx_ret;

If it's a stret call and the invocation actually has a buffer to hold the return value, the value in the invocation's return value buffer is copied into the memory pointed to by r->rdi, which is where the caller specified that it wanted the return value placed:

        if(r->isStretCall && inv->_stretBuffer)
            memcpy((void *)r->rdi, inv->_stretBuffer, [inv returnValueSize]);

Everything is now complete, so the invocation is released, and control is returned to the assembly language glue:

        [inv release];

The glue code will now copy the rax and rdx fields back into the respective CPU registers, then return control to the original method caller, which will see the return value either in those registers or in the stret buffer that it passed in rdi.

That wraps up the implementation of MAInvocation. It's enormously complicated and involved, despite only supporting x86-64 and ignoring struct parameters and floating point of all kinds, which are a large part of the x86-64 calling conventions. NSInvocation not only supports all types of parameters and return values (aside from a few corner cases like union parameters), it also supports them on at least three different architectures: i386, x86-64, and ARM.

However, despite the complication, it's all very much doable. Covering all of the cases requires a lot of time and effort, but there's nothing mysterious or magical. It would require equivalents of the assembly glue functions for the other architectures, expanding the glue functions to cover the floating-point registers, and implementing all of the logic for which arguments go where in the MAInvocation Objective-C code.

MAInvocation was a lot of fun to build and gives great insight on just what NSInvocation is doing. It should be obvious, but don't use MAInvocation for any real work. NSInvocation does all the same stuff and more, and no doubt does it better.

That's it for today. Come back next time for another breathtaking adventure. Friday Q&A is driven by reader ideas, so until then, please keep sending in your ideas for topics to cover.

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Jean-Daniel at 2013-03-23 22:17:20:
I know it would kill all the fun, but just for the record, it is possible to "easily" implement an Invocation class using libff to handle most calling convention subtleties.

Mark Aufflick at 2013-04-04 23:08:39:
@Jean-Daniel I assume you mean libffi ?

Carl Lindberg at 2013-08-04 23:52:23:
I'm not sure the isStretReturn implementation is quite right for all-float or all-double structures; I think such structures <= 16 bytes stay with the normal objc_msgSend and not the stret version. I'm pretty sure I've seen this in practice with CGPoint vs CGRect (only the latter uses _stret), and the debugDescription of NSMethodSignature (which prints out Apple's isStret value) seems to agree with that.

In terms of the spec, it says "If the size of the aggregate exceeds two eightbytes and the first eightbyte isn’t SSE or any other eightbyte isn’t SSEUP, the whole argument is passed in memory." So if the size does not exceed two eightbytes, a combination of all-SSE elements seems to not need a stret. NSMethodSignature does seem to allow {foo=dd} and {foo=ffff} as regular returns but changes to the stret return on say {foo=fffff}.

Minor nit, but... just in case you were interested ;-)

Carl Lindberg at 2013-08-05 20:22:04:
In fact, after some experimentation... looks like clang uses the stret return versions for basically every structure above 16 bytes, even structures which just have three or four long longs. Not sure I can figure out why reading the ABI spec but NSMethodSignature reports that, and it will crash at runtime using the wrong one -- NSMethodSignature seems accurate per what the compiler does.

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