Consuming the Objective-C Runtime - objc_msgSend Struct edition

28 Sep 2014

objc_msgSend is a hand optimized, magical, function of the Objective-C runtime. It is how messages are implemented. In order to consume it’s API in C, a user must select the correct message send variant and explicitly cast the function. The compiler can abstract the machine details away with the information provided by static typing. While implementing the message send API, I found a bit of compile time magic in the C and Objective-C implementations.

Cupertino.js, a “JavaScript to Cocoa compiler” I’ve been excited about lately, uses objc_msgSend to dispatch call invocations. Since the JavaScript runtime is implemented on top of the Objective-C runtime, all variants of message send need to be supported.

Armed with the public runtime header for guidance:

* Sends a message with a data-structure return value to an instance of a class.
* @see objc_msgSend
OBJC_EXPORT void objc_msgSend_stret(id self, SEL op, ...)

I derived a broken prototype:

Much like the C , it declared a pointer to objc_msgSend_stret and casted it to the proper type. It was invoked with the target, selector, and other arguments like objc_msgSend. It accessed the return value like objc_msgSend.

My test device was a 32 bit X86 machine and the message send worked great for most cases. Things got interesting when I started testing calls to -[UIScreen bounds], which return a struct CGRect. The test machine had a 32 bit arm cpu, formally armv7. It was obvious the implementation was wrong after objc_msgSend_stret exploded.

So I coded a msgSend_stret in Objective-C.

UIView *view = [UIView new];
CGRect frame = view.frame;

Here’s part of the ARM (armv7) assembly Clang emitted for the above code:

	movw	r0, :lower16:(L_OBJC_SELECTOR_REFERENCES_7-(LPC2_2+4))
	mov	r1, r4
	movt	r0, :upper16:(L_OBJC_SELECTOR_REFERENCES_7-(LPC2_2+4))
	add	r0, pc
	ldr	r2, [r0]
	mov	r0, sp
	blx	_objc_msgSend_stret

The standard arguments are shifted over by one!

The standard argument registers are: r0, r1, r2, and r4. The stack pointer, which is used for the struct variable, is at r0, the UIView instance view is at r1, and the selector frame at r3. More specifically, directly before objc_msgSend_stret is called, the stack pointer is moved to r1. Up a few lines, the selector is loaded in r2 and the local variable view is moved to r1.

From the perspective of the C API, this seems odd. However, in practice, the API is intuitive. The temporary struct frame is just a stack variable. Its lifetime is the scope of this frame. As a result, its memory location must be passed to the function.

When it comes to the Objective-C runtime, there is no better reference than a working implementation. I read the message send implementation in Clang. There’s a comment that explains the proper use of objc_msgSend. Take a look at CGObjCMac.cpp:

/// void objc_msgSend_stret (id, SEL, ...)
/// The messenger used when the return value is an aggregate returned
/// by indirect reference in the first argument, and therefore the
/// self and selector parameters are shifted over by one.

There are other nuances of implementation objc_msgSend. Not all architectures use objc_msgSend_stret for struct returns and some conditionally use objc_msgSend for depending on the struct size.

It’s instructive to read the logic of CGObjCCommonMac::EmitMessageSend, specifically this bit:

  llvm::Constant *Fn = nullptr;
  if (CGM.ReturnSlotInterferesWithArgs(MSI.CallInfo)) {
    if (!IsSuper) nullReturn.init(CGF, Arg0);
    Fn = (ObjCABI == 2) ?  ObjCTypes.getSendStretFn2(IsSuper)
      : ObjCTypes.getSendStretFn(IsSuper);
  } else if (CGM.ReturnTypeUsesFPRet(ResultType)) {
    Fn = (ObjCABI == 2) ? ObjCTypes.getSendFpretFn2(IsSuper)
      : ObjCTypes.getSendFpretFn(IsSuper);
  } else if (CGM.ReturnTypeUsesFP2Ret(ResultType)) {
    Fn = (ObjCABI == 2) ? ObjCTypes.getSendFp2RetFn2(IsSuper)
      : ObjCTypes.getSendFp2retFn(IsSuper);
  } else {
    // arm64 uses objc_msgSend for stret methods and yet null receiver check
    // must be made for it.
    if (!IsSuper && CGM.ReturnTypeUsesSRet(MSI.CallInfo))
      nullReturn.init(CGF, Arg0);
    Fn = (ObjCABI == 2) ? ObjCTypes.getSendFn2(IsSuper)
      : ObjCTypes.getSendFn(IsSuper);


The runtime’s nice user API is quite deceptive: There’s magic happening under the hood for objc_msgSend! It’s a sensible design to allocate stack memory for this variable and function of the prolog. To use the C API, the user must cast the function to the correct type because, the compiler needs the type information to allocate the correct amount of memory and select a messenger. Higher level languages abstract machine away. Static typing is required for objc_msgSend in C and Objective-C.

When in doubt, use the source Luke.

It’s worth noting that official runtime documentation is quite useful. objc_msgSend and its many variants are well documented:

Sends a message with a data-structure return value to an instance of a class.
void objc_msgSend_stret(void * stretAddr, id theReceiver, SEL theSelector,  ...)

Conclusion Pt 2: What would this look like in JavaScript?

Cupertino.js introduces a “cast operator” to add static typing to JavaScript function calls. The cast operator is named after the corresponding type and syntactically works like a function call:

var frame = CGRect(view.frame)

This provides enough information for the compiler to allocate the local variable and select the correct messenger.

The cast operator is necessary becase the static typing information provided by -[UIView frame] isn’t enough. In a completely dynamic language it would be incorrect to assume that all methods with the name frame return a CGRect. A method could legally have the name frame but not return a struct.

If you spotted any errors in this post, please let me know!

Published on 28 Sep 2014 Find me on Twitter!