InspectiveCarm64.mm

struct ObjcMsgSendAndEnabled {
  uintptr_t msgSendPtr;
  uintptr_t enabled;
};

// arm64 hooking magic.

// Called in our replacementObjc_msgSend before calling the original objc_msgSend.
// This pushes a CallRecord to our stack, most importantly saving the lr.
// Returns orig_objc_msgSend in x0 and isLoggingEnabled in x1.
struct ObjcMsgSendAndEnabled preObjc_msgSend(id self, uintptr_t lr, SEL _cmd, struct RegState_ *rs) {
  ThreadCallStack *cs = getThreadCallStack();
  if (!cs->isLoggingEnabled) { // Not enabled, just return.
    return (struct ObjcMsgSendAndEnabled) {reinterpret_cast<uintptr_t>(orig_objc_msgSend), 0};
  }
  pushCallRecord(self, lr, _cmd, cs);
  pa_list args = (pa_list){ rs, ((unsigned char *)rs) + 208, 2, 0 }; // 208 is the offset of rs from the top of the stack.

  preObjc_msgSend_common(self, lr, _cmd, cs, args);

  return (struct ObjcMsgSendAndEnabled) {reinterpret_cast<uintptr_t>(orig_objc_msgSend), 1};
}

// Our replacement objc_msgSend (arm64).
//
// See:
// https://blog.nelhage.com/2010/10/amd64-and-va_arg/
// http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf
// https://developer.apple.com/library/ios/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARM64FunctionCallingConventions.html
__attribute__((__naked__))
static void replacementObjc_msgSend() {
  __asm__ volatile (
    // push {q0-q7}
      "stp q6, q7, [sp, #-32]!\n"
      "stp q4, q5, [sp, #-32]!\n"
      "stp q2, q3, [sp, #-32]!\n"
      "stp q0, q1, [sp, #-32]!\n"
    // push {x0-x8, lr}
      "stp x8, lr, [sp, #-16]!\n"
      "stp x6, x7, [sp, #-16]!\n"
      "stp x4, x5, [sp, #-16]!\n"
      "stp x2, x3, [sp, #-16]!\n"
      "stp x0, x1, [sp, #-16]!\n"
    // Swap args around for call.
      "mov x2, x1\n"
      "mov x1, lr\n"
      "mov x3, sp\n"
    // Call preObjc_msgSend which puts orig_objc_msgSend into x0 and isLoggingEnabled into x1.
      "bl __Z15preObjc_msgSendP11objc_objectmP13objc_selectorP9RegState_\n"
      "mov x9, x0\n"
      "mov x10, x1\n"
      "tst x10, x10\n" // Set condition code for later branch.
    // pop {x0-x8, lr}
      "ldp x0, x1, [sp], #16\n"
      "ldp x2, x3, [sp], #16\n"
      "ldp x4, x5, [sp], #16\n"
      "ldp x6, x7, [sp], #16\n"
      "ldp x8, lr, [sp], #16\n"
    // pop {q0-q7}
      "ldp q0, q1, [sp], #32\n"
      "ldp q2, q3, [sp], #32\n"
      "ldp q4, q5, [sp], #32\n"
      "ldp q6, q7, [sp], #32\n"
    // Make sure it's enabled.
      "b.eq Lpassthrough\n"
    // Call through to the original objc_msgSend.
      "blr x9\n"
    // push {x0-x9}
      "stp x0, x1, [sp, #-16]!\n"
      "stp x2, x3, [sp, #-16]!\n"
      "stp x4, x5, [sp, #-16]!\n"
      "stp x6, x7, [sp, #-16]!\n"
      "stp x8, x9, [sp, #-16]!\n" // Not sure if needed - push for alignment.
    // push {q0-q7}
      "stp q0, q1, [sp, #-32]!\n"
      "stp q2, q3, [sp, #-32]!\n"
      "stp q4, q5, [sp, #-32]!\n"
      "stp q6, q7, [sp, #-32]!\n"
    // Call our postObjc_msgSend hook.
      "bl __Z16postObjc_msgSendv\n"
      "mov lr, x0\n"
    // pop {q0-q7}
      "ldp q6, q7, [sp], #32\n"
      "ldp q4, q5, [sp], #32\n"
      "ldp q2, q3, [sp], #32\n"
      "ldp q0, q1, [sp], #32\n"
    // pop {x0-x9}
      "ldp x8, x9, [sp], #16\n"
      "ldp x6, x7, [sp], #16\n"
      "ldp x4, x5, [sp], #16\n"
      "ldp x2, x3, [sp], #16\n"
      "ldp x0, x1, [sp], #16\n"
      "ret\n"

    // Pass through to original objc_msgSend.
      "Lpassthrough:\n"
      "br x9"
    );
}