[PLAT-8737] Use __attribute__((objc_direct_members))

[nickdowell]

Goal

Reduce binary code size.

Design

Use __attribute__((objc_direct_members)) on private @interfaces and @implementations to cause their methods to have no associated Objective-C metadata and use C function calling convention.

For information about this attribute, see

• https://reviews.llvm.org/D69991
• https://clang.llvm.org/docs/AttributeReference.html#objc-direct

Changeset

Adds BSG_OBJC_DIRECT_MEMBERS macro to BSGDefines.h.

Applies BSG_OBJC_DIRECT_MEMBERS to private @interfaces and @implementations not delared in the public headers or BugsnagInternals.h.

Testing

Tested via existing unit and E2E tests.

Use of __attribute__((objc_direct_members)) is disabled when building for testing to allow linking to private methods - direct methods have enforced hidden visibility and hence unreachable cross image.

[github-actions]

Bugsnag.framework binary size decreased by 54,504 bytes from 757,680 to 703,176 🎉

    FILE SIZE        VM SIZE    
 --------------  -------------- 
 -36.3% -4.97Ki  +264% +11.0Ki    [__DATA]
  +6.1% +1.55Ki  +6.1% +1.55Ki    [__TEXT]
  -0.6%     -12  -0.6%     -12    __TEXT,__stub_helper
  -0.6%     -12  -0.6%     -12    __TEXT,__stubs
  -0.5%     -21  -0.5%     -21    [3 Others]
  -0.2%     -32  -0.2%     -32    __DATA,__cfstring
  -0.9%     -40  -0.9%     -40    Lazy Binding Info
  -0.3%     -56  -0.3%     -56    __TEXT,__cstring
  -5.3%     -64  -5.3%     -64    Export Info
  -3.9%     -72  -3.9%     -72    Rebase Info
  -3.4%    -104  -3.4%    -104    __TEXT,__unwind_info
  -7.9%    -136  -7.9%    -136    Function Start Addresses
  -5.3%    -172  -5.3%    -172    __TEXT,__gcc_except_tab
 -15.5%    -377 -15.5%    -377    __TEXT,__objc_methtype
 -29.4% -1.88Ki -29.4% -1.88Ki    __DATA,__objc_selrefs
 -30.2% -7.70Ki -30.2% -7.70Ki    __TEXT,__objc_methname
 -20.1% -9.11Ki -20.1% -9.11Ki    __DATA,__objc_const
  -3.9% -9.12Ki  -3.9% -9.12Ki    __TEXT,__text
  -7.9% -10.4Ki  -7.9% -10.4Ki    Symbol Table
  -6.4% -10.5Ki  -6.4% -10.5Ki    String Table
  [ = ]       0 -89.2% -10.8Ki    [__LINKEDIT]
  -7.2% -53.2Ki  -6.4% -48.0Ki    TOTAL

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