[llvm-dev] objc object file generated for gnustep runtime for ELF target is too big

Perevalov Alexey via llvm-dev
Mon, 26 Sep 2016 13:33:11 -0000

Dear community,


I'm using gnustep runtime -fobjc-runtime=gnustep with       gnustep-libobjc2
(https://github.com/gnustep/libobjc2) and

Cocotron/Chameleon.

For following source file

#import <Foundation/Foundation.h>
      #import <UIKit/UIKit.h>

int main(void)
        {
                NSString *str = [NSString stringWithCString:"TEST"];
                NSLog(@"test object %@", str);
                return 0;
        }

Compilation in xcode's clang
xcrun -sdk iphoneos clang -arch armv7s -mios-version-min=6.1 -c
test_foundation.m -o test_foundation_apple.o
gives following result:
1. object file has only 3156 bytes size
2. and in disassembler only main function
3. the final executable will be 49992 bytes size in this case


But compilation in llvm's clang downloaded from
http://llvm.org/releases/3.8.0/clang+llvm-3.8.0-x86_64-apple-darwin.tar.xz

(
clang version 3.8.0 (tags/RELEASE_380/final)
      Target: x86_64-apple-darwin15.0.0
      Thread model: posix
      )
gives another results:
1. object file has 159044 bytes size
2. in disassembler a lot of unnecessary .objc_property_list,
.objc_method_list, .objc_protocol_list and .objc_protocol  in .bss       and
.data section
3. the final binary size is 101837 bytes size



If comment in above sample #import <UIKit/UIKit.h, the size of       object
file and binary file in xcode compilation will be the same,
but size binary and object file in case of
clang+llvm-3.8.0-x86_64-apple-darwin compilation will be reduced
sufficiently:
binary 28569 and object 36952 accordingly.



Looks like all protocols from UIKit.h is placed into       binary/object even
it's not used there.
I got 3.9.0 from public git (branch release_39 of clang git       repository)
and found that all protocols interpret as definition
(isThisDeclarationADefinition),
and in CGObjCGNU.cpp it's placed into object file whenever it's
used or not. With the same open source compiler and       -fobjc-runtime=ios
protocols are not putting into
object file even in compilation for ELF (I used -target
arm-linux-gnueabi).
I used default binutil linker, not llvm's ld.ldd, due to I have problem with
relocation on ARM.
I didn't use LTO, due to my linker doesn't have plugin support (I
didnt' yet recompile it).

Could somebody point me where to solve that problem, is it AST       generation
stage, or linkage stage or maybe it's not a problem and just LTO could be
solution here?




p.s. It's not only one issue found by me in gnustep runtime
generation, and I'll report these issues in next emails.





         Best regards,
        Alexey Perevalov