What does ENABLE BITCODE do in xcode 7

Question

I have a problem with the embedded bitcode term  What is embedded bitcode  When to enable  ENABLE BITCODE in new Xcode  What happens to the binary when enabled  ENABLE BITCODE in Xcode 7

User · Answer

Since the exact question is  what does enable bitcode do   I d like to give a few thin technical details I ve figured out thus far  Most of this is practically impossible to figure out with 100  certainty until Apple releases the source code for this compiler  First  Apple s bitcode does not appear to be the same thing as LLVM bytecode  At least  I ve not been able to figure out any resemblance between them  It appears to have a proprietary header  always starts with  xar    and probably some link-time reference magic that prevents data duplications  If you write out a hardcoded string  this string will only be put into the data once  rather than twice as would be expected if it was normal LLVM bytecode    Second  bitcode is not really shipped in the binary archive as a separate architecture as might be expected  It is not shipped in the same way as say x86 and ARM are put into one binary  FAT archive   Instead  they use a special section in the architecture specific MachO binary named    LLVM  which is shipped with every architecture supported  ie  duplicated   I assume this is a short coming with their compiler system and may be fixed in the future to avoid the duplication    C code  compiled with clang -fembed-bitcode hi c -S -emit-llvm     include  lt stdio h gt   int main         printf  hi there         return 0      LLVM IR output      ModuleID     var folders rd sv6v2 f50nzbrn4f64gnd4gh0000gq T hi-a8c16c bc  target datalayout    e-m o-i64 64-f80 128-n8 16 32 64-S128  target triple    x86 64-apple-macosx10 10 0     str   private unnamed addr constant  10 x i8  c hi there  00   align 1  llvm embedded module   appending constant  1600 x i8  c  DE C0 17 0B 00 00 00 00 14 00 00 00  06 00 00 07 00 00 01BC C0 DE  0C 00 00 86 01 00 00 0B 82  00 02 00 00 00 12 00 00 00 07 81  91A C8 04I 06 1029 92 01 84 0C  05 08 19 1E 04 8Bb 80 10E 02B 92 0BB 84 102 148 08 18I 0A2D H 0A 90   C4R 80 0C 19 r  07 C8 08 11b A8 A0 A8  C6 F0 01 00 00 00Q 18 00 00 C7 00 00 00 1Bp  F8 FF FF FF FF 01 90 00 0D 08 03 82 1D CAa 1E E6 A1 0D E0A 1E CAa 1C D2a 1E CA A1 0D CC 01 1E DA  1C C8 010 87p  87y  07 80p 87wh 03s 90 87ph 87rh 03xx 87tp 07z  07yh 83r  87th 07 80 1E E4 A1 1E CA 01 18 DC E1 1D DA C0 1C E4  1C DA A1 1C DA 00 1E DE  1D DC 81 1E CAA 1E DA A0 1C D8  1D DA A1 0D DC E1 1D DC A1 0D D8 A1 1C C2 C1 1C 00 C2 1D DE A1 0D D2 C1 1D CCa 1E DA C0 1C E0 A1 0D DA  1C E8 01 1D 00s 08 07v 98 87r 00 08wx 876p 87pp 87yh 03s 80 876h 87p A0 07t 00 CC  1C D8a 1E CA 01  E6 81 1E C2a 1C D6 A1 0D E0A 1E DE 81 1E CAa 1C E8 E1 1D E4 A1 0D C4 A1 1E CC C1 1C CAA 1E DA  1E D2A 1F CA 01 C0 03 80 A0 87p 90 87s  07zh 83q 80 87z 00 C6 E1 1D E4 A1 1C E4 00  E8  1C E4 E1 1C CA 81 1E DA C0 1C CA  1C E8 A1 1E E4 A1 1C E6 01X 83y 98 87y  879  835 18 07  88 03   835 98 87y  076X 83y 98 87r 90 036X 83y 98 87r 98 03 80 A8 07w 98 87p0 87rh 03s 80 876h 87p A0 07t 00 CC  1C D8a 1E CA 01  EAa 1E CA A1 0D E6 E1 1D CC 81 1E DA C0 1C D8 E1 1D C2 81 1E 00s 08 07v 98 87r 006 C8 88 F0 FF FF FF FF 03 C1 0E E50 0F F3 D0 06 F0  0F E50 0E E90 0F E5 D0 06 E6 00 0F ED 10 0E E4 00 98C8 B0 C3 lt  94 03  B8 C3  B4 819 C8C8 B4C9 B4 01 lt  BCC  B8 03  94 83 lt  B4A9 B0C  B4 03  0F F2P 0F E5 00 0C EE F0 0Em  0E F2 10 0E EDP 0Em 00 0F EF 90 0E EE  0F E5  0FmP 0E EC 90 0E ED D0 06 EE F0 0E EE D0 06 ECP 0E E1  0E 00 E1 0E EF D0 06 E9 E0 0E E60 0Fm  0E F0 D0 06 ED 10 0E F4 80 0E 809 84 03  CCC9 00 84  BCC 1B B8C8 B8 C3 lt  B4 819 C0C 1B B4C8 D0 03  00 E6 10 0E EC0 0F E5 00 10 F3  0F E10 0E EB D0 06 F0  0F EF  0F E50 0E F4 F0 0E F2 D0 06 E2P 0F E6  0E E5  0Fm0 0F E9 A0 0F E5 00 E0 01  D0C8 C8 C39 94 03  B4 C18 C0C  00 E3 F0 0E F2P 0Er 00 10 F4 10 0E F2p 0E E5  0Fm  0E E5 10 0E F4P 0F F2P 0E F3 00 AC C1 lt  CC C3 lt  94 C3 1C B0 C1 1A 8C 03 gt  C4 81 1D B0 C1 1A CC C3 lt  94 03 1B AC C1 lt  CCC9 C8 01 1B AC C1 lt  CCC9 CC 01  D4 83  CCC8 98C9 B4 819 C0C 1B B4C8 D0 03  00 E6 10 0E EC0 0F E5 00 10 F50 0F E5 D0 06 F3 F0 0E E6  0Fm  0E EC F0 0E E1  0F 809 84 03  CCC9 00 00I 18 00 00 02 00 00 00 13 82 B  00 00 00 89  00 00 0D 00 00 002 22 08 09 d 85 04 13 22 A4 84 04 13 22 E3 84 A1 90 14 12L 88 8C 0B 84 84L 100s 04H  00 C5 1C 01 18 94  88 08 AA0F7 10 3 02 00 134  C0 03  F8 05  A0 836 08 07x 80 07v  876h 87p 18 87w 98 07  88 038p 838 80 037 80 83 0DeP 0Em D0 0Ez F0 0Em 90 0Ev  07z  07t D0 06 E6 80 07p A0 07q  07x D0 06 EE 80 07z 10 07v A0 07s  07z  07t D0 06 B3 10 07r 80 07  0FDH  EB 80 1D 8C 10 18I 00 00  00 00 C0 10 A7 00 00  00 00 00 00 00 00 00 868 08 10 00 02 00 00 00 00 00 00 90 05 02 00 00 08 00 00 002 1E 98 0C 19 11L 90 8C 09 amp G C6 04C 9A 22  01 0AM D0i 10 1D  96 97C 00 00 00y 18 00 00 1C 00 00 00 1A 03L 90F 02 134A 18 08 amp PIC Level 13 84a D80 04 C2 C05 08 82 83c  03ab B2j 02 B1  93 9BK s 03 B9q 81q 81 01A 19c 0Bs k B9 81 81q 81q A9 99q 99I D9 10 14 8D D8 D8 EC DA 5C DA DE C8 EA D8 CA 5C CC D8 C2 CE E6 A6 04C 1566 BB6 974 B227 BA A 01 00y 18 00 002 00 00 003 08 80 1C C4 E1 1Cf 14 01  88C8 84 C3 8CB 80 07yx 07s 98q 0C E6 00 0F ED 10 0E F4 80 0E3 0CB 1E C2 C1 1D CE A1 1Cf0 05  88C8 84 83 1B CC 03  C8C  8C 03  CCx 8Ctp 07  08 07yH 87pp 07zp 03vx 87p  87 19 CC 11 0E EC 90 0E E10 0Fn0 0F E3 F0 0E F0P 0E3 10 C4 1D DE  1C D8  1D C2a 1Ef0 89  BC 83  D0C9 B4 03 lt  BC 83 lt  84 03  CC F0 14v  07 h 077h 87rh 077 80 87p 90 87p  07v  07v F8 05vx 87w 80 87  08 87q 18 87r 98 87y 98 81  EE F0 0E EE E0 0E F5 C0 0E EC 00q  00 00 05 00 00 00 amp   lt  11 D2L 85 05 10 0C 804 06  F8 D2 14 01 00 00a  00 00 0B 00 00 00 13 04A  10 00 00 00 03 00 00 004  00dC 19 020 18 83 01 003 11 CA  0C 83 11 C1 00 00  06 04 00 1CB 12 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   section    LLVM   bitcode   llvm cmdline   appending constant  67 x i8  c -triple 00x86 64-apple-macosx10 10 0 00-emit-llvm 00-disable-llvm-optzns 00   section    LLVM   cmdline     Function Attrs  nounwind ssp uwtable define i32  main    0      1   alloca i32  align 4   store i32 0  i32   1    2   call i32  i8          printf i8  getelementptr inbounds   10 x i8     str  i32 0  i32 0     ret i32 0    declare i32  printf i8         1  attributes  0     nounwind ssp uwtable  less-precise-fpmad   false   no-frame-pointer-elim   true   no-frame-pointer-elim-non-leaf   no-infs-fp-math   false   no-nans-fp-math   false   stack-protector-buffer-size   8   target-cpu   core2   target-features    ssse3  cx16  sse  sse2  sse3   unsafe-fp-math   false   use-soft-float   false    attributes  1      less-precise-fpmad   false   no-frame-pointer-elim   true   no-frame-pointer-elim-non-leaf   no-infs-fp-math   false   no-nans-fp-math   false   stack-protector-buffer-size   8   target-cpu   core2   target-features    ssse3  cx16  sse  sse2  sse3   unsafe-fp-math   false   use-soft-float   false      llvm module flags      0   llvm ident      1    0     i32 1    PIC Level   i32 2   1       Apple LLVM version 7 0 0  clang-700 0 53 3      The data array that is in the IR also changes depending on the optimization and other code generation settings of clang  It s completely unknown to me what format or anything that this is in   EDIT   Following the hint on Twitter  I decided to revisit this and to confirm it  I followed this blog post and used his bitcode extractor tool to get the Apple Archive binary out of the MachO executable  And after extracting the Apple Archive with the xar utility  I got this  converted to text with llvm-dis of course     ModuleID    1  target datalayout    e-m o-i64 64-f80 128-n8 16 32 64-S128  target triple    x86 64-apple-macosx10 10 0     str   private unnamed addr constant  10 x i8  c hi there  00   align 1    Function Attrs  nounwind ssp uwtable define i32  main    0      1   alloca i32  align 4   store i32 0  i32   1    2   call i32  i8         printf i8  getelementptr inbounds   10 x i8    10 x i8     str  i32 0  i32 0     ret i32 0    declare i32  printf i8         1  attributes  0     nounwind ssp uwtable  less-precise-fpmad   false   no-frame-pointer-elim   true   no-frame-pointer-elim-non-leaf   no-infs-fp-math   false   no-nans-fp-math   false   stack-protector-buffer-size   8   target-cpu   core2   target-features    ssse3  cx16  sse  sse2  sse3   unsafe-fp-math   false   use-soft-float   false    attributes  1      less-precise-fpmad   false   no-frame-pointer-elim   true   no-frame-pointer-elim-non-leaf   no-infs-fp-math   false   no-nans-fp-math   false   stack-protector-buffer-size   8   target-cpu   core2   target-features    ssse3  cx16  sse  sse2  sse3   unsafe-fp-math   false   use-soft-float   false      llvm module flags      0   llvm ident      1    0     i32 1    PIC Level   i32 2   1       Apple LLVM version 7 0 0  clang-700 1 76      The only notable difference really between the non-bitcode IR and the bitcode IR is that filenames have been stripped to just 1  2  etc for each architecture    I also confirmed that the bitcode embedded in a binary is generated after optimizations  If you compile with -O3 and extract out the bitcode  it ll be different than if you compile with -O0    And just to get extra credit  I also confirmed that Apple does not ship bitcode to devices when you download an iOS 9 app  They include a number of other strange sections that I don t recognized like   LINKEDIT  but they do not include   LLVM   bundle  and thus do not appear to include bitcode in the final binary that runs on a device  Oddly enough  Apple still ships fat binaries with separate 32 64bit code to iOS 8 devices though

User · Answer

Bitcode refers to to the type of code   LLVM Bitcode  that is sent to iTunes Connect  This allows Apple to use certain calculations to re-optimize apps further  e g  possibly downsize executable sizes   If Apple needs to alter your executable then they can do this without a new build being uploaded   This differs from  Slicing which is the process of Apple optimizing your app for a user s device based on the device s resolution and architecture  Slicing does not require Bitcode   Ex  only including  2x images on a 5s   App Thinning is the combination of slicing  bitcode  and on-demand resources     Bitcode is an intermediate representation of a compiled program  Apps   you upload to iTunes Connect that contain bitcode will be compiled and   linked on the App Store  Including bitcode will allow Apple to   re-optimize your app binary in the future without the need to submit a   new version of your app to the store    Apple Documentation on App Thinning

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What is embedded bitcode   According to docs      Bitcode is an intermediate representation of a compiled program  Apps you upload to iTunes Connect that contain bitcode will be compiled and linked on the App Store  Including bitcode will allow Apple to re-optimize your app binary in the future without the need to submit a new version of your app to the store    Update  This phrase in  New Features in Xcode 7  made me to think for a long time that Bitcode is needed for Slicing to reduce app size       When you archive for submission to the App Store  Xcode will compile your app into an intermediate representation  The App Store will then compile the bitcode down into the 64 or 32 bit executables as necessary    However that s not true  Bitcode and Slicing work independently  Slicing is about reducing app size and generating app bundle variants  and Bitcode is about certain binary optimizations  I ve verified this by checking included architectures in executables of non-bitcode apps and founding that they only include necessary ones   Bitcode allows other App Thinning component called Slicing to generate app bundle variants with particular executables for particular architectures  e g  iPhone 5S variant will include only arm64 executable  iPad Mini armv7 and so on   When to enable ENABLE BITCODE in new Xcode      For iOS apps  bitcode is the default  but optional  If you provide bitcode  all apps and frameworks in the app bundle need to include bitcode  For watchOS and tvOS apps  bitcode is required    What happens to the binary when ENABLE BITCODE is enabled in the new Xcode   From Xcode 7 reference       Activating this setting indicates that the target or project should generate bitcode during compilation for platforms and architectures which support it   For Archive builds  bitcode will be generated in the linked binary for submission to the app store   For other builds  the compiler and linker will check whether the code complies with the requirements for bitcode generation  but will not generate actual bitcode     Here s a couple of links that will help in deeper understanding of Bitcode    Bitcode Demystified Why I m not enabling Bitcode

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Bitcode  iOS  watchOS       Bitcode is an intermediate representation of a compiled program  Apps you upload to iTunes Connect that contain bitcode will be compiled and linked on the App Store  Including bitcode will allow Apple to re-optimize your app binary in the future without the need to submit a new version of your app to the store      Basically this concept is somewhat similar to java where byte code is run on different JVM s and in this case the bitcode is placed on iTune store and instead of giving the intermediate code to different platforms devices  it provides the compiled code which don t need any virtual machine to run   Thus we need to create the bitcode once and it will be available for existing or coming devices  It s the Apple s headache to compile an make it compatible with each platform they have   Devs don t have to make changes and submit the app again to support new platforms   Let s take the example of iPhone 5s when apple introduced x64 chip in it  Although x86 apps were totally compatible with x64 architecture but to fully utilise the x64 platform the developer has to change the architecture or some code  Once s he s done the app is submitted to the app store for the review    If this bitcode concept was launched earlier then we the developers doesn t have to make any changes to support the x64 bit architecture

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Update  Apple has clarified that slicing occurs independent of enabling bitcode  I ve observed this in practice as well where a non-bitcode enabled app will only be downloaded as the architecture appropriate for the target device   Original  More specifically      Bitcode  Archive your app for submission to the App Store in an   intermediate representation  which is compiled into 64- or 32-bit   executables for the target devices when delivered       Slicing  Artwork incorporated into the Asset Catalog and tagged for a   platform allows the App Store to deliver only what is needed for   installation    The way I read this  if you support bitcode  downloaders of your app will only get the compiled architecture needed for their own device

[ios] What does ENABLE_BITCODE do in xcode 7?

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