Commits on Apr 29, 2014

1. rustc: Enable -f{function,data}-sections …

   The compiler has previously been producing binaries on the order of 1.8MB for
   hello world programs "fn main() {}". This is largely a result of the compilation
   model used by compiling entire libraries into a single object file and because
   static linking is favored by default.
   
   When linking, linkers will pull in the entire contents of an object file if any
   symbol from the object file is used. This means that if any symbol from a rust
   library is used, the entire library is pulled in unconditionally, regardless of
   whether the library is used or not.
   
   Traditional C/C++ projects do not normally encounter these large executable
   problems because their archives (rust's rlibs) are composed of many objects.
   Because of this, linkers can eliminate entire objects from being in the final
   executable. With rustc, however, the linker does not have the opportunity to
   leave out entire object files.
   
   In order to get similar benefits from dead code stripping at link time, this
   commit enables the -ffunction-sections and -fdata-sections flags in LLVM, as
   well as passing --gc-sections to the linker *by default*. This means that each
   function and each global will be placed into its own section, allowing the
   linker to GC all unused functions and data symbols.
   
   By enabling these flags, rust is able to generate much smaller binaries default.
   On linux, a hello world binary went from 1.8MB to 597K (a 67% reduction in
   size). The output size of dynamic libraries remained constant, but the output
   size of rlibs increased, as seen below:
   
       libarena         -  2.27% bigger (   292872 =>    299508)
       libcollections   -  0.64% bigger (  6765884 =>   6809076)
       libflate         -  0.83% bigger (   186516 =>    188060)
       libfourcc        - 14.71% bigger (   307290 =>    352498)
       libgetopts       -  4.42% bigger (   761468 =>    795102)
       libglob          -  2.73% bigger (   899932 =>    924542)
       libgreen         -  9.63% bigger (  1281718 =>   1405124)
       libhexfloat      - 13.88% bigger (   333738 =>    380060)
       liblibc          - 10.79% bigger (   551280 =>    610736)
       liblog           - 10.93% bigger (   218208 =>    242060)
       libnative        -  8.26% bigger (  1362096 =>   1474658)
       libnum           -  2.34% bigger (  2583400 =>   2643916)
       librand          -  1.72% bigger (  1608684 =>   1636394)
       libregex         -  6.50% bigger (  1747768 =>   1861398)
       librustc         -  4.21% bigger (151820192 => 158218924)
       librustdoc       -  8.96% bigger ( 13142604 =>  14320544)
       librustuv        -  4.13% bigger (  4366896 =>   4547304)
       libsemver        -  2.66% bigger (   396166 =>    406686)
       libserialize     -  1.91% bigger (  6878396 =>   7009822)
       libstd           -  3.59% bigger ( 39485286 =>  40902218)
       libsync          -  3.95% bigger (  1386390 =>   1441204)
       libsyntax        -  4.96% bigger ( 35757202 =>  37530798)
       libterm          - 13.99% bigger (   924580 =>   1053902)
       libtest          -  6.04% bigger (  2455720 =>   2604092)
       libtime          -  2.84% bigger (  1075708 =>   1106242)
       liburl           -  6.53% bigger (   590458 =>    629004)
       libuuid          -  4.63% bigger (   326350 =>    341466)
       libworkcache     -  8.45% bigger (  1230702 =>   1334750)
   
   This increase in size is a result of encoding many more section names into each
   object file (rlib). These increases are moderate enough that this change seems
   worthwhile to me, due to the drastic improvements seen in the final artifacts.
   The overall increase of the stage2 target folder (not the size of an install)
   went from 337MB to 348MB (3% increase).
   
   Additionally, linking is generally slower when executed with all these new
   sections plus the --gc-sections flag. The stage0 compiler takes 1.4s to link the
   `rustc` binary, where the stage1 compiler takes 1.9s to link the binary. Three
   megabytes are shaved off the binary. I found this increase in link time to be
   acceptable relative to the benefits of code size gained.
   
   This commit only enables --gc-sections for *executables*, not dynamic libraries.
   LLVM does all the heavy lifting when producing an object file for a dynamic
   library, so there is little else for the linker to do (remember that we only
   have one object file).
   
   I conducted similar experiments by putting a *module's* functions and data
   symbols into its own section (granularity moved to a module level instead of a
   function/static level). The size benefits of a hello world were seen to be on
   the order of 400K rather than 1.2MB. It seemed that enough benefit was gained
   using ffunction-sections that this route was less desirable, despite the lesser
   increases in binary rlib size.

   

   alexcrichton committed Apr 29, 2014
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