building musl libc.so with gcc -flto

[Andre McCurdy <armccurdy@gmail.com>]

Hi all,

Below are some observations from building musl libc.so with gcc's -flto
(link time optimization) option.

1) With today's master (afbcac68), adding -flto to CFLAGS causes the
build to fail:

 | `_dlstart_c' referenced in section `.text' of /tmp/cc8ceNIy.ltrans0.ltrans.o: defined in discarded section `.text' of src/ldso/dlstart.lo (symbol from plugin)
 | collect2: error: ld returned 1 exit status
 | make: *** [lib/libc.so] Error 1

Reverting f1faa0e1 (make _dlstart_c function use hidden visibility)
seems to be a workaround.

2) With f1faa0e1 reverted, the build succeeds, but with a warning about
differing declarations for dummy_tsd and __pthread_tsd_main:

 | src/thread/pthread_create.c:169:1: warning: type of '__pthread_tsd_main' does not match original declaration
 |  weak_alias(dummy_tsd, __pthread_tsd_main);
 |  ^
 | src/thread/pthread_key_create.c:4:7: note: previously declared here
 |  void *__pthread_tsd_main[PTHREAD_KEYS_MAX] = { 0 };
 |        ^

3) Overall build times are similar, but archieving the best results
with -flto relies on manually duplicating any 'make -j' options for
the linker. Times below are from a quad core + hyperthreading system
running 'make -j8 lib/libc.so':

  original : real 0m8.501s
  -flto    : real 0m18.034s
  -flto=4  : real 0m9.885s
  -flto=8  : real 0m8.876s

4) Changes in code size seem to be minor, except when compiling with
-O3, where the code gets noticably larger (presumably due to -flto
giving a lot more scope for inlining?). Results below are from building
with gcc 4.9.2 for 32bit x86:

    text    data     bss     dec     hex filename

  536405    1416    8800  546621   8573d lib/libc.so      ( -Os )
  536324    1324    8780  546428   8567c lib/libc.so.lto  ( -Os )

  612028    1416    8928  622372   97f24 lib/libc.so      ( -O2 )
  611701    1304    9132  622137   97e39 lib/libc.so.lto  ( -O2 )

  687708    1416    8992  698116   aa704 lib/libc.so      ( -O3 )
  713704    1312    9208  724224   b0d00 lib/libc.so.lto  ( -O3 )

Andre
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