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Message-Id: <1429742932-6026-1-git-send-email-armccurdy@gmail.com>
Date: Wed, 22 Apr 2015 15:48:52 -0700
From: Andre McCurdy <armccurdy@...il.com>
To: musl@...ts.openwall.com
Cc: Andre McCurdy <armccurdy@...il.com>
Subject: building musl libc.so with gcc -flto
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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