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Date: Wed, 28 Oct 2015 10:53:41 +0100
From: Denys Vlasenko <vda.linux@...glemail.com>
To: musl <musl@...ts.openwall.com>, Rich Felker <dalias@...c.org>
Subject: Re: [PATCH] configure: add gcc flags for better link-time optimization
On Tue, Oct 27, 2015 at 10:01 PM, Rich Felker <dalias@...c.org> wrote:
>> After -ffunction-sections -fdata-sections are added to gcc command line,
>> "nm" output does change, the entire difference is as follows:
>>
>> --- libc.so.nm.OLD 2015-10-27 19:57:52.971964518 +0100
>> +++ libc.so.nm 2015-10-27 19:58:28.544115009 +0100
>> @@ -18,7 +18,6 @@
>> 0000000000000001 T setutxent
>> 0000000000000001 W updwtmp
>> 0000000000000001 T updwtmpx
>> -0000000000000002 T dlclose
>> 0000000000000002 T __stack_chk_fail
>> 0000000000000003 T catclose
>> 0000000000000003 T dirfd
>> @@ -84,6 +83,7 @@
>> 0000000000000005 T catopen
>> 0000000000000005 T creall
>> 0000000000000005 T dladdr
>> +0000000000000005 T dlclose
>> 0000000000000005 T dlinfo
>> 0000000000000005 T __fbufsize
>> 0000000000000005 T __freadptrinc
>> @@ -328,7 +328,6 @@
>> 000000000000000b T recv
>> 000000000000000b T send
>> 000000000000000b T setprotoent
>> -000000000000000b T tfind
>> 000000000000000b T __xstat
>> 000000000000000b W __xstat64
>> 000000000000000c T __acquire_ptc
>> @@ -417,6 +416,7 @@
>> 000000000000000e T tcflow
>> 000000000000000e T tcflush
>> 000000000000000e T tcsendbreak
>> +000000000000000e T tfind
>> 000000000000000f T dcgettext
>> 000000000000000f T execv
>> 000000000000000f T execvp
>> @@ -986,8 +986,6 @@
>> 0000000000000032 T pthread_rwlock_init
>> 0000000000000032 T putchar_unlocked
>> 0000000000000032 T sem_init
>> -0000000000000032 T __stdio_exit
>> -0000000000000032 W __stdio_exit_needed
>> 0000000000000032 T wcschr
>> 0000000000000033 T pthread_rwlock_tryrdlock
>> 0000000000000033 T pthread_setcanceltype
>> @@ -1011,6 +1009,8 @@
>> 0000000000000035 T pthread_sigmask
>> 0000000000000035 T pwritev
>> 0000000000000035 W pwritev64
>> +0000000000000035 T __stdio_exit
>> +0000000000000035 W __stdio_exit_needed
>> 0000000000000035 W thrd_detach
>> 0000000000000035 T __tre_mem_destroy
>> 0000000000000035 T tsearch
>>
>> As you see, not a single label was eliminated.
>>
>>
>> The visible small differences in size for a few functions are caused
>> by the need to always use "near" jumps (not "short" ones)
>> for tail call optimizations now, since they now jump across sections:
>>
>> Before:
>> 00000000000274a6 <dlclose>:
>> 274a6: eb c8 jmp 27470 <__reset_tls+0x1f0>
>> After:
>> 000000000002795f <dlclose>:
>> 2795f: e9 c5 ff ff ff jmpq 27929 <__reset_tls+0x1f0>
>
> I see. That's probably not a big deal.
>
> Did you see any symbols disappear when adding --gc-sections?
Yes, I do.
$ nm --size-sort busybox_unstripped >busybox_unstripped.nm
$ nm --size-sort busybox_unstripped--gc-sections
>busybox_unstripped--gc-sections.nm
$ diff -u busybox_unstripped.nm busybox_unstripped--gc-sections.nm |
grep '^[^ @]'
--- busybox_unstripped.nm 2015-10-28 10:48:16.362304813 +0100
+++ busybox_unstripped--gc-sections.nm 2015-10-28 10:48:26.056294599 +0100
-0000000000000001 t reinit_unicode_for_ash
-0000000000000001 t reinit_unicode_for_hush
-0000000000000007 T xmalloc_sockaddr2host
-0000000000000008 b cur.1926
-0000000000000008 b dummy
-0000000000000008 b dummy_file
-0000000000000008 b end.1927
-0000000000000008 b lock.1928
-0000000000000008 T xstrtoi_range
-0000000000000008 T xstrtoll_range
-000000000000000a T bb_internal_getpwnam_r
-000000000000000a T ipneigh_main
-000000000000000c T xsocket_stream
-000000000000000e T xgid2group
-0000000000000010 T selinux_or_die
-0000000000000011 T xatoi_range_sfx
-0000000000000011 T xatou_range_sfx
-0000000000000012 T xstrtoi
-0000000000000013 T xatoll_range_sfx
-0000000000000015 T replace
-0000000000000017 T xatoi_sfx
-0000000000000017 T xspawn
-0000000000000018 T replace_underscores
-000000000000001a T bb_iswspace
-000000000000001b T bb_internal_setpwent
-000000000000001b T xgetgrgid
-000000000000001c T llist_rev
-000000000000001c T xstrtoll
-000000000000001d T xread_char
-000000000000001e T monotonic_ns
-0000000000000021 T xatoll_sfx
-0000000000000021 T xmalloc_fgetline_str
-0000000000000022 T bb_iswpunct
-0000000000000023 T bb_iswalnum
-000000000000002f T bb_internal_endpwent
-000000000000002f T isrv_want_wr
-0000000000000033 T bb_delete_module
-000000000000003c T index_in_str_array
-000000000000003e T rewind
-0000000000000043 T is_suffixed_with
-000000000000004b T moderror
-0000000000000054 T executable_exists
-000000000000005a T rta_addattr_l
-0000000000000062 T string_to_llist
-0000000000000088 T bb_init_module
-00000000000000ae T bb_herror_msg
-00000000000000c3 T parse_cmdline_module_options
-000000000000010d T __simple_malloc
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