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Date: Mon, 27 Apr 2015 17:16:12 -0700
From: Andre McCurdy <armccurdy@...il.com>
To: musl@...ts.openwall.com
Subject: Re: building musl libc.so with gcc -flto
On Thu, Apr 23, 2015 at 2:45 AM, Rich Felker <dalias@...c.org> wrote:
> On Wed, Apr 22, 2015 at 10:34:40PM -0700, Andre McCurdy wrote:
>> On Wed, Apr 22, 2015 at 7:23 PM, Rich Felker <dalias@...c.org> wrote:
>> > On Wed, Apr 22, 2015 at 03:48:52PM -0700, Andre McCurdy wrote:
>> >> Hi all,
>> >>
>> >> Below are some observations from building musl libc.so with gcc's -flto
>> >> (link time optimization) option.
>> >
>> > Interesting!
>> >
>> >> 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.
>> >
>> > I think the problem is that LTO is garbage collecting "unused" symbols
>> > before it gets to the step of linking with asm for which there is no
>> > IR code, thereby losing anything that's only referenced from asm. A
>> > better workaround might be to define _dlstart_c with a different name
>> > as a non-hidden function (e.g. call it __dls1) and then make
>> > _dlstart_c a hidden alias for it via:
>> >
>> > __attribute__((__visibility__("hidden")))
>> > void _dlstart_c(size_t *, size_t *);
>> >
>> > weak_alias(__dls1, _dlstart_c);
>> >
>> > If you get a chance to try that, let me know if it works.
>>
>> That change does fix the build, but the resulting binary fails to run:
>>
>> $ gdb ./lib/libc.so
>> ....
>> (gdb) run
>> Starting program: /home/andre/.../lib/libc.so
>>
>> Program received signal SIGILL, Illegal instruction.
>> 0x56572ab8 in _dlstart ()
>> (gdb) disassemble
>> Dump of assembler code for function _dlstart:
>> 0x56572aa0 <+0>: xor %ebp,%ebp
>> 0x56572aa2 <+2>: mov %esp,%eax
>> 0x56572aa4 <+4>: and $0xfffffff0,%esp
>> 0x56572aa7 <+7>: push %eax
>> 0x56572aa8 <+8>: push %eax
>> 0x56572aa9 <+9>: call 0x56572aae <_dlstart+14>
>> 0x56572aae <+14>: addl $0x7864a,(%esp)
>> 0x56572ab5 <+21>: push %eax
>> 0x56572ab6 <+22>: call 0x56572ab7 <_dlstart+23>
>> 0x56572abb <+27>: nop
>> 0x56572abc <+28>: lea 0x0(%esi,%eiz,1),%esi
>> End of assembler dump.
>> (gdb)
>
> OK, it looks like the _dlstart_c symbol got removed before linking the
> asm. What about selectively compiling this file with -fno-lto via
> something like this in config.mak:
>
> src/ldso/dlstart.lo: CFLAGS += -fno-lto
That works. Should I send a patch?
>> > Also seems rather like what I would expect. Any idea if performance is
>> > significantly better? It's not very comprehensive but you could try
>> > libc-bench.
>>
>> I modified libc-bench so that it loops though everything in main() ten
>> times and then ran the same libc-bench binary with each version of
>> libc.so, sending output to /dev/null.
>>
>> The -O3 -flto build seems to be consistently very slightly *slower*
>> than the non -flto version...
>
> That makes the whole thing somewhat less interesting. LTO is probably
> more interesting for static libc.
Yes, quite disappointing...
I'll try to experiment a little with static linking.
>
> Rich
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