linux-ext4 - Re: [PATCH 0/4] e2fprogs: compiler fun

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Date:   Thu, 9 Aug 2018 16:28:13 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH 0/4] e2fprogs: compiler fun

On Thu, Aug 09, 2018 at 12:18:32PM -0700, Darrick J. Wong wrote:
> 
> Now there's a side effect I didn't notice before -- LTO means we switch
> to static libraries, presumably if the linker can find static libs with
> LTO bytecode inside.

Hmm, I'm not seeing this with my compiler toolchain (binaries built
using dpkg-buildpackage -us -uc -b on a recently updated Debian
unstable system):

<tytso@...c> {/tmp/debian/e2fsprogs/debian/e2fsprogs/sbin}   (next)
1391% ldd debugfs 
	linux-vdso.so.1 (0x00007ffc816f7000)
	libext2fs.so.2 => /lib/x86_64-linux-gnu/libext2fs.so.2 (0x00007f395fdcb000)
	libe2p.so.2 => /lib/x86_64-linux-gnu/libe2p.so.2 (0x00007f395fdc1000)
	libss.so.2 => /lib/x86_64-linux-gnu/libss.so.2 (0x00007f395fdb7000)
	libcom_err.so.2 => /lib/x86_64-linux-gnu/libcom_err.so.2 (0x00007f395fdb1000)
	libblkid.so.1 => /lib/x86_64-linux-gnu/libblkid.so.1 (0x00007f395fb62000)
	libuuid.so.1 => /lib/x86_64-linux-gnu/libuuid.so.1 (0x00007f395f95b000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f395f954000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f395f797000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f395f776000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f395fe7c000)
<tytso@...c> {/tmp/debian/e2fsprogs/debian/e2fsprogs/sbin}   (next)
1392% ldd e2fsck
	linux-vdso.so.1 (0x00007fffae2e7000)
	libext2fs.so.2 => /lib/x86_64-linux-gnu/libext2fs.so.2 (0x00007f35f07cf000)
	libcom_err.so.2 => /lib/x86_64-linux-gnu/libcom_err.so.2 (0x00007f35f07c9000)
	libblkid.so.1 => /lib/x86_64-linux-gnu/libblkid.so.1 (0x00007f35f057a000)
	libuuid.so.1 => /lib/x86_64-linux-gnu/libuuid.so.1 (0x00007f35f0373000)
	libe2p.so.2 => /lib/x86_64-linux-gnu/libe2p.so.2 (0x00007f35f0369000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f35f0364000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f35f01a5000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f35f0184000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f35f089c000)
<tytso@...c> {/tmp/debian/e2fsprogs/debian/e2fsprogs/sbin}   (next)
1393% grep -- -lto ../../rules
COMMON_CONF_FLAGS = --enable-lto --disable-ubsan --disable-addrsan \

And given the bloated size of libext2fs.a, I know that we actually
were building with LTO:

<tytso@...c> {/tmp/debian/e2fsprogs/debian/e2fsprogs/sbin}   (next)
1394% ls ../../libext2fs-dev/usr/lib/x86_64-linux-gnu/libext2fs.a 
3696 ../../libext2fs-dev/usr/lib/x86_64-linux-gnu/libext2fs.a

And just in case the problem was shared object files with -fpic
weren't built with LTO, I can confirm they were:

<tytso@...c> {/tmp/debian/e2fsprogs/debian/e2fsprogs/sbin}   (next)
1395% cd ../../BUILD-STD/lib/ext2fs/
<tytso@...c> {/tmp/debian/e2fsprogs/debian/BUILD-STD/lib/ext2fs}   (next)
1396% objdump -x elfshared/alloc.o | grep lto | head -10
  3 .gnu.lto_.profile.dc6c1beb1dcdd1cc 00000014  0000000000000000  0000000000000000  00000e1a  2**0
  4 .gnu.lto_.icf.dc6c1beb1dcdd1cc 00000082  0000000000000000  0000000000000000  00000e2e  2**0
  5 .gnu.lto_.jmpfuncs.dc6c1beb1dcdd1cc 0000033c  0000000000000000  0000000000000000  00000eb0  2**0
  6 .gnu.lto_.inline.dc6c1beb1dcdd1cc 000004c1  0000000000000000  0000000000000000  000011ec  2**0
  7 .gnu.lto_.pureconst.dc6c1beb1dcdd1cc 00000035  0000000000000000  0000000000000000  000016ad  2**0
  8 .gnu.lto_check_inode_uninit.dc6c1beb1dcdd1cc 000006e8  0000000000000000  0000000000000000  000016e2  2**0
  9 .gnu.lto_ext2fs_clear_block_uninit.part.7.dc6c1beb1dcdd1cc 000002bf  0000000000000000  0000000000000000  00001dca  2**0
 10 .gnu.lto_ext2fs_get_free_blocks2.part.8.dc6c1beb1dcdd1cc 00000703  0000000000000000  0000000000000000  00002089  2**0
 11 .gnu.lto_ext2fs_alloc_range.part.9.dc6c1beb1dcdd1cc 000003ca  0000000000000000  0000000000000000  0000278c  2**0
 12 .gnu.lto_ext2fs_clear_block_uninit.dc6c1beb1dcdd1cc 000003c6  0000000000000000  0000000000000000  00002b56  2**0

And just double checking to be sure:

<tytso@...c> {/tmp/debian/e2fsprogs/debian/BUILD-STD/e2fsck}   (next)
1399% objdump -x e2fsck | grep lto | head -10
000000000000f6d3 l     F .text	00000000000002c9              new_table_block.lto_priv.31
000000000000f610 l     F .text	00000000000000c3              finish_processing_inode.part.27.lto_priv.28
0000000000016730 l     F .text	00000000000000c7              mark_inode_bad.lto_priv.30
00000000000503a0 l     O .bss	0000000000000038              ext2_max_sizes.lto_priv.15
00000000000503d8 l     O .bss	0000000000000008              inodes_to_process.lto_priv.16
0000000000034900 l     F .text	0000000000000197              e2fsck_handle_read_error.lto_priv.22
0000000000050740 l     O .bss	0000000000000008              operation.lto_priv.8
0000000000012e60 l     F .text	000000000000006a              pass1_read_inode.lto_priv.19
000000000002bcc0 l     F .text	00000000000001fb              check_large_ea_inode.lto_priv.13
000000000000f99c l     F .text	000000000000014a              adjust_extattr_refcount.lto_priv.27
<tytso@...c> {/tmp/debian/e2fsprogs/debian/BUILD-STD/e2fsck}   (next)
1400% ldd e2fsck
	linux-vdso.so.1 (0x00007fff889a6000)
	libext2fs.so.2 => /lib/x86_64-linux-gnu/libext2fs.so.2 (0x00007f2512abe000)
	libcom_err.so.2 => /lib/x86_64-linux-gnu/libcom_err.so.2 (0x00007f2512ab8000)
	libblkid.so.1 => /lib/x86_64-linux-gnu/libblkid.so.1 (0x00007f2512869000)
	libuuid.so.1 => /lib/x86_64-linux-gnu/libuuid.so.1 (0x00007f2512662000)
	libe2p.so.2 => /lib/x86_64-linux-gnu/libe2p.so.2 (0x00007f2512658000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f2512653000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f2512494000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f2512473000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f2512b8b000)

> That's an interesting implication -- libraries which are shared widely
> among the "usual" set of running programs on a computer should not be
> LTO'd because we're better off (in terms of memory consumption) to share
> the library code; but libraries which are not widely shared should be
> LTO if the reduction in code size outweighs the loss of amortization
> possibilities.  I guess?

I think the bigger take home is that LTO is very much an emerging
technology, and may be true for one version of the toolchain may not
be true for another.

So I'm wondering if we should default LTO to be off, rather than
--enable-lto=probe for now.  Mainly because I'm a bit more nervous how
mature the LTO support is for all distributions.

       	       	       	      	  - Ted



> 
> --D
> 
> > 
> > 						- Ted