uselex.rb as a tiny tool to find dead code

From: Sergei Trofimovich <[email protected]>

Hi guys!

TL;DR:

the patches remove (currently or already) dead code and localize symbol visibility used only in one module.

Thanks!

The Story:

Once upon a time I've got very simple idea to find dead code in large C/C++ mixed project at work.

The things of my primary interest were things of too large visibility scope, like:
- externally visible global variables used only in module defining the variable
- and functions of the same property

Once you identify those functions and mark as 'static' you achieve two goals:

- minor: give compiler possibility to inline code a bit better (especially for one-caller case)
- more interesting: possibility to spot that the symbol is no longer used at all

Hence the project idea:
1. scan all intermediate object (.o) files for exported symbols
2. find such symbols that are not imported into any .o files

Meet uselex.rb: one-file script to parse 'nm' output:

https://github.com/trofi/uselex/blob/master/uselex.rb

For linux's kernel it's use is as simple as:

    $ make $config && make
    $ uselex.rb `find -name '*.o'`

and you've got material to look at!

It outputs 3000 symbols thus I've started small:

    $ uselex.rb `find fs/btrfs -name '*.o'

To get an impression it reports such things:

set_state_private: [R]: exported from: fs/btrfs/built-in.o fs/btrfs/btrfs.o fs/btrfs/extent_io.o
btrfs_get_inode_ref_index: [R]: exported from: fs/btrfs/built-in.o fs/btrfs/inode-item.o fs/btrfs/btrfs.o
btrfs_print_tree: [R]: exported from: fs/btrfs/built-in.o fs/btrfs/print-tree.o fs/btrfs/btrfs.o
btrfs_reada_detach: [R]: exported from: fs/btrfs/built-in.o fs/btrfs/reada.o fs/btrfs/btrfs.o __btrfs_getxattr: [R]: exported from: fs/btrfs/built-in.o fs/btrfs/xattr.o fs/btrfs/btrfs.o __btrfs_setxattr: [R]: exported from: fs/btrfs/built-in.o fs/btrfs/xattr.o fs/btrfs/btrfs.o
btrfs_read_root_item: [R]: exported from: fs/btrfs/root-tree.o fs/btrfs/built-in.o fs/btrfs/btrfs.o __tracepoint_btrfs_sync_fs: [R]: exported from: fs/btrfs/super.o fs/btrfs/built-in.o fs/btrfs/btrfs.o btrfs_start_transaction_lflush: [R]: exported from: fs/btrfs/transaction.o fs/btrfs/built-in.o fs/btrfs/btrfs.o btrfs_write_and_wait_marked_extents: [R]: exported from: fs/btrfs/transaction.o fs/btrfs/built-in.o fs/btrfs/btrfs.o
ulist_fini: [R]: exported from: fs/btrfs/ulist.o fs/btrfs/built-in.o fs/btrfs/btrfs.o
ulist_init: [R]: exported from: fs/btrfs/ulist.o fs/btrfs/built-in.o fs/btrfs/btrfs.o

You just need to make sure it does not lie too much.

For example '__btrfs_getxattr' needs to be brought under '#ifdef CONFIG_BTRFS_FS_POSIX_ACL' and not removed entirely.

Thanks for your patience!