Re: [PATCH] btrfs: remove redundant local var in read_block_for_search

[David Sterba <dsterba@suse.cz>]

On Wed, May 27, 2020 at 01:10:59PM +0300, Nikolay Borisov wrote:
> The local 'b' variable is only used to directly read values from passed
> extent buffer. So eliminate  it and directly use the input parameter. Furthermore
> this shrinks the size of the following functions:
> 
> ./scripts/bloat-o-meter ctree.orig fs/btrfs/ctree.o
> add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-73 (-73)
> Function                                     old     new   delta
> read_block_for_search.isra                   876     871      -5
> push_node_left                              1112    1044     -68
> -	struct extent_buffer *b = *eb_ret;

> -	blocknr = btrfs_node_blockptr(b, slot);
> -	gen = btrfs_node_ptr_generation(b, slot);
> -	parent_level = btrfs_header_level(b);
> -	btrfs_node_key_to_cpu(b, &first_key, slot);
> +	blocknr = btrfs_node_blockptr(*eb_ret, slot);
> +	gen = btrfs_node_ptr_generation(*eb_ret, slot);
> +	parent_level = btrfs_header_level(*eb_ret);
> +	btrfs_node_key_to_cpu(*eb_ret, &first_key, slot);

It's interesting how such trivial and obvious change can improve the
code generation. The removed indirection resulted in smaller function
push_node_left that is not even called directly, but as there are lot of
static helpers it's some intra-procedural pass that can utilize the
reduced variable indirection.