linux-kernel - Re: Bcache v. whatever

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Date:	Thu, 25 Apr 2013 16:17:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	tj@...nel.org, axboe@...nel.dk,
	James.Bottomley@...senpartnership.com, snitzer@...hat.com
Subject: Re: Bcache v. whatever

On Mon, 14 Jan 2013 14:32:02 -0800 Kent Overstreet <koverstreet@...gle.com> wrote:

> Bcache: a block layer SSD cache

sparc64 gcc-3.4.5:

drivers/md/bcache/btree.c: In function `bch_btree_read':
drivers/md/bcache/btree.c:266: error: invalid operands to binary +
drivers/md/bcache/btree.c: In function `__btree_write':
drivers/md/bcache/btree.c:379: error: invalid operands to binary +
drivers/md/bcache/btree.c: In function `btree_node_free':
drivers/md/bcache/btree.c:980: error: invalid operands to binary +
drivers/md/bcache/btree.c: In function `btree_insert_key':
drivers/md/bcache/btree.c:1857: error: invalid operands to binary +
drivers/md/bcache/btree.c:1857: error: invalid operands to binary +
drivers/md/bcache/btree.c:1859: error: invalid operands to binary +
drivers/md/bcache/btree.c:1859: error: invalid operands to binary +
drivers/md/bcache/btree.c:1864: error: invalid operands to binary +
drivers/md/bcache/btree.c:1864: error: invalid operands to binary +
drivers/md/bcache/btree.c: In function `btree_split':
drivers/md/bcache/btree.c:1934: error: invalid operands to binary +
drivers/md/bcache/btree.c: In function `bch_btree_set_root':
drivers/md/bcache/btree.c:2159: error: invalid operands to binary +
drivers/md/bcache/btree.c: In function `bch_btree_search_recurse':
drivers/md/bcache/btree.c:2262: error: invalid operands to binary +
drivers/md/bcache/btree.c: In function `bch_btree_refill_keybuf':
drivers/md/bcache/btree.c:2330: error: invalid operands to binary +

due to

#define pbtree(b)       (&bch_pbtree(b).s[0])

I don't know why this is happening (presumably a gcc glitch), but
returning an 80-byte struct by value from bch_pkey() and bch_pbtree()
is just gruesome.  The compiler has to allocate the space on the caller
stack, pass a hidden pointer into the callee and the callee copies its
return value into that caller stack slot.  It's slow and consumes stack.

Something different, please.


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