Re: Bcache v. whatever

[Kent Overstreet <koverstreet@google.com>]

On Thu, Apr 25, 2013 at 04:17:04PM -0700, Andrew Morton wrote:
> On Mon, 14 Jan 2013 14:32:02 -0800 Kent Overstreet <koverstreet@google.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.

Well, it is kind of... perverse but really the compiler's doing exactly
what I would've had to do otherwise - stick a char buf[80] on the
caller's stack and pass it to bch_pbtree(). With the caveat that I
haven't looked at the generated code.

As far as I can tell the only real improvement would be to add a %p
format string to vsnprintf, but adding a global extension would obviously be
inappropriate for this. It'd be really nice to have a mechanism for
adding file/module private format strings to vsnprintf, but I haven't
cared enough yet to implement it myself.

Of course if you know a better solution I'm all ears.

Uhm, as for the actual bug - that is a fairly ancient gcc, I wasn't
aware we were supporting compilers that old but I'm sure you wouldn't be
bugging me about it if we weren't...

If you _really_ want me to rip out the macro/struct return hack I
will... but this is just debug code and I hate making it more verbose if
I don't have to.

Otherwise, I'll set up gcc-3.4.5 (hopefully it doesn't have to be a
sparc compiler :P) and see if I can get gcc to stop complaining.