Re: Bcache v. whatever

From: Andrew Morton
Date: Fri Apr 26 2013 - 16:24:44 EST

On Fri, 26 Apr 2013 12:46:42 -0700 Kent Overstreet <koverstreet@xxxxxxxxxx> wrote:

> 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@xxxxxxxxxx> wrote:
> >
> ...
> > 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.

That's the more idiomatic way of doing things and yes, the code
generation will be similarly awful.

> 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...

Documentation/Changes is the official status. It says gcc-3.2+. We're
very slow in updating those version numbers because it's hard.
gcc-3.4.5 for mips fails in the same way.

Why are those things macros anyway? urgh, it's because we want to jam
a string into the caller's stack frame without declaring any of it.

Really I do think it would be better to do away with the C party tricks
and have callers do

char btree_buf[BTREE_BUF_SIZE];

btree_to_text(btree_buf, b);
pr_debug("%s\n", btree_buf);

Nice, simple, explicit, direct and stupid. It might generate
unused-var warnings if DEBUG is undefined but from my reading of
pr_debug() things will be OK.

Then we can poke around at btree_to_text() until gcc-3.4.5 is happy
with it.
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