Re: [2.6.37-rc1] Build failure: __divdi3

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 6 Nov 2010 03:36:27 +0900
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> Andrew Morton wrote:
> > > >   #define roundup(x, y) (                                 \
> > > >   {                                                       \
> > > >   	typeof(y) __y = y;                              \
> > > >   	(((x) + (__y - 1)) / __y) * __y;                \
> > > >   }                                                       \
> > > >   )
> > > > 
> > > > for roundup((long long), (const)) case.
> > > 
> > > I guess this is my fault (I'm not completely convinced) since my testing
> > > was done with gcc 4.  I just retested to make sure that
> > > gcc-4.5.1-4.fc14.x86_64 is able to optimize both (and it does).  Is the
> > > fact that it requires compiler optimizations for the code to build a
> > > good thing?
> > 
> > It does make us pretty fragile against future compiler versions, but we
> > do rely quite a lot on such compiler things.  And things do break, such
> > as the several-month outbreak of calls to
> > you_cannot_kmalloc_that_much() a while back.
> > 
> > I wonder if there's some way of tricking gcc back into doing the right
> > thing here.  Make __y const or something?
> 
> Nice suggestion.
> 
> -typeof(y) __y = y;
> +const typeof(y) __y = y;
> 
> solved __divdi3 problem with GCC 3.3.5.
> 

OK, thanks.  Pending any better ideas, I typed this in:


From: Andrew Morton <akpm@linux-foundation.org>

gcc-3.3.5 (at least) is failing to optimise away the divide when `y' is a
constant 64-bit power-of-two value - it emits a call to __divdi3().

It turns out that defining `y' as const prevents this.

Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/kernel.h |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff -puN include/linux/kernel.h~include-linux-kernelh-roundup-work-around-a-gcc-33-miscompile include/linux/kernel.h
--- a/include/linux/kernel.h~include-linux-kernelh-roundup-work-around-a-gcc-33-miscompile
+++ a/include/linux/kernel.h
@@ -58,9 +58,11 @@ extern const char linux_proc_banner[];
 
 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
 #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
+
+/* The `const' in roundup() prevents gcc-3.3 from calling __divdi3 */
 #define roundup(x, y) (					\
 {							\
-	typeof(y) __y = y;				\
+	const typeof(y) __y = y;			\
 	(((x) + (__y - 1)) / __y) * __y;		\
 }							\
 )
_