On Sat, 6 Nov 2010 03:36:27 +0900
Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxxx>
Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Randy Dunlap <randy.dunlap@xxxxxxxxxx>
Cc: Eric Paris <eparis@xxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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; \
} \
)
_
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