[Mesa-dev] [PATCH] util/macros: Simplify DIV_ROUND_UP() definition

[Matt Turner]

On Thu, Dec 17, 2015 at 11:04 AM, Nanley Chery <nanleychery at gmail.com> wrote:
> On Thu, Dec 17, 2015 at 12:05:46PM +0100, Glenn Kennard wrote:
>> On Wed, 16 Dec 2015 20:57:51 +0100, Nanley Chery <nanleychery at gmail.com> wrote:
>>
>> >From: Nanley Chery <nanley.g.chery at intel.com>
>> >
>> >Commit 64880d073ab21ae1abad0c049ea2d6a1169a3cfa consolidated two
>> >DIV_ROUND_UP() definitions to one, but chose the more
>> >compute-intensive version in the process. Use the simpler version
>> >instead. Reduces .text size by 1360 bytes.
>> >
>> >Output of `size lib/i965_dri.so`:
>> >      text    data     bss     dec     hex filename
>> >   7850440  219264   27240 8096944  7b8cb0 lib/i965_dri.so (before)
>> >   7849080  219264   27240 8095584  7b8760 lib/i965_dri.so (after)
>> >
>> >Cc: Axel Davy <axel.davy at ens.fr>
>> >Signed-off-by: Nanley Chery <nanley.g.chery at intel.com>
>> >---
>> > src/util/macros.h | 2 +-
>> > 1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> >diff --git a/src/util/macros.h b/src/util/macros.h
>> >index 0c8958f..53a98a0 100644
>> >--- a/src/util/macros.h
>> >+++ b/src/util/macros.h
>> >@@ -211,6 +211,6 @@ do {                       \
>> > #endif
>> >/** Compute ceiling of integer quotient of A divided by B. */
>> >-#define DIV_ROUND_UP( A, B )  ( (A) % (B) == 0 ? (A)/(B) : (A)/(B)+1 )
>> >+#define DIV_ROUND_UP(A, B)  (((A) + (B) - 1) / (B))
>> >#endif /* UTIL_MACROS_H */
>>
>> I'll point out that these are not equivalent, one can overflow and the other doesn't. You
>> probably want to check if the call sites have sufficient checks for that before
>> substituting one for the other.
>>
>
> Good point. As I mentioned in another email, I'll leave the current
> macro untouched.

I think the chances of us relying on overflow behavior are exceedingly
small, and nearly all uses of DIV_ROUND_UP are in the i965 driver. I
think it's sufficiently safe to go ahead with the patch (but I am
still interested to know about your compiler and CFLAGS).