On Fri, Feb 07, 2020 at 10:07:32AM -0800, Linus Torvalds wrote:
> On Fri, Feb 7, 2020 at 12:18 AM Martin KaFai Lau <kafai@xxxxxx> wrote:
> >
> > It was reported that the max_t, ilog2, and roundup_pow_of_two macros have
> > exponential effects on the number of states in the sparse checker.
>
> Patch looks good, but I'd like to point out that it's not just sparse.
>
> You can see it with a simple
>
> make net/core/bpf_sk_storage.i
> grep 'smap->bucket_log = ' net/core/bpf_sk_storage.i | wc
>
> and see the end result:
>
> 1 365071 2686974
>
> That's one line (the assignment line) that is 2,686,974 characters in length.
In addition to this patch I've tried:
diff --git a/include/linux/log2.h b/include/linux/log2.h
index 83a4a3ca3e8a..7363abf60854 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -74,74 +74,76 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
* Use this where sparse expects a true constant expression, e.g. for array
* indices.
*/
-#define const_ilog2(n) \
-( \
- __builtin_constant_p(n) ? ( \
- (n) < 2 ? 0 : \
- (n) & (1ULL << 63) ? 63 : \
- (n) & (1ULL << 62) ? 62 : \
...
+#define __const_ilog2(n, unique_n) ({ \
+ typeof(n) unique_n = (n); \
+ __builtin_constant_p(unique_n) ? ( \
+ (unique_n) < 2 ? 0 : \
+ (unique_n) & (1ULL << 63) ? 63 : \
+ (unique_n) & (1ULL << 62) ? 62 : \
+ (unique_n) & (1ULL << 61) ? 61 : \
+ (unique_n) & (1ULL << 60) ? 60 : \
+ (unique_n) & (1ULL << 59) ? 59 : \
...
+ (unique_n) & (1ULL << 3) ? 3 : \
+ (unique_n) & (1ULL << 2) ? 2 : \
1) : \
- -1)
+ -1; })
+
+#define const_ilog2(n) __const_ilog2(n, __UNIQUE_ID(__n))
and for this nested ilog2() case that caused this explosion
the line got shorter: from 2.6M characters to 144k.
Still a lot.
Unfortunately this approach doesn't work in all cases:
../include/linux/log2.h:77:36: error: braced-group within expression allowed only inside a function
77 | #define __const_ilog2(n, unique_n) ({ \
| ^
../include/linux/log2.h:146:24: note: in expansion of macro ‘__const_ilog2’
146 | #define const_ilog2(n) __const_ilog2(n, __UNIQUE_ID(__n))
| ^~~~~~~~~~~~~
../include/linux/log2.h:161:2: note: in expansion of macro ‘const_ilog2’
161 | const_ilog2(n) : \
| ^~~~~~~~~~~
../include/linux/blockgroup_lock.h:14:27: note: in expansion of macro ‘ilog2’
14 | #define NR_BG_LOCKS (4 << ilog2(NR_CPUS < 32 ? NR_CPUS : 32))
| ^~~~~
../include/linux/blockgroup_lock.h:24:24: note: in expansion of macro ‘NR_BG_LOCKS’
24 | struct bgl_lock locks[NR_BG_LOCKS];
Just fyi for folks who're looking at ilog2 and wondering
why it was done this way without ({ })
