[PATCH v5 8/9] bitmap: don't assume compiler evaluates small mem*() builtins calls

Alexander Lobakin
2022-06-24T12:14:17Z

Intel kernel bot triggered the build bug on ARC architecture that
in fact is as follows:

DECLARE_BITMAP(bitmap, BITS_PER_LONG);

bitmap_clear(bitmap, 0, BITS_PER_LONG);
BUILD_BUG_ON(!__builtin_constant_p(*bitmap));

which can be expanded to:

unsigned long bitmap[1];

memset(bitmap, 0, sizeof(*bitmap));
BUILD_BUG_ON(!__builtin_constant_p(*bitmap));

In most cases, a compiler is able to expand small/simple mem*()
calls to simple assignments or bitops, in this case that would mean:

unsigned long bitmap[1] = { 0 };

BUILD_BUG_ON(!__builtin_constant_p(*bitmap));

and on most architectures this works, but not on ARC, despite having
-O3 for every build.
So, to make this work, in case when the last bit to modify is still
within the first long (small_const_nbits()), just use plain
assignments for the rest of bitmap_*() functions which still use
mem*(), but didn't receive such compile-time optimizations yet.
This doesn't have the same coverage as compilers provide, but at
least something to start:

text: add/remove: 3/7 grow/shrink: 43/78 up/down: 1848/-3370 (-1546)
data: add/remove: 1/11 grow/shrink: 0/8 up/down: 4/-356 (-352)

notably cpumask_*() family when NR_CPUS <= BITS_PER_LONG:

netif_get_num_default_rss_queues 38 4 -34
cpumask_copy 90 - -90
cpumask_clear 146 - -146

and the abovementioned assertion started passing.

Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
---
include/linux/bitmap.h | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 2e6cd5681040..a0f4f3af8d30 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -238,20 +238,32 @@ extern int bitmap_print_list_to_buf(char *buf, const unsigned long *maskp,
static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
{
unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
- memset(dst, 0, len);
+
+ if (small_const_nbits(nbits))
+ *dst = 0;
+ else
+ memset(dst, 0, len);
}

static inline void bitmap_fill(unsigned long *dst, unsigned int nbits)
{
unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
- memset(dst, 0xff, len);
+
+ if (small_const_nbits(nbits))
+ *dst = ~0UL;
+ else
+ memset(dst, 0xff, len);
}

static inline void bitmap_copy(unsigned long *dst, const unsigned long *src,
unsigned int nbits)
{
unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
- memcpy(dst, src, len);
+
+ if (small_const_nbits(nbits))
+ *dst = *src;
+ else
+ memcpy(dst, src, len);
}

/*
@@ -431,6 +443,8 @@ static __always_inline void bitmap_set(unsigned long *map, unsigned int start,
{
if (__builtin_constant_p(nbits) && nbits == 1)
__set_bit(start, map);
+ else if (small_const_nbits(start + nbits))
+ *map |= GENMASK(start + nbits - 1, start);
else if (__builtin_constant_p(start & BITMAP_MEM_MASK) &&
IS_ALIGNED(start, BITMAP_MEM_ALIGNMENT) &&
__builtin_constant_p(nbits & BITMAP_MEM_MASK) &&
@@ -445,6 +459,8 @@ static __always_inline void bitmap_clear(unsigned long *map, unsigned int start,
{
if (__builtin_constant_p(nbits) && nbits == 1)
__clear_bit(start, map);
+ else if (small_const_nbits(start + nbits))
+ *map &= ~GENMASK(start + nbits - 1, start);
else if (__builtin_constant_p(start & BITMAP_MEM_MASK) &&
IS_ALIGNED(start, BITMAP_MEM_ALIGNMENT) &&
__builtin_constant_p(nbits & BITMAP_MEM_MASK) &&
--
2.36.1