Re: [PATCH 03/14] bitops: let the compiler optimize __assign_bit()
From: Yury Norov <yury.norov@gmail.com>
Date: 2023-10-09 16:18:47
Also in:
dm-devel, linux-btrfs, linux-s390, lkml, ntfs3
On Mon, Oct 09, 2023 at 05:10:15PM +0200, Alexander Lobakin wrote:
Since commit b03fc1173c0c ("bitops: let optimize out non-atomic bitops
on compile-time constants"), the compilers are able to expand inline
bitmap operations to compile-time initializers when possible.
However, during the round of replacement if-__set-else-__clear with
__assign_bit() as per Andy's advice, bloat-o-meter showed +1024 bytes
difference in object code size for one module (even one function),
where the pattern:
DECLARE_BITMAP(foo) = { }; // on the stack, zeroed
if (a)
__set_bit(const_bit_num, foo);
if (b)
__set_bit(another_const_bit_num, foo);
...
is heavily used, although there should be no difference: the bitmap is
zeroed, so the second half of __assign_bit() should be compiled-out as
a no-op.
I either missed the fact that __assign_bit() has bitmap pointer marked
as `volatile` (as we usually do for bitmaps) or was hoping that the
No, we usually don't. Atomic ops on individual bits is a notable exception
for bitmaps, as the comment for generic_test_bit() says, for example:
/*
* Unlike the bitops with the '__' prefix above, this one *is* atomic,
* so `volatile` must always stay here with no cast-aways. See
* `Documentation/atomic_bitops.txt` for the details.
*/
For non-atomic single-bit operations and all multi-bit ops, volatile is
useless, and generic___test_and_set_bit() in the same file casts the
*addr to a plain 'unsigned long *'.
quoted hunk ↗ jump to hunk
compilers would at least try to look past the `volatile` for __always_inline functions. Anyhow, due to that attribute, the compilers were always compiling the whole expression and no mentioned compile-time optimizations were working. Convert __assign_bit() to a macro since it's a very simple if-else and all of the checks are performed inside __set_bit() and __clear_bit(), thus that wrapper has to be as transparent as possible. After that change, despite it showing only -20 bytes change for vmlinux (due to that it's still relatively unpopular), no drastic code size changes happen when replacing if-set-else-clear for onstack bitmaps with __assign_bit(), meaning the compiler now expands them to the actual operations will all the expected optimizations. Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com> --- include/linux/bitops.h | 10 ++-------- 1 file changed, 2 insertions(+), 8 deletions(-)diff --git a/include/linux/bitops.h b/include/linux/bitops.h index e0cd09eb91cd..f98f4fd1047f 100644 --- a/include/linux/bitops.h +++ b/include/linux/bitops.h@@ -284,14 +284,8 @@ static __always_inline void assign_bit(long nr, volatile unsigned long *addr, clear_bit(nr, addr); } -static __always_inline void __assign_bit(long nr, volatile unsigned long *addr, - bool value) -{ - if (value) - __set_bit(nr, addr); - else - __clear_bit(nr, addr); -} +#define __assign_bit(nr, addr, value) \ + ((value) ? __set_bit(nr, addr) : __clear_bit(nr, addr))
Can you protect nr and addr with braces just as well? Can you convert the atomic version too, to keep them synchronized ?
/** * __ptr_set_bit - Set bit in a pointer's value -- 2.41.0