On Fri, 17 Jun 2022 at 19:00, Alexander Lobakin <alexandr.lobakin@xxxxxxxxx> wrote: > > Currently, many architecture-specific non-atomic bitop > implementations use inline asm or other hacks which are faster or > more robust when working with "real" variables (i.e. fields from > the structures etc.), but the compilers have no clue how to optimize > them out when called on compile-time constants. That said, the > following code: > > DECLARE_BITMAP(foo, BITS_PER_LONG) = { }; // -> unsigned long foo[1]; > unsigned long bar = BIT(BAR_BIT); > unsigned long baz = 0; > > __set_bit(FOO_BIT, foo); > baz |= BIT(BAZ_BIT); > > BUILD_BUG_ON(!__builtin_constant_p(test_bit(FOO_BIT, foo)); > BUILD_BUG_ON(!__builtin_constant_p(bar & BAR_BIT)); > BUILD_BUG_ON(!__builtin_constant_p(baz & BAZ_BIT)); > > triggers the first assertion on x86_64, which means that the > compiler is unable to evaluate it to a compile-time initializer > when the architecture-specific bitop is used even if it's obvious. > In order to let the compiler optimize out such cases, expand the > bitop() macro to use the "constant" C non-atomic bitop > implementations when all of the arguments passed are compile-time > constants, which means that the result will be a compile-time > constant as well, so that it produces more efficient and simple > code in 100% cases, comparing to the architecture-specific > counterparts. > > The savings are architecture, compiler and compiler flags dependent, > for example, on x86_64 -O2: > > GCC 12: add/remove: 78/29 grow/shrink: 332/525 up/down: 31325/-61560 (-30235) > LLVM 13: add/remove: 79/76 grow/shrink: 184/537 up/down: 55076/-141892 (-86816) > LLVM 14: add/remove: 10/3 grow/shrink: 93/138 up/down: 3705/-6992 (-3287) > > and ARM64 (courtesy of Mark): > > GCC 11: add/remove: 92/29 grow/shrink: 933/2766 up/down: 39340/-82580 (-43240) > LLVM 14: add/remove: 21/11 grow/shrink: 620/651 up/down: 12060/-15824 (-3764) > > Cc: Mark Rutland <mark.rutland@xxxxxxx> > Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx> Reviewed-by: Marco Elver <elver@xxxxxxxxxx> > --- > include/linux/bitops.h | 18 +++++++++++++++++- > 1 file changed, 17 insertions(+), 1 deletion(-) > > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index 3c3afbae1533..26a43360c4ae 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -33,8 +33,24 @@ extern unsigned long __sw_hweight64(__u64 w); > > #include <asm-generic/bitops/generic-non-atomic.h> > > +/* > + * Many architecture-specific non-atomic bitops contain inline asm code and due > + * to that the compiler can't optimize them to compile-time expressions or > + * constants. In contrary, gen_*() helpers are defined in pure C and compilers > + * optimize them just well. > + * Therefore, to make `unsigned long foo = 0; __set_bit(BAR, &foo)` effectively > + * equal to `unsigned long foo = BIT(BAR)`, pick the generic C alternative when > + * the arguments can be resolved at compile time. That expression itself is a > + * constant and doesn't bring any functional changes to the rest of cases. > + * The casts to `uintptr_t` are needed to mitigate `-Waddress` warnings when > + * passing a bitmap from .bss or .data (-> `!!addr` is always true). > + */ > #define bitop(op, nr, addr) \ > - op(nr, addr) > + ((__builtin_constant_p(nr) && \ > + __builtin_constant_p((uintptr_t)(addr) != (uintptr_t)NULL) && \ > + (uintptr_t)(addr) != (uintptr_t)NULL && \ > + __builtin_constant_p(*(const unsigned long *)(addr))) ? \ > + const##op(nr, addr) : op(nr, addr)) > > #define __set_bit(nr, addr) bitop(___set_bit, nr, addr) > #define __clear_bit(nr, addr) bitop(___clear_bit, nr, addr) > -- > 2.36.1 >