Re: [PATCH v3 6/7] bitops: let optimize out non-atomic bitops on compile-time constants

[Alexander Lobakin]

From: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Date: Mon, 20 Jun 2022 13:05:06 +0300

> On Fri, Jun 17, 2022 at 04:40:30PM +0200, Alexander Lobakin 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)
> 
> ...
> 
> > +/*
> > + * 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
> 
> generic_*() ?

Ah right, bah, forgot to change that in v2. Will fix in v4, as
__builtin_constant_p() test from v7 triggered build bugs on ARC,
will look into that.

> 
> > + * 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).
> > + */
> 
> -- 
> With Best Regards,
> Andy Shevchenko

Thanks,
Al