From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Lobakin Subject: Re: [PATCH v2 6/6] bitops: let optimize out non-atomic bitops on compile-time constants Date: Wed, 15 Jun 2022 16:00:30 +0200 Message-ID: <20220615140030.1265068-1-alexandr.lobakin@intel.com> References: <20220610113427.908751-1-alexandr.lobakin@intel.com> <20220610113427.908751-7-alexandr.lobakin@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1655301764; x=1686837764; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=wB8IdGjwWR4VpCGztcdlKcCnZSDfsh9h9lU6StlhX6Q=; b=njhk0eETAY8TQd5C9oLIIhlM2Hdrshc02LDrFFc/7POKC+ssOIMN3Dtq eHX9Rio5q5TsytbqpGHVVgMTpG+qYXNlAA8kWOh2Ty0DBNXsKLG51Bkm6 h7Cb6e9eMmRTrOcelo9qQd0qRwnuGkLoVufGBDvwnqRK2T/6SGuRimIrK AhGJNmMtNsrtHsxj/NF1dhYPwNcl+KQGOxPs2ymC6ZD3RAZUqZqZ7gTc6 OX37OM5+GXEqCstu4szaVBVlP0iDKsCO1LKRZ7tImH/w9weGpkiAsbdDf 7pPRAE+7FXUguRRywamz7ZM7gI4D5e90ehMOCGuSo7nAvCL9LrbP2i+zh g==; In-Reply-To: List-ID: To: Yury Norov Cc: Alexander Lobakin , Arnd Bergmann , Andy Shevchenko , Mark Rutland , Matt Turner , Brian Cain , Geert Uytterhoeven , Yoshinori Sato , Rich Felker , "David S. Miller" , Kees Cook , "Peter Zijlstra (Intel)" , Marco Elver , Borislav Petkov , Tony Luck , Greg Kroah-Hartman , linux-alpha@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-sh@vger.ke From: Yury Norov Date: Tue, 14 Jun 2022 20:26:54 -0700 > Hi Alexander, > > On Fri, Jun 10, 2022 at 01:34:27PM +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)); > > Can you put this snippet into lib/test_bitops.c? Great idea, sure! > > Thanks, > Yury > > > 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 > > Signed-off-by: Alexander Lobakin > > --- > > 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 753f98e0dcf5..364bdc3606b4 100644 > > --- a/include/linux/bitops.h > > +++ b/include/linux/bitops.h > > @@ -33,8 +33,24 @@ extern unsigned long __sw_hweight64(__u64 w); > > > > #include > > > > +/* > > + * 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 Thanks, Olek