Messages in this thread Patch in this message | | | Subject | Re: [PATCH 11/12] tools: sync lib/find_bit implementation | From | Tetsuo Handa <> | Date | Tue, 11 May 2021 20:53:53 +0900 |
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On 2021/05/11 0:44, Andy Shevchenko wrote: > And I'm a bit lost here, because I can't imagine the offset being > constant along with a size of bitmap. What do we want to achieve by > this? Any examples to better understand the case?
Because I feel that the GENMASK() macro cannot be evaluated without both arguments being a constant.
The usage is
unsigned long find_next_bit(const unsigned long *addr, unsigned long size, unsigned long offset) { + if (small_const_nbits(size)) { + unsigned long val; + + if (unlikely(offset >= size)) + return size; + + val = *addr & GENMASK(size - 1, offset); + return val ? __ffs(val) : size; + } + return _find_next_bit(addr, NULL, size, offset, 0UL, 0); }
where GENMASK() might be called even if "offset" is not a constant.
#define GENMASK(h, l) \ (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
#define __GENMASK(h, l) \ (((~UL(0)) - (UL(1) << (l)) + 1) & \ (~UL(0) >> (BITS_PER_LONG - 1 - (h))))
#define GENMASK_INPUT_CHECK(h, l) \ (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ __builtin_constant_p((l) > (h)), (l) > (h), 0)))
__GENMASK() does not need "h" and "l" being a constant.
Yes, small_const_nbits(size) in find_next_bit() can guarantee that "size" is a constant and hence "h" argument in GENMASK_INPUT_CHECK() call is also a constant. But nothing can guarantee that "offset" is a constant, and hence nothing can guarantee that "l" argument in GENMASK_INPUT_CHECK() call is also a constant.
Then, how can (l) > (h) in __builtin_constant_p((l) > (h)) be evaluated at build time if either l or h (i.e. "offset" and "size - 1" in find_next_bit()) lacks a guarantee of being a constant?
But what a surprise,
On 2021/05/11 7:51, Rikard Falkeborn wrote: > Does the following work for you? For simplicity, I copied__is_constexpr from > include/linux/minmax.h (which isn't available in tools/). A proper patch > would reuse __is_constexpr (possibly refactoring it to a separate > header since bits.h including minmax.h for that only seems smelly) and fix > bits.h in the kernel header as well, to keep the files in sync.
this works for me.
> > diff --git a/tools/include/linux/bits.h b/tools/include/linux/bits.h > index 7f475d59a097..7bc4c31a7df0 100644 > --- a/tools/include/linux/bits.h > +++ b/tools/include/linux/bits.h > @@ -19,10 +19,13 @@ > * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000. > */ > #if !defined(__ASSEMBLY__) > + > +#define __is_constexpr(x) \ > + (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8))) > #include <linux/build_bug.h> > #define GENMASK_INPUT_CHECK(h, l) \ > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ > - __builtin_constant_p((l) > (h)), (l) > (h), 0))) > + __is_constexpr((l) > (h)), (l) > (h), 0))) > #else > /* > * BUILD_BUG_ON_ZERO is not available in h files included from asm files, >
On 2021/05/11 7:52, Yury Norov wrote: > I tested the objtool build with the 8.4.0 and 7.5.0 compilers from > ubuntu 21 distro, and it looks working. Can you please share more > details about your system?
Nothing special. A plain x86_64 CentOS 7.9 system with devtoolset-8.
$ /opt/rh/devtoolset-8/root/bin/gcc --version gcc (GCC) 8.3.1 20190311 (Red Hat 8.3.1-3) Copyright (C) 2018 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ rpm -qi devtoolset-8-gcc Name : devtoolset-8-gcc Version : 8.3.1 Release : 3.2.el7 Architecture: x86_64 Install Date: Wed Apr 22 07:58:16 2020 Group : Development/Languages Size : 74838011 License : GPLv3+ and GPLv3+ with exceptions and GPLv2+ with exceptions and LGPLv2+ and BSD Signature : RSA/SHA1, Thu Apr 16 19:44:43 2020, Key ID 4eb84e71f2ee9d55 Source RPM : devtoolset-8-gcc-8.3.1-3.2.el7.src.rpm Build Date : Sat Mar 28 00:06:45 2020 Build Host : c1be.rdu2.centos.org Relocations : (not relocatable) Packager : CBS <cbs@centos.org> Vendor : CentOS URL : http://gcc.gnu.org Summary : GCC version 8 Description : The devtoolset-8-gcc package contains the GNU Compiler Collection version 7.
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