Re: [PATCH 11/12] tools: sync lib/find_bit implementation

Rikard Falkeborn
2021-05-11T20:38:10Z

On Tue, May 11, 2021 at 08:53:53PM +0900, Tetsuo Handa wrote:
> 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?
>

So the idea is that if (l > h) is constant, __builtin_constant_p should
evaluate that, and if it is not it should use zero instead as input to
__builtin_chose_expr(). This works with non-const inputs in many other
places in the kernel, but apparently in this case with a certain
compiler, it doesn't so I guess we need to work around it.

> 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.
>

Great, thanks for testing!

I sent a patch for this here:
https://lore.kernel.org/lkml/20210511203716.117010-1-rikard.falkeborn@gmail.com/

Rikard

> >
> > 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.
>