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

Tetsuo Handa
2021-05-11T11:54:28Z

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.