diff mbox series

[v2] lib: test_bitops: add compile-time optimization/evaluations assertions

Message ID 20231130102717.1297492-1-mailhol.vincent@wanadoo.fr
State New
Headers
Series [v2] lib: test_bitops: add compile-time optimization/evaluations assertions |

Commit Message

Vincent Mailhol Nov. 30, 2023, 10:27 a.m. UTC 
  Add a function in the bitops test suite to assert that the bitops
helper correctly fold constant expressions (or trigger a build bug
otherwise). This should work on all the optimization levels supported
by Kbuild.

The function doesn't perform any runtime tests and gets optimized out to
nothing after passing the build assertions.

Architectures which fail that test should adjust their
arch/*/include/asm/bitops.h in order to use the compiler's generic
__builitn implementation if the argument is a constant expression
(similar to [1]).

[1] commit 146034fed6ee ("x86/asm/bitops: Use __builtin_ffs() to evaluate
    constant expressions")

Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
This is tested on x86, x86_64, arm and arm64.

Other architectures were not tested. My idea would be to add this
patch to any kind of CI which runs on all architecture (linux-next?)
and see if anything breaks.

Or maybe I should send a message to the maintainers of all of the
arch/*/include/asm/bitops.h files?

Tell me what you think.

** Changelog **

v1 -> v2:

  - Drop the RFC patch. v1 was not ready to be applied on x86 because
    of pending changes in arch/x86/include/asm/bitops.h. This was
    finally fixed by Nick in commit 3dae5c43badf ("x86/asm/bitops: Use
    __builtin_clz{l|ll} to evaluate constant expressions"). Thanks Nick!

  - Update the commit description.

  - Introduce the test_const_eval() macro to factorize code.

  - No functional changes.

  Link: https://lore.kernel.org/all/20221111081316.30373-1-mailhol.vincent@wanadoo.fr/
---
 lib/Kconfig.debug |  4 ++++
 lib/test_bitops.c | 31 +++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

Comments

kernel test robot Nov. 30, 2023, 4:25 p.m. UTC | #1 
Hi Vincent,

kernel test robot noticed the following build errors:

[auto build test ERROR on akpm-mm/mm-everything]
[also build test ERROR on linus/master v6.7-rc3 next-20231130]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Vincent-Mailhol/lib-test_bitops-add-compile-time-optimization-evaluations-assertions/20231130-182837
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20231130102717.1297492-1-mailhol.vincent%40wanadoo.fr
patch subject: [PATCH v2] lib: test_bitops: add compile-time optimization/evaluations assertions
config: m68k-allmodconfig (https://download.01.org/0day-ci/archive/20231201/202312010058.JJKeeqvE-lkp@intel.com/config)
compiler: m68k-linux-gcc (GCC) 13.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231201/202312010058.JJKeeqvE-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202312010058.JJKeeqvE-lkp@intel.com/

All errors (new ones prefixed by >>):

   In file included from <command-line>:
   In function 'test_bitops_const_eval',
       inlined from 'test_bitops_startup' at lib/test_bitops.c:126:2:
>> include/linux/compiler_types.h:435:45: error: call to '__compiletime_assert_183' declared with attribute error: BUILD_BUG_ON failed: !__builtin_constant_p(__test_expr)
     435 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
         |                                             ^
   include/linux/compiler_types.h:416:25: note: in definition of macro '__compiletime_assert'
     416 |                         prefix ## suffix();                             \
         |                         ^~~~~~
   include/linux/compiler_types.h:435:9: note: in expansion of macro '_compiletime_assert'
     435 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
         |         ^~~~~~~~~~~~~~~~~~~
   include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
      39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
         |                                     ^~~~~~~~~~~~~~~~~~
   include/linux/build_bug.h:50:9: note: in expansion of macro 'BUILD_BUG_ON_MSG'
      50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
         |         ^~~~~~~~~~~~~~~~
   lib/test_bitops.c:59:9: note: in expansion of macro 'BUILD_BUG_ON'
      59 |         BUILD_BUG_ON(!__builtin_constant_p(__test_expr));       \
         |         ^~~~~~~~~~~~
   lib/test_bitops.c:74:9: note: in expansion of macro 'test_const_eval'
      74 |         test_const_eval(__ffs(BIT(n)) == n);
         |         ^~~~~~~~~~~~~~~


vim +/__compiletime_assert_183 +435 include/linux/compiler_types.h

eb5c2d4b45e3d2 Will Deacon 2020-07-21  421  
eb5c2d4b45e3d2 Will Deacon 2020-07-21  422  #define _compiletime_assert(condition, msg, prefix, suffix) \
eb5c2d4b45e3d2 Will Deacon 2020-07-21  423  	__compiletime_assert(condition, msg, prefix, suffix)
eb5c2d4b45e3d2 Will Deacon 2020-07-21  424  
eb5c2d4b45e3d2 Will Deacon 2020-07-21  425  /**
eb5c2d4b45e3d2 Will Deacon 2020-07-21  426   * compiletime_assert - break build and emit msg if condition is false
eb5c2d4b45e3d2 Will Deacon 2020-07-21  427   * @condition: a compile-time constant condition to check
eb5c2d4b45e3d2 Will Deacon 2020-07-21  428   * @msg:       a message to emit if condition is false
eb5c2d4b45e3d2 Will Deacon 2020-07-21  429   *
eb5c2d4b45e3d2 Will Deacon 2020-07-21  430   * In tradition of POSIX assert, this macro will break the build if the
eb5c2d4b45e3d2 Will Deacon 2020-07-21  431   * supplied condition is *false*, emitting the supplied error message if the
eb5c2d4b45e3d2 Will Deacon 2020-07-21  432   * compiler has support to do so.
eb5c2d4b45e3d2 Will Deacon 2020-07-21  433   */
eb5c2d4b45e3d2 Will Deacon 2020-07-21  434  #define compiletime_assert(condition, msg) \
eb5c2d4b45e3d2 Will Deacon 2020-07-21 @435  	_compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
eb5c2d4b45e3d2 Will Deacon 2020-07-21  436
kernel test robot Dec. 6, 2023, 1:27 p.m. UTC | #2 
Hi Vincent,

kernel test robot noticed the following build errors:

[auto build test ERROR on akpm-mm/mm-everything]
[also build test ERROR on linus/master v6.7-rc4 next-20231206]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Vincent-Mailhol/lib-test_bitops-add-compile-time-optimization-evaluations-assertions/20231130-182837
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20231130102717.1297492-1-mailhol.vincent%40wanadoo.fr
patch subject: [PATCH v2] lib: test_bitops: add compile-time optimization/evaluations assertions
config: hexagon-allmodconfig (https://download.01.org/0day-ci/archive/20231206/202312062158.60c1e6yt-lkp@intel.com/config)
compiler: clang version 17.0.0 (https://github.com/llvm/llvm-project.git 4a5ac14ee968ff0ad5d2cc1ffa0299048db4c88a)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231206/202312062158.60c1e6yt-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202312062158.60c1e6yt-lkp@intel.com/

All errors (new ones prefixed by >>):

>> lib/test_bitops.c:74:2: error: call to '__compiletime_assert_178' declared with 'error' attribute: BUILD_BUG_ON failed: !__builtin_constant_p(__test_expr)
      74 |         test_const_eval(__ffs(BIT(n)) == n);
         |         ^
   lib/test_bitops.c:59:2: note: expanded from macro 'test_const_eval'
      59 |         BUILD_BUG_ON(!__builtin_constant_p(__test_expr));       \
         |         ^
   include/linux/build_bug.h:50:2: note: expanded from macro 'BUILD_BUG_ON'
      50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
         |         ^
   include/linux/build_bug.h:39:37: note: expanded from macro 'BUILD_BUG_ON_MSG'
      39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
         |                                     ^
   note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
   include/linux/compiler_types.h:423:2: note: expanded from macro '_compiletime_assert'
     423 |         __compiletime_assert(condition, msg, prefix, suffix)
         |         ^
   include/linux/compiler_types.h:416:4: note: expanded from macro '__compiletime_assert'
     416 |                         prefix ## suffix();                             \
         |                         ^
   <scratch space>:18:1: note: expanded from here
      18 | __compiletime_assert_178
         | ^
   1 error generated.


vim +74 lib/test_bitops.c

    62	
    63	static void test_bitops_const_eval(void)
    64	{
    65		/*
    66		 * On any supported optimization level (-O2, -Os) and if
    67		 * invoked with a compile-time constant argument, the compiler
    68		 * must be able to fold into a constant expression all the bit
    69		 * find functions. Namely: __ffs(), ffs(), ffz(), __fls(),
    70		 * fls() and fls64(). Otherwise, trigger a build bug.
    71		 */
    72		const int n = 10;
    73	
  > 74		test_const_eval(__ffs(BIT(n)) == n);
    75		test_const_eval(ffs(BIT(n)) == n + 1);
    76		test_const_eval(ffz(~BIT(n)) == n);
    77		test_const_eval(__fls(BIT(n)) == n);
    78		test_const_eval(fls(BIT(n)) == n + 1);
    79		test_const_eval(fls64(BIT_ULL(n)) == n + 1);
    80	}
    81
diff mbox series

Patch

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index cc7d53d9dc01..c97d818dbc30 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2454,6 +2454,10 @@  config TEST_BITOPS
 	  compilations. It has no dependencies and doesn't run or load unless
 	  explicitly requested by name.  for example: modprobe test_bitops.
 
+	  In addition, check that the compiler is able to fold the bitops
+	  function into a compile-time constant (given that the argument is also
+	  a compile-time constant) and trigger a build bug otherwise.
+
 	  If unsure, say N.
 
 config TEST_VMALLOC
diff --git a/lib/test_bitops.c b/lib/test_bitops.c
index 3b7bcbee84db..49e2f76575e4 100644
--- a/lib/test_bitops.c
+++ b/lib/test_bitops.c
@@ -50,6 +50,35 @@  static unsigned long order_comb_long[][2] = {
 };
 #endif
 
+/* Assert that a boolean expression can be folded in a constant and is true. */
+#define test_const_eval(test_expr)				\
+({								\
+	/* Evaluate once so that compiler can fold it. */	\
+	bool __test_expr = test_expr;				\
+								\
+	BUILD_BUG_ON(!__builtin_constant_p(__test_expr));	\
+	BUILD_BUG_ON(!__test_expr);				\
+})
+
+static void test_bitops_const_eval(void)
+{
+	/*
+	 * On any supported optimization level (-O2, -Os) and if
+	 * invoked with a compile-time constant argument, the compiler
+	 * must be able to fold into a constant expression all the bit
+	 * find functions. Namely: __ffs(), ffs(), ffz(), __fls(),
+	 * fls() and fls64(). Otherwise, trigger a build bug.
+	 */
+	const int n = 10;
+
+	test_const_eval(__ffs(BIT(n)) == n);
+	test_const_eval(ffs(BIT(n)) == n + 1);
+	test_const_eval(ffz(~BIT(n)) == n);
+	test_const_eval(__fls(BIT(n)) == n);
+	test_const_eval(fls(BIT(n)) == n + 1);
+	test_const_eval(fls64(BIT_ULL(n)) == n + 1);
+}
+
 static int __init test_bitops_startup(void)
 {
 	int i, bit_set;
@@ -94,6 +123,8 @@  static int __init test_bitops_startup(void)
 	if (bit_set != BITOPS_LAST)
 		pr_err("ERROR: FOUND SET BIT %d\n", bit_set);
 
+	test_bitops_const_eval();
+
 	pr_info("Completed bitops test\n");
 
 	return 0;