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

[Vincent Mailhol]

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@xxxxxxxxx>
Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
---
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@xxxxxxxxxx/
---
 lib/Kconfig.debug |  4 ++++
 lib/test_bitops.c | 31 +++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

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;
-- 
2.25.1