linux-kernel - [PATCH v4 8/8] lib: test_bitmap: add compile-time optimization/evaluations assertions

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Date:   Tue, 21 Jun 2022 21:15:53 +0200
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Arnd Bergmann <arnd@...db.de>, Yury Norov <yury.norov@...il.com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Matt Turner <mattst88@...il.com>,
        Brian Cain <bcain@...cinc.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Kees Cook <keescook@...omium.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-alpha@...r.kernel.org, linux-hexagon@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v4 8/8] lib: test_bitmap: add compile-time optimization/evaluations assertions

Add a function to the bitmap test suite, which will ensure that
compilers are able to evaluate operations performed by the
bitops/bitmap helpers to compile-time constants when all of the
arguments are compile-time constants as well, or trigger a build
bug otherwise. This should work on all architectures and 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.

Suggested-by: Yury Norov <yury.norov@...il.com>
Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---
 lib/test_bitmap.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/lib/test_bitmap.c b/lib/test_bitmap.c
index d5923a640457..3a7b09b82794 100644
--- a/lib/test_bitmap.c
+++ b/lib/test_bitmap.c
@@ -869,6 +869,50 @@ static void __init test_bitmap_print_buf(void)
 	}
 }
 
+static void __init test_bitmap_const_eval(void)
+{
+	DECLARE_BITMAP(bitmap, BITS_PER_LONG);
+	unsigned long initvar = BIT(2);
+	unsigned long bitopvar = 0;
+	unsigned long var = 0;
+	int res;
+
+	/*
+	 * Compilers must be able to optimize all of those to compile-time
+	 * constants on any supported optimization level (-O2, -Os) and any
+	 * architecture. Otherwise, trigger a build bug.
+	 * The whole function gets optimized out then, there's nothing to do
+	 * in runtime.
+	 */
+
+	/* Equals to `unsigned long bitmap[1] = { BIT(5), }` */
+	bitmap_clear(bitmap, 0, BITS_PER_LONG);
+	if (!test_bit(7, bitmap))
+		bitmap_set(bitmap, 5, 1);
+
+	/* Equals to `unsigned long bitopvar = BIT(20)` */
+	__change_bit(31, &bitopvar);
+	bitmap_shift_right(&bitopvar, &bitopvar, 11, BITS_PER_LONG);
+
+	/* Equals to `unsigned long var = BIT(25)` */
+	var |= BIT(25);
+	if (var & BIT(0))
+		var ^= GENMASK(9, 6);
+
+	/* __const_hweight<32|64>(BIT(5)) == 1 */
+	res = bitmap_weight(bitmap, 20);
+	BUILD_BUG_ON(!__builtin_constant_p(res));
+
+	/* !(BIT(31) & BIT(18)) == 1 */
+	res = !test_bit(18, &bitopvar);
+	BUILD_BUG_ON(!__builtin_constant_p(res));
+
+	/* BIT(2) & GENMASK(14, 8) == 0 */
+	BUILD_BUG_ON(!__builtin_constant_p(initvar & GENMASK(14, 8)));
+	/* ~BIT(25) */
+	BUILD_BUG_ON(!__builtin_constant_p(~var));
+}
+
 static void __init selftest(void)
 {
 	test_zero_clear();
@@ -884,6 +928,7 @@ static void __init selftest(void)
 	test_for_each_set_clump8();
 	test_bitmap_cut();
 	test_bitmap_print_buf();
+	test_bitmap_const_eval();
 }
 
 KSTM_MODULE_LOADERS(test_bitmap);
-- 
2.36.1