Z_POW2_CEIL: simplify implementation

Avoid potentially calling __builtin_clz() twice with non-constant
values. Also add a test for it.

Clang produces false positive vla warnings so disable them. GCC will
spot real vla's already.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

CMakeLists.txt

@@ -381,6 +381,9 @@ if(NOT CMAKE_C_COMPILER_ID STREQUAL "Clang")
   if(NOT ${ZEPHYR_TOOLCHAIN_VARIANT} STREQUAL "xcc")
     zephyr_cc_option(-fno-defer-pop)
   endif()
+else()
+  # Clang produces false positive vla warnings
+  zephyr_cc_option(-Wno-vla)
 endif()
 
 zephyr_cc_option_ifdef(CONFIG_STACK_USAGE            -fstack-usage)

include/zephyr/toolchain/gcc.h

@@ -595,15 +595,8 @@ do {                                                                    \
  * @param x Nonzero unsigned long value
  * @return X rounded up to the next power of two
  */
-#ifdef CONFIG_64BIT
+#define Z_POW2_CEIL(x) \
-#define Z_POW2_CEIL(x) ((1UL << (63U - __builtin_clzl(x))) < x ?  \
+	((x) <= 2UL ? (x) : (1UL << (8 * sizeof(long) - __builtin_clzl((x) - 1))))
-		1UL << (63U - __builtin_clzl(x) + 1U) : \
-		1UL << (63U - __builtin_clzl(x)))
-#else
-#define Z_POW2_CEIL(x) ((1UL << (31U - __builtin_clz(x))) < x ?  \
-		1UL << (31U - __builtin_clz(x) + 1U) : \
-		1UL << (31U - __builtin_clz(x)))
-#endif
 
 /**
  * @brief Check whether or not a value is a power of 2

tests/kernel/common/CMakeLists.txt

@@ -22,7 +22,8 @@ target_sources(app PRIVATE
 	src/main.c
 	src/timeout_order.c
 	src/multilib.c
-  src/errno.c
+	src/errno.c
-  src/boot_delay.c
+	src/boot_delay.c
-  src/irq_offload.c
+	src/irq_offload.c
+	src/pow2.c
   )

tests/kernel/common/src/main.c

@@ -52,6 +52,7 @@ extern void test_bitarray_alloc_free(void);
 extern void test_bitarray_region_set_clear(void);
 extern void test_nop(void);
 extern void test_ffs(void);
+extern void test_pow2_ceil(void);
 
 /**
  * @defgroup kernel_common_tests Common Tests
@@ -168,7 +169,8 @@ void test_main(void)
 			 ztest_unit_test(test_ms_time_duration),
 			 ztest_unit_test(test_bounds_check_mitigation),
 			 ztest_unit_test(test_nop),
-			 ztest_unit_test(test_ffs)
+			 ztest_unit_test(test_ffs),
+			 ztest_unit_test(test_pow2_ceil)
 			 );
 
 	ztest_run_test_suite(common);

tests/kernel/common/src/pow2.c

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2022 BayLibre SAS
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <ztest.h>
+#include <zephyr.h>
+
+/**
+ * @brief Test the Z_POW2_CEIL() macro
+ *
+ * @defgroup test_pow2_ceil Z_POW2_CEIL() tests
+ *
+ * @ingroup all_tests
+ *
+ * @{
+ * @}
+ */
+
+/**
+ * @brief Verify compile-time constant results
+ *
+ * @ingroup test_pow2_ceil
+ *
+ * @details Check if static array allocations are sized as expected.
+ */
+
+char static_array1[Z_POW2_CEIL(1)];
+char static_array2[Z_POW2_CEIL(2)];
+char static_array3[Z_POW2_CEIL(3)];
+char static_array4[Z_POW2_CEIL(4)];
+char static_array5[Z_POW2_CEIL(5)];
+char static_array7[Z_POW2_CEIL(7)];
+char static_array8[Z_POW2_CEIL(8)];
+char static_array9[Z_POW2_CEIL(9)];
+
+BUILD_ASSERT(sizeof(static_array1) == 1);
+BUILD_ASSERT(sizeof(static_array2) == 2);
+BUILD_ASSERT(sizeof(static_array3) == 4);
+BUILD_ASSERT(sizeof(static_array4) == 4);
+BUILD_ASSERT(sizeof(static_array5) == 8);
+BUILD_ASSERT(sizeof(static_array7) == 8);
+BUILD_ASSERT(sizeof(static_array8) == 8);
+BUILD_ASSERT(sizeof(static_array9) == 16);
+
+/**
+ * @brief Verify run-time non-constant results
+ *
+ * @ingroup test_pow2_ceil
+ *
+ * @details Check if run-time non-constant results are as expected.
+ *          Use a volatile variable to prevent compiler optimizations.
+ */
+
+static void test_pow2_ceil_x(unsigned long test_value,
+			     unsigned long expected_result)
+{
+	volatile unsigned int x = test_value;
+	unsigned int result = Z_POW2_CEIL(x);
+
+	zassert_equal(result, expected_result,
+		      "ZPOW2_CEIL(%lu) returned %lu, expected %lu",
+		      test_value, result, expected_result);
+}
+
+void test_pow2_ceil(void)
+{
+	test_pow2_ceil_x(1, 1);
+	test_pow2_ceil_x(2, 2);
+	test_pow2_ceil_x(3, 4);
+	test_pow2_ceil_x(4, 4);
+	test_pow2_ceil_x(5, 8);
+	test_pow2_ceil_x(7, 8);
+	test_pow2_ceil_x(8, 8);
+	test_pow2_ceil_x(9, 16);
+}