On Wed, May 17, 2023 at 11:06:57AM +0100, Richard W.M. Jones wrote:
Previously it only worked for 64 bit ints on 64 bit platforms, but we can easily adjust the function to work on any platform. --- common/include/ispowerof2.h | 4 ++-- common/include/test-ispowerof2.c | 6 ++---- 2 files changed, 4 insertions(+), 6 deletions(-)
diff --git a/common/include/ispowerof2.h b/common/include/ispowerof2.h index a4cb52de3..8f56c8dbf 100644 --- a/common/include/ispowerof2.h +++ b/common/include/ispowerof2.h @@ -54,9 +54,9 @@ is_power_of_2 (unsigned long v) * __builtin_clzl is available in GCC and clang. */ static inline int -log_2_bits (unsigned long v) +log_2_bits (uint64_t v) { - return SIZEOF_LONG*8 - __builtin_clzl (v) - 1; + return 64 - __builtin_clzll (v) - 1;
We already documented that this is undefined for v==0, and your change does not alter that. What we don't document is whether this is well-defined for a non-power-of-2...
}
/* Round up to next power of 2. diff --git a/common/include/test-ispowerof2.c b/common/include/test-ispowerof2.c index 1221ac09c..09d248889 100644 --- a/common/include/test-ispowerof2.c +++ b/common/include/test-ispowerof2.c @@ -63,10 +63,8 @@ main (void) assert (log_2_bits (512) == 9); assert (log_2_bits (4096) == 12); assert (log_2_bits (0x80000000) == 31); -#if SIZEOF_LONG == 8 - assert (log_2_bits (0x100000000) == 32); - assert (log_2_bits (0x8000000000000000) == 63); -#endif + assert (log_2_bits (UINT64_C (0x100000000)) == 32); + assert (log_2_bits (UINT64_C (0x8000000000000000)) == 63);
...we only test it on values where is_power_of_2() returns true. We should probably take this opportunity to either document that it is not just 0, but all non-power-of-2 values that have undefined behavior; or else to test some values with more than one bit set. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org