RFR: 8257985: count_trailing_zeros doesn't handle 64-bit values on 32-bit JVM [v4]
Kim Barrett
kbarrett at openjdk.java.net
Fri Dec 11 13:46:59 UTC 2020
On Fri, 11 Dec 2020 13:35:10 GMT, Claes Redestad <redestad at openjdk.org> wrote:
>> Re-implement count_trailing_zeros as a template function similar to count_leading_zeros, adding support for 8 and 16 bit types as well as for 64-bit types on 32-bit builds, which is currently not supported. This prohibits implementing the log2i_exact proposed by #1663 using count_trailing_zeros, as suggested during review.
>>
>> Compared to count_leading_zeros the implementation can be slightly simpler since subword specializations can simply use the internal 32-bit variant.
>>
>> Windows doesn't define _BitScanForward64 on 32-bit targets, so the 32-bit variant for handling 64-bit values becomes more convoluted.
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
>
> Explicit cast to unsigned
This can be much simpler:
#if defined(TARGET_COMPILER_gcc)
inline unsigned count_trailing_zeros_32(uint32_t x) {
return __builtin_ctz(x);
}
inline unsigned count_trailing_zeros_64(uint64_t x) {
return __builtin_ctzll(x);
}
#elif ...
...
#endif
template<typename T,
ENABLE_IF(std::is_integral<T>::value),
ENABLE_IF(sizeof(T) <= sizeof(uint64_t))>
inline unsigned count_trailing_zeros(T x) {
assert(x != 0, "precondition");
return (sizeof(x) <= sizeof(uint32_t)) ?
count_trailing_zeros_32(x) :
count_trailing_zeros_64(x);
}
}
-------------
Changes requested by kbarrett (Reviewer).
PR: https://git.openjdk.java.net/jdk/pull/1747
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