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https://issues.apache.org/jira/browse/ARROW-1749?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16239234#comment-16239234
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ASF GitHub Bot commented on ARROW-1749:
---------------------------------------
cpcloud commented on a change in pull request #1282: ARROW-1749: [C++] Handle
range of Decimal128 values that require 39 digits to be displayed
URL: https://github.com/apache/arrow/pull/1282#discussion_r148941301
##########
File path: cpp/src/arrow/util/bit-util.h
##########
@@ -296,6 +297,25 @@ static inline int Log2(uint64_t x) {
return result;
}
+/// \brief Count the number of leading zeros in a 32 bit integer.
+static inline int64_t CountLeadingZeros(uint32_t value) {
+ DCHECK_NE(value, 0);
+#if defined(__clang__) || defined(__GNUC__)
+ return static_cast<int64_t>(__builtin_clz(value));
Review comment:
Interesting tidbit here is that `__builtin_clz` compiles to `31 - bit scan
reverse` on gcc and clang.
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> [C++] Handle range of Decimal128 values that require 39 digits to be displayed
> ------------------------------------------------------------------------------
>
> Key: ARROW-1749
> URL: https://issues.apache.org/jira/browse/ARROW-1749
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++
> Affects Versions: 0.7.1
> Reporter: Phillip Cloud
> Assignee: Phillip Cloud
> Priority: Major
> Labels: pull-request-available
> Fix For: 0.8.0
>
>
> {{2 ** 127 - 1}} and {{-2 ** 127}} both require a 39th digit to be displayed
> as a decimal string.
> This means we do not have the ability to _display_ decimal values above {{10
> ** 38 - 1}} even though we have the ability to _store_ them.
> For reference, Impala returns {{NULL}} when trying to cast {{2 ** 127 - 1}}
> to {{DECIMAL(38, 0)}}.
> A few next steps are in order:
> # Explicitly test this behavior
> # Avoid crashing or displaying garbage when we cannot display a number that
> can be stored
> # Make a decision about whether we want to eventually support range of values
> from {{10 ** 38}} to {{2 ** 127 - 1}} positive and {{-2 ** 127}} to {{-10 **
> 38}} negative. This would require a larger integer type to hold the value
> just before being converted to a string.
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