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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[CDRIVER-4163] clang version checks for __builtin_bswap are incorrect</title>
                <link>https://jira.mongodb.org/browse/CDRIVER-4163</link>
                <project id="10030" key="CDRIVER">C Driver</project>
                    <description>&lt;p&gt;&lt;b&gt;Background and Motivation&lt;/b&gt;&lt;/p&gt;

&lt;p&gt;The compiler check in bson-endian.h for clang support of __builtin_bswap is as follows:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;#elif defined(__clang__) &amp;amp;&amp;amp; defined(__clang_major__) &amp;amp;&amp;amp;  \&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;   defined(__clang_minor__) &amp;amp;&amp;amp; (__clang_major__ &amp;gt;= 3) &amp;amp;&amp;amp; \&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;   (__clang_minor__ &amp;gt;= 1)&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;#if __has_builtin(__builtin_bswap16)&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;#define BSON_UINT16_SWAP_LE_BE(v) __builtin_bswap16 (v)&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;#endif&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;
&lt;p&gt;The intent appears to check that the version of clang is &amp;gt;= 3.1. But this asserts that both major version is &amp;gt;= 3 and minor version is &amp;gt;= 1. This excludes clang versions with a 0 minor version, like 9.0.1.&lt;/p&gt;

&lt;p&gt;Checking the version does not seem necessary. I think it is sufficient to only use &lt;tt&gt;__has_builtin&lt;/tt&gt; for clang. A caveat is that the &lt;a href=&quot;https://clang.llvm.org/docs/LanguageExtensions.html#feature-checking-macros&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;Clang Language Extensions&lt;/a&gt; notes:&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;Prior to Clang 10, __has_builtin could not be used to detect most builtin pseudo-functions.&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;I believe &lt;tt&gt;__builtin_bswap16&lt;/tt&gt;, &lt;tt&gt;_builtin_bswap32&lt;/tt&gt;, and &lt;tt&gt;_builtin_bswap64&lt;/tt&gt; _classify as pseudo-functions since they accept an argument. But &lt;a href=&quot;https://godbolt.org/z/nT91EzG7d&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;this test with Compiler Explorer&lt;/a&gt; shows that &lt;tt&gt;_has_builtin&lt;/tt&gt; detects support back to clang 3.0.0.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;Impact&lt;/b&gt;&lt;/p&gt;

&lt;p&gt;The impact of this bug is that byte swapping may be slower when clang is used to compile.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;Scope&lt;/b&gt;&lt;br/&gt;
 Update the check for &lt;tt&gt;_&lt;em&gt;builtin_bswap&lt;/tt&gt;&#160;support on clang in bson-endian.h to only use &lt;tt&gt;&lt;/em&gt;_has_builtin&lt;/tt&gt;.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1876139">CDRIVER-4163</key>
            <summary>clang version checks for __builtin_bswap are incorrect</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</priority>
                        <status id="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="-1">Unassigned</assignee>
                                    <reporter username="kevin.albertson@mongodb.com">Kevin Albertson</reporter>
                        <labels>
                    </labels>
                <created>Thu, 16 Sep 2021 15:06:34 +0000</created>
                <updated>Fri, 10 Feb 2023 16:25:49 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>2</watches>
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