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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>
    </build-info>


<item>
            <title>[CDRIVER-2681] bson_context_get_default optimized out when compiling with LTO</title>
                <link>https://jira.mongodb.org/browse/CDRIVER-2681</link>
                <project id="10030" key="CDRIVER">C Driver</project>
                    <description>&lt;p&gt;Drew noticed when we compile the C driver with link time optimization our tests fail. Repro:&lt;/p&gt;
&lt;p/&gt;
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&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;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;cd mongo-c-driver&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;mkdir cmake-build &amp;amp;&amp;amp; cd cmake-build&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;cmake -DCMAKE_C_FLAGS=&quot;-flto&quot; ../&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;make -j8 test-libmongoc&lt;/span&gt;&lt;/pre&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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;cd ../&lt;/span&gt;&lt;/pre&gt;
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		&lt;/tr&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;./cmake-build/src/libmongoc/test-libmongoc -d --no-fork -l /bson/bcon/test_oid&lt;/span&gt;&lt;/pre&gt;
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&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;fails with a segfault.&lt;/p&gt;

&lt;p&gt;It looks like some function definitions are getting optimized out when they shouldn&apos;t be. It&apos;s a little difficult to debug since -flto removes debug info, but after some printf debugging, it became clear that &lt;tt&gt;bson_context_get_default&lt;/tt&gt; was not getting called. Using &lt;tt&gt;nm&lt;/tt&gt; you can also see there&apos;s no symbol in the binary:&lt;/p&gt;
&lt;p/&gt;
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&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;   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;nm ./cmake-build/src/libmongoc/test-libmongoc | grep &apos;bson_context_get_default&apos;&lt;/span&gt;&lt;/pre&gt;
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		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;comes up with nothing.&lt;/p&gt;

&lt;p&gt;Here&apos;s the definition of&lt;/p&gt;
&lt;p/&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;BSON_EXPORT (bson_context_t *)&lt;/span&gt;&lt;/pre&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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;bson_context_get_default (void) BSON_GNUC_CONST;&lt;/span&gt;&lt;/pre&gt;
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		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;&lt;tt&gt;BSON_GNUC_CONST&lt;/tt&gt; is defined as &lt;tt&gt;_&lt;em&gt;attribute&lt;/em&gt;_ ((const))&lt;/tt&gt; for gcc/clang. The &lt;a href=&quot;https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;gcc function attributes docs&lt;/a&gt; give this explanation for the &lt;tt&gt;const&lt;/tt&gt; attribute:&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;Many functions do not examine any values except their arguments, and have no effects except to return a value. Calls to such functions lend themselves to optimization such as common subexpression elimination. &lt;b&gt;The const attribute imposes greater restrictions on a function&#8217;s definition than the similar pure attribute below because it prohibits the function from reading global variables&lt;/b&gt;. Consequently, the presence of the attribute on a function declaration allows GCC to emit more efficient code for some calls to the function. Decorating the same function with both the const and the pure attribute is diagnosed.&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;But &lt;tt&gt;bson_context_get_default&lt;/tt&gt; does read from global variables (PTHREAD_ONCE_INIT}} and &lt;tt&gt;gContextDefault&lt;/tt&gt;. Since &lt;tt&gt;const&lt;/tt&gt; functions shouldn&apos;t have side effects, the linker thinks it can optimize it out. Sure enough, removing the &lt;tt&gt;const&lt;/tt&gt; attribute results in this test passing.&lt;/p&gt;

&lt;p&gt;There are other functions marked as &lt;tt&gt;const&lt;/tt&gt; that probably shouldn&apos;t be, e.g. &lt;tt&gt;mongoc_ssl_opt_get_default&lt;/tt&gt;, which returns a global.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;</description>
                <environment></environment>
        <key id="553462">CDRIVER-2681</key>
            <summary>bson_context_get_default optimized out when compiling with LTO</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="kevin.albertson@mongodb.com">Kevin Albertson</assignee>
                                    <reporter username="kevin.albertson@mongodb.com">Kevin Albertson</reporter>
                        <labels>
                    </labels>
                <created>Sat, 2 Jun 2018 17:26:08 +0000</created>
                <updated>Sat, 28 Oct 2023 11:29:49 +0000</updated>
                            <resolved>Thu, 7 Jun 2018 17:19:13 +0000</resolved>
                                                    <fixVersion>1.11.0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="1914064" author="xgen-internal-githook" created="Thu, 7 Jun 2018 17:18:14 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;username&apos;: &apos;kevinAlbs&apos;, &apos;name&apos;: &apos;Kevin Albertson&apos;, &apos;email&apos;: &apos;kevin.albertson@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/CDRIVER-2681&quot; title=&quot;bson_context_get_default optimized out when compiling with LTO&quot; class=&quot;issue-link&quot; data-issue-key=&quot;CDRIVER-2681&quot;&gt;&lt;del&gt;CDRIVER-2681&lt;/del&gt;&lt;/a&gt; fix functions with const attr&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo-c-driver/commit/8c2a814b99e08bebede7276dbaa4b7d5f9bba317&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo-c-driver/commit/8c2a814b99e08bebede7276dbaa4b7d5f9bba317&lt;/a&gt;&lt;/p&gt;</comment>
                    </comments>
                <issuelinks>
                            <issuelinktype id="10011">
                    <name>Depends</name>
                                                                <inwardlinks description="is depended on by">
                                        <issuelink>
            <issuekey id="548221">SERVER-35184</issuekey>
        </issuelink>
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                            <issuelinktype id="10012">
                    <name>Related</name>
                                            <outwardlinks description="related to">
                                        <issuelink>
            <issuekey id="595327">CDRIVER-2805</issuekey>
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