Hi,<div><br></div><div><div>Unlike gcc and clang, MSVC pads each consecutive member variables of the same type in bitfields. e.g. if you have:</div><div>struct AB {</div><div>    unsigned m_1 : 31;</div><div>    bool m_2 : 1;</div>


<div>}</div><div>then <b>MSVC pads m_1 and allocates sizeof(unsigned) * 2 for AB</b> whereas gcc and clang only allocate sizeof(unsigned) * 1 for AB.</div></div><div><br></div><div>This is <b>not a compiler bug</b>. It&#39;s a spec. compliant behavior, and may in fact have a better run-time performance in some situations. However, for core objects like RenderObject and InlineBox, allocating extra 4-8 bytes per each object is prohibitory expensive.</div>


<div><br></div><div>In such cases, please <b>use the same POD type for all bitfield member variables</b>. (Storage class classifiers and variable qualifiers seem to have no effect on how variables are packed; e.g. mutable, const, etc...). For example, MSVC will allocate sizeof(unsigned) * 1 for AB if we rewrite the above code as:</div>

<div><div>struct AB {</div>
<div>    unsigned m_1 : 31;</div><div>    unsigned m_2 : 1;</div><div>}</div></div><div><br></div><div>When you&#39;re making this change, <b>be sure to audit all code that assigns a non-boolean value to m_2</b> because implicit type coercion into boolean is no longer in effect. For example,</div>


<div><br></div><div>AB ab;</div><div>ab.m_2 = 2;</div><div>puts(ab.m_2 ? &quot;true&quot; : &quot;false&quot;);</div><div><br></div><div>will print &quot;true&quot; before the change and will print &quot;false&quot; after the change. An easy way to ensure you&#39;ve audited all such code is to add getters and setters for all bitfield member variables or wrap them in a special structure or class as done in <a href="http://trac.webkit.org/changeset/103353" target="_blank">http://trac.webkit.org/changeset/103353</a> and <a href="http://trac.webkit.org/changeset/104254" target="_blank">http://trac.webkit.org/changeset/104254</a>.</div>


<div><br></div><div>Best regards,</div><div>Ryosuke Niwa<br><font color="#999999">Software Engineer</font><div><font color="#999999">Google Inc.</font></div><div><font color="#999999"><br></font></div><br>
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