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<div class="section" id="llvm-extensions">
<h1>LLVM Extensions<a class="headerlink" href="#llvm-extensions" title="Permalink to this headline">¶</a></h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#introduction" id="id5">Introduction</a></li>
<li><a class="reference internal" href="#general-assembly-syntax" id="id6">General Assembly Syntax</a><ul>
<li><a class="reference internal" href="#c99-style-hexadecimal-floating-point-constants" id="id7">C99-style Hexadecimal Floating-point Constants</a></li>
</ul>
</li>
<li><a class="reference internal" href="#machine-specific-assembly-syntax" id="id8">Machine-specific Assembly Syntax</a><ul>
<li><a class="reference internal" href="#x86-coff-dependent" id="id9">X86/COFF-Dependent</a><ul>
<li><a class="reference internal" href="#relocations" id="id10">Relocations</a></li>
<li><a class="reference internal" href="#linkonce-directive" id="id11"><code class="docutils literal notranslate"><span class="pre">.linkonce</span></code> Directive</a></li>
<li><a class="reference internal" href="#section-directive" id="id12"><code class="docutils literal notranslate"><span class="pre">.section</span></code> Directive</a></li>
</ul>
</li>
<li><a class="reference internal" href="#arm64-coff-dependent" id="id13">ARM64/COFF-Dependent</a><ul>
<li><a class="reference internal" href="#id1" id="id14">Relocations</a></li>
</ul>
</li>
<li><a class="reference internal" href="#elf-dependent" id="id15">ELF-Dependent</a><ul>
<li><a class="reference internal" href="#id2" id="id16"><code class="docutils literal notranslate"><span class="pre">.section</span></code> Directive</a></li>
<li><a class="reference internal" href="#linker-options-section-linker-options" id="id17"><code class="docutils literal notranslate"><span class="pre">.linker-options</span></code> Section (linker options)</a></li>
<li><a class="reference internal" href="#sht-llvm-call-graph-profile-section-call-graph-profile" id="id18"><code class="docutils literal notranslate"><span class="pre">SHT_LLVM_CALL_GRAPH_PROFILE</span></code> Section (Call Graph Profile)</a></li>
<li><a class="reference internal" href="#sht-llvm-addrsig-section-address-significance-table" id="id19"><code class="docutils literal notranslate"><span class="pre">SHT_LLVM_ADDRSIG</span></code> Section (address-significance table)</a></li>
</ul>
</li>
<li><a class="reference internal" href="#codeview-dependent" id="id20">CodeView-Dependent</a><ul>
<li><a class="reference internal" href="#cv-file-directive" id="id21"><code class="docutils literal notranslate"><span class="pre">.cv_file</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-func-id-directive" id="id22"><code class="docutils literal notranslate"><span class="pre">.cv_func_id</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-inline-site-id-directive" id="id23"><code class="docutils literal notranslate"><span class="pre">.cv_inline_site_id</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-loc-directive" id="id24"><code class="docutils literal notranslate"><span class="pre">.cv_loc</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-linetable-directive" id="id25"><code class="docutils literal notranslate"><span class="pre">.cv_linetable</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-inline-linetable-directive" id="id26"><code class="docutils literal notranslate"><span class="pre">.cv_inline_linetable</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-def-range-directive" id="id27"><code class="docutils literal notranslate"><span class="pre">.cv_def_range</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-stringtable-directive" id="id28"><code class="docutils literal notranslate"><span class="pre">.cv_stringtable</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-filechecksums-directive" id="id29"><code class="docutils literal notranslate"><span class="pre">.cv_filechecksums</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-filechecksumoffset-directive" id="id30"><code class="docutils literal notranslate"><span class="pre">.cv_filechecksumoffset</span></code> Directive</a></li>
<li><a class="reference internal" href="#cv-fpo-data-directive" id="id31"><code class="docutils literal notranslate"><span class="pre">.cv_fpo_data</span></code> Directive</a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference internal" href="#target-specific-behaviour" id="id32">Target Specific Behaviour</a><ul>
<li><a class="reference internal" href="#x86" id="id33">X86</a><ul>
<li><a class="reference internal" href="#id3" id="id34">Relocations</a></li>
</ul>
</li>
<li><a class="reference internal" href="#windows-on-arm" id="id35">Windows on ARM</a><ul>
<li><a class="reference internal" href="#stack-probe-emission" id="id36">Stack Probe Emission</a></li>
<li><a class="reference internal" href="#variable-length-arrays" id="id37">Variable Length Arrays</a></li>
</ul>
</li>
<li><a class="reference internal" href="#windows-on-arm64" id="id38">Windows on ARM64</a><ul>
<li><a class="reference internal" href="#id4" id="id39">Stack Probe Emission</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
<div class="toctree-wrapper compound">
</div>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id5">Introduction</a><a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>This document describes extensions to tools and formats LLVM seeks compatibility
with.</p>
</div>
<div class="section" id="general-assembly-syntax">
<h2><a class="toc-backref" href="#id6">General Assembly Syntax</a><a class="headerlink" href="#general-assembly-syntax" title="Permalink to this headline">¶</a></h2>
<div class="section" id="c99-style-hexadecimal-floating-point-constants">
<h3><a class="toc-backref" href="#id7">C99-style Hexadecimal Floating-point Constants</a><a class="headerlink" href="#c99-style-hexadecimal-floating-point-constants" title="Permalink to this headline">¶</a></h3>
<p>LLVM’s assemblers allow floating-point constants to be written in C99’s
hexadecimal format instead of decimal if desired.</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="no">.data</span>
<span class="na">.float</span> <span class="mi">0x1c2</span><span class="no">.2ap3</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="machine-specific-assembly-syntax">
<h2><a class="toc-backref" href="#id8">Machine-specific Assembly Syntax</a><a class="headerlink" href="#machine-specific-assembly-syntax" title="Permalink to this headline">¶</a></h2>
<div class="section" id="x86-coff-dependent">
<h3><a class="toc-backref" href="#id9">X86/COFF-Dependent</a><a class="headerlink" href="#x86-coff-dependent" title="Permalink to this headline">¶</a></h3>
<div class="section" id="relocations">
<h4><a class="toc-backref" href="#id10">Relocations</a><a class="headerlink" href="#relocations" title="Permalink to this headline">¶</a></h4>
<p>The following additional relocation types are supported:</p>
<p><strong>@IMGREL</strong> (AT&T syntax only) generates an image-relative relocation that
corresponds to the COFF relocation types <code class="docutils literal notranslate"><span class="pre">IMAGE_REL_I386_DIR32NB</span></code> (32-bit) or
<code class="docutils literal notranslate"><span class="pre">IMAGE_REL_AMD64_ADDR32NB</span></code> (64-bit).</p>
<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>.text
fun:
mov foo@IMGREL(%ebx, %ecx, 4), %eax
.section .pdata
.long fun@IMGREL
.long (fun@imgrel + 0x3F)
.long $unwind$fun@imgrel
</pre></div>
</div>
<p><strong>.secrel32</strong> generates a relocation that corresponds to the COFF relocation
types <code class="docutils literal notranslate"><span class="pre">IMAGE_REL_I386_SECREL</span></code> (32-bit) or <code class="docutils literal notranslate"><span class="pre">IMAGE_REL_AMD64_SECREL</span></code> (64-bit).</p>
<p><strong>.secidx</strong> relocation generates an index of the section that contains
the target. It corresponds to the COFF relocation types
<code class="docutils literal notranslate"><span class="pre">IMAGE_REL_I386_SECTION</span></code> (32-bit) or <code class="docutils literal notranslate"><span class="pre">IMAGE_REL_AMD64_SECTION</span></code> (64-bit).</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>.section .debug$S,"rn"
.long 4
.long 242
.long 40
.secrel32 _function_name + 0
.secidx _function_name
...
</pre></div>
</div>
</div>
<div class="section" id="linkonce-directive">
<h4><a class="toc-backref" href="#id11"><code class="docutils literal notranslate"><span class="pre">.linkonce</span></code> Directive</a><a class="headerlink" href="#linkonce-directive" title="Permalink to this headline">¶</a></h4>
<p>Syntax:</p>
<blockquote>
<div><code class="docutils literal notranslate"><span class="pre">.linkonce</span> <span class="pre">[</span> <span class="pre">comdat</span> <span class="pre">type</span> <span class="pre">]</span></code></div></blockquote>
<p>Supported COMDAT types:</p>
<dl class="docutils">
<dt><code class="docutils literal notranslate"><span class="pre">discard</span></code></dt>
<dd>Discards duplicate sections with the same COMDAT symbol. This is the default
if no type is specified.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">one_only</span></code></dt>
<dd>If the symbol is defined multiple times, the linker issues an error.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">same_size</span></code></dt>
<dd>Duplicates are discarded, but the linker issues an error if any have
different sizes.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">same_contents</span></code></dt>
<dd>Duplicates are discarded, but the linker issues an error if any duplicates
do not have exactly the same content.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">largest</span></code></dt>
<dd>Links the largest section from among the duplicates.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">newest</span></code></dt>
<dd>Links the newest section from among the duplicates.</dd>
</dl>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="no">.text$foo</span>
<span class="na">.linkonce</span>
<span class="na">...</span>
</pre></div>
</div>
</div>
<div class="section" id="section-directive">
<h4><a class="toc-backref" href="#id12"><code class="docutils literal notranslate"><span class="pre">.section</span></code> Directive</a><a class="headerlink" href="#section-directive" title="Permalink to this headline">¶</a></h4>
<p>MC supports passing the information in <code class="docutils literal notranslate"><span class="pre">.linkonce</span></code> at the end of
<code class="docutils literal notranslate"><span class="pre">.section</span></code>. For example, these two codes are equivalent</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="no">secName</span><span class="p">,</span> <span class="s">"dr"</span><span class="p">,</span> <span class="no">discard</span><span class="p">,</span> <span class="s">"Symbol1"</span>
<span class="na">.globl</span> <span class="no">Symbol1</span>
<span class="nl">Symbol1:</span>
<span class="na">.long</span> <span class="mi">1</span>
</pre></div>
</div>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="no">secName</span><span class="p">,</span> <span class="s">"dr"</span>
<span class="na">.linkonce</span> <span class="no">discard</span>
<span class="na">.globl</span> <span class="no">Symbol1</span>
<span class="nl">Symbol1:</span>
<span class="na">.long</span> <span class="mi">1</span>
</pre></div>
</div>
<p>Note that in the combined form the COMDAT symbol is explicit. This
extension exists to support multiple sections with the same name in
different COMDATs:</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="no">secName</span><span class="p">,</span> <span class="s">"dr"</span><span class="p">,</span> <span class="no">discard</span><span class="p">,</span> <span class="s">"Symbol1"</span>
<span class="na">.globl</span> <span class="no">Symbol1</span>
<span class="nl">Symbol1:</span>
<span class="na">.long</span> <span class="mi">1</span>
<span class="na">.section</span> <span class="no">secName</span><span class="p">,</span> <span class="s">"dr"</span><span class="p">,</span> <span class="no">discard</span><span class="p">,</span> <span class="s">"Symbol2"</span>
<span class="na">.globl</span> <span class="no">Symbol2</span>
<span class="nl">Symbol2:</span>
<span class="na">.long</span> <span class="mi">1</span>
</pre></div>
</div>
<p>In addition to the types allowed with <code class="docutils literal notranslate"><span class="pre">.linkonce</span></code>, <code class="docutils literal notranslate"><span class="pre">.section</span></code> also accepts
<code class="docutils literal notranslate"><span class="pre">associative</span></code>. The meaning is that the section is linked if a certain other
COMDAT section is linked. This other section is indicated by the comdat symbol
in this directive. It can be any symbol defined in the associated section, but
is usually the associated section’s comdat.</p>
<blockquote>
<div><p>The following restrictions apply to the associated section:</p>
<ol class="arabic simple">
<li>It must be a COMDAT section.</li>
<li>It cannot be another associative COMDAT section.</li>
</ol>
</div></blockquote>
<p>In the following example the symobl <code class="docutils literal notranslate"><span class="pre">sym</span></code> is the comdat symbol of <code class="docutils literal notranslate"><span class="pre">.foo</span></code>
and <code class="docutils literal notranslate"><span class="pre">.bar</span></code> is associated to <code class="docutils literal notranslate"><span class="pre">.foo</span></code>.</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="no">.foo</span><span class="p">,</span><span class="s">"bw"</span><span class="p">,</span><span class="no">discard</span><span class="p">,</span> <span class="s">"sym"</span>
<span class="na">.section</span> <span class="no">.bar</span><span class="p">,</span><span class="s">"rd"</span><span class="p">,</span><span class="no">associative</span><span class="p">,</span> <span class="s">"sym"</span>
</pre></div>
</div>
<p>MC supports these flags in the COFF <code class="docutils literal notranslate"><span class="pre">.section</span></code> directive:</p>
<blockquote>
<div><ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">b</span></code>: BSS section (<code class="docutils literal notranslate"><span class="pre">IMAGE_SCN_CNT_INITIALIZED_DATA</span></code>)</li>
<li><code class="docutils literal notranslate"><span class="pre">d</span></code>: Data section (<code class="docutils literal notranslate"><span class="pre">IMAGE_SCN_CNT_UNINITIALIZED_DATA</span></code>)</li>
<li><code class="docutils literal notranslate"><span class="pre">n</span></code>: Section is not loaded (<code class="docutils literal notranslate"><span class="pre">IMAGE_SCN_LNK_REMOVE</span></code>)</li>
<li><code class="docutils literal notranslate"><span class="pre">r</span></code>: Read-only</li>
<li><code class="docutils literal notranslate"><span class="pre">s</span></code>: Shared section</li>
<li><code class="docutils literal notranslate"><span class="pre">w</span></code>: Writable</li>
<li><code class="docutils literal notranslate"><span class="pre">x</span></code>: Executable section</li>
<li><code class="docutils literal notranslate"><span class="pre">y</span></code>: Not readable</li>
<li><code class="docutils literal notranslate"><span class="pre">D</span></code>: Discardable (<code class="docutils literal notranslate"><span class="pre">IMAGE_SCN_MEM_DISCARDABLE</span></code>)</li>
</ul>
</div></blockquote>
<p>These flags are all compatible with gas, with the exception of the <code class="docutils literal notranslate"><span class="pre">D</span></code> flag,
which gnu as does not support. For gas compatibility, sections with a name
starting with “.debug” are implicitly discardable.</p>
</div>
</div>
<div class="section" id="arm64-coff-dependent">
<h3><a class="toc-backref" href="#id13">ARM64/COFF-Dependent</a><a class="headerlink" href="#arm64-coff-dependent" title="Permalink to this headline">¶</a></h3>
<div class="section" id="id1">
<h4><a class="toc-backref" href="#id14">Relocations</a><a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h4>
<p>The following additional symbol variants are supported:</p>
<p><strong>:secrel_lo12:</strong> generates a relocation that corresponds to the COFF relocation
types <code class="docutils literal notranslate"><span class="pre">IMAGE_REL_ARM64_SECREL_LOW12A</span></code> or <code class="docutils literal notranslate"><span class="pre">IMAGE_REL_ARM64_SECREL_LOW12L</span></code>.</p>
<p><strong>:secrel_hi12:</strong> generates a relocation that corresponds to the COFF relocation
type <code class="docutils literal notranslate"><span class="pre">IMAGE_REL_ARM64_SECREL_HIGH12A</span></code>.</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="nf">add</span> <span class="no">x0</span><span class="p">,</span> <span class="no">x0</span><span class="p">,</span> <span class="p">:</span><span class="no">secrel_hi12</span><span class="p">:</span><span class="no">symbol</span>
<span class="nf">ldr</span> <span class="no">x0</span><span class="p">,</span> <span class="p">[</span><span class="no">x0</span><span class="p">,</span> <span class="p">:</span><span class="no">secrel_lo12</span><span class="p">:</span><span class="no">symbol</span><span class="p">]</span>
<span class="nf">add</span> <span class="no">x1</span><span class="p">,</span> <span class="no">x1</span><span class="p">,</span> <span class="p">:</span><span class="no">secrel_hi12</span><span class="p">:</span><span class="no">symbol</span>
<span class="nf">add</span> <span class="no">x1</span><span class="p">,</span> <span class="no">x1</span><span class="p">,</span> <span class="p">:</span><span class="no">secrel_lo12</span><span class="p">:</span><span class="no">symbol</span>
<span class="na">...</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="elf-dependent">
<h3><a class="toc-backref" href="#id15">ELF-Dependent</a><a class="headerlink" href="#elf-dependent" title="Permalink to this headline">¶</a></h3>
<div class="section" id="id2">
<h4><a class="toc-backref" href="#id16"><code class="docutils literal notranslate"><span class="pre">.section</span></code> Directive</a><a class="headerlink" href="#id2" title="Permalink to this headline">¶</a></h4>
<p>In order to support creating multiple sections with the same name and comdat,
it is possible to add an unique number at the end of the <code class="docutils literal notranslate"><span class="pre">.seciton</span></code> directive.
For example, the following code creates two sections named <code class="docutils literal notranslate"><span class="pre">.text</span></code>.</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="no">.text</span><span class="p">,</span><span class="s">"ax"</span><span class="p">,</span><span class="na">@progbits</span><span class="p">,</span><span class="no">unique</span><span class="p">,</span><span class="mi">1</span>
<span class="nf">nop</span>
<span class="na">.section</span> <span class="no">.text</span><span class="p">,</span><span class="s">"ax"</span><span class="p">,</span><span class="na">@progbits</span><span class="p">,</span><span class="no">unique</span><span class="p">,</span><span class="mi">2</span>
<span class="nf">nop</span>
</pre></div>
</div>
<p>The unique number is not present in the resulting object at all. It is just used
in the assembler to differentiate the sections.</p>
<p>The ‘o’ flag is mapped to SHF_LINK_ORDER. If it is present, a symbol
must be given that identifies the section to be placed is the
.sh_link.</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="no">.foo</span><span class="p">,</span><span class="s">"a"</span><span class="p">,</span><span class="na">@progbits</span>
<span class="nl">.Ltmp:</span>
<span class="na">.section</span> <span class="no">.bar</span><span class="p">,</span><span class="s">"ao"</span><span class="p">,</span><span class="na">@progbits</span><span class="p">,.</span><span class="no">Ltmp</span>
</pre></div>
</div>
<p>which is equivalent to just</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="no">.foo</span><span class="p">,</span><span class="s">"a"</span><span class="p">,</span><span class="na">@progbits</span>
<span class="na">.section</span> <span class="no">.bar</span><span class="p">,</span><span class="s">"ao"</span><span class="p">,</span><span class="na">@progbits</span><span class="p">,.</span><span class="no">foo</span>
</pre></div>
</div>
</div>
<div class="section" id="linker-options-section-linker-options">
<h4><a class="toc-backref" href="#id17"><code class="docutils literal notranslate"><span class="pre">.linker-options</span></code> Section (linker options)</a><a class="headerlink" href="#linker-options-section-linker-options" title="Permalink to this headline">¶</a></h4>
<p>In order to support passing linker options from the frontend to the linker, a
special section of type <code class="docutils literal notranslate"><span class="pre">SHT_LLVM_LINKER_OPTIONS</span></code> (usually named
<code class="docutils literal notranslate"><span class="pre">.linker-options</span></code> though the name is not significant as it is identified by
the type). The contents of this section is a simple pair-wise encoding of
directives for consideration by the linker. The strings are encoded as standard
null-terminated UTF-8 strings. They are emitted inline to avoid having the
linker traverse the object file for retrieving the value. The linker is
permitted to not honour the option and instead provide a warning/error to the
user that the requested option was not honoured.</p>
<p>The section has type <code class="docutils literal notranslate"><span class="pre">SHT_LLVM_LINKER_OPTIONS</span></code> and has the <code class="docutils literal notranslate"><span class="pre">SHF_EXCLUDE</span></code>
flag to ensure that the section is treated as opaque by linkers which do not
support the feature and will not be emitted into the final linked binary.</p>
<p>This would be equivalent to the follow raw assembly:</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.section</span> <span class="s">".linker-options"</span><span class="p">,</span><span class="s">"e"</span><span class="p">,</span><span class="na">@llvm_linker_options</span>
<span class="na">.asciz</span> <span class="s">"option 1"</span>
<span class="na">.asciz</span> <span class="s">"value 1"</span>
<span class="na">.asciz</span> <span class="s">"option 2"</span>
<span class="na">.asciz</span> <span class="s">"value 2"</span>
</pre></div>
</div>
<p>The following directives are specified:</p>
<blockquote>
<div><ul>
<li><p class="first">lib</p>
<p>The parameter identifies a library to be linked against. The library will
be looked up in the default and any specified library search paths
(specified to this point).</p>
</li>
<li><p class="first">libpath</p>
<p>The paramter identifies an additional library search path to be considered
when looking up libraries after the inclusion of this option.</p>
</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="sht-llvm-call-graph-profile-section-call-graph-profile">
<h4><a class="toc-backref" href="#id18"><code class="docutils literal notranslate"><span class="pre">SHT_LLVM_CALL_GRAPH_PROFILE</span></code> Section (Call Graph Profile)</a><a class="headerlink" href="#sht-llvm-call-graph-profile-section-call-graph-profile" title="Permalink to this headline">¶</a></h4>
<p>This section is used to pass a call graph profile to the linker which can be
used to optimize the placement of sections. It contains a sequence of
(from symbol, to symbol, weight) tuples.</p>
<p>It shall have a type of <code class="docutils literal notranslate"><span class="pre">SHT_LLVM_CALL_GRAPH_PROFILE</span></code> (0x6fff4c02), shall
have the <code class="docutils literal notranslate"><span class="pre">SHF_EXCLUDE</span></code> flag set, the <code class="docutils literal notranslate"><span class="pre">sh_link</span></code> member shall hold the section
header index of the associated symbol table, and shall have a <code class="docutils literal notranslate"><span class="pre">sh_entsize</span></code> of
16. It should be named <code class="docutils literal notranslate"><span class="pre">.llvm.call-graph-profile</span></code>.</p>
<p>The contents of the section shall be a sequence of <code class="docutils literal notranslate"><span class="pre">Elf_CGProfile</span></code> entries.</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">typedef</span> <span class="k">struct</span> <span class="p">{</span>
<span class="n">Elf_Word</span> <span class="n">cgp_from</span><span class="p">;</span>
<span class="n">Elf_Word</span> <span class="n">cgp_to</span><span class="p">;</span>
<span class="n">Elf_Xword</span> <span class="n">cgp_weight</span><span class="p">;</span>
<span class="p">}</span> <span class="n">Elf_CGProfile</span><span class="p">;</span>
</pre></div>
</div>
<dl class="docutils">
<dt>cgp_from</dt>
<dd>The symbol index of the source of the edge.</dd>
<dt>cgp_to</dt>
<dd>The symbol index of the destination of the edge.</dd>
<dt>cgp_weight</dt>
<dd>The weight of the edge.</dd>
</dl>
<p>This is represented in assembly as:</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.cg_profile</span> <span class="no">from</span><span class="p">,</span> <span class="no">to</span><span class="p">,</span> <span class="mi">42</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">.cg_profile</span></code> directives are processed at the end of the file. It is an error
if either <code class="docutils literal notranslate"><span class="pre">from</span></code> or <code class="docutils literal notranslate"><span class="pre">to</span></code> are undefined temporary symbols. If either symbol
is a temporary symbol, then the section symbol is used instead. If either
symbol is undefined, then that symbol is defined as if <code class="docutils literal notranslate"><span class="pre">.weak</span> <span class="pre">symbol</span></code> has been
written at the end of the file. This forces the symbol to show up in the symbol
table.</p>
</div>
<div class="section" id="sht-llvm-addrsig-section-address-significance-table">
<h4><a class="toc-backref" href="#id19"><code class="docutils literal notranslate"><span class="pre">SHT_LLVM_ADDRSIG</span></code> Section (address-significance table)</a><a class="headerlink" href="#sht-llvm-addrsig-section-address-significance-table" title="Permalink to this headline">¶</a></h4>
<p>This section is used to mark symbols as address-significant, i.e. the address
of the symbol is used in a comparison or leaks outside the translation unit. It
has the same meaning as the absence of the LLVM attributes <code class="docutils literal notranslate"><span class="pre">unnamed_addr</span></code>
and <code class="docutils literal notranslate"><span class="pre">local_unnamed_addr</span></code>.</p>
<p>Any sections referred to by symbols that are not marked as address-significant
in any object file may be safely merged by a linker without breaking the
address uniqueness guarantee provided by the C and C++ language standards.</p>
<p>The contents of the section are a sequence of ULEB128-encoded integers
referring to the symbol table indexes of the address-significant symbols.</p>
<p>There are two associated assembly directives:</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.addrsig</span>
</pre></div>
</div>
<p>This instructs the assembler to emit an address-significance table. Without
this directive, all symbols are considered address-significant.</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="na">.addrsig_sym</span> <span class="no">sym</span>
</pre></div>
</div>
<p>This marks <code class="docutils literal notranslate"><span class="pre">sym</span></code> as address-significant.</p>
</div>
</div>
<div class="section" id="codeview-dependent">
<h3><a class="toc-backref" href="#id20">CodeView-Dependent</a><a class="headerlink" href="#codeview-dependent" title="Permalink to this headline">¶</a></h3>
<div class="section" id="cv-file-directive">
<h4><a class="toc-backref" href="#id21"><code class="docutils literal notranslate"><span class="pre">.cv_file</span></code> Directive</a><a class="headerlink" href="#cv-file-directive" title="Permalink to this headline">¶</a></h4>
<dl class="docutils">
<dt>Syntax:</dt>
<dd><code class="docutils literal notranslate"><span class="pre">.cv_file</span></code> <em>FileNumber FileName</em> [ <em>checksum</em> ] [ <em>checksumkind</em> ]</dd>
</dl>
</div>
<div class="section" id="cv-func-id-directive">
<h4><a class="toc-backref" href="#id22"><code class="docutils literal notranslate"><span class="pre">.cv_func_id</span></code> Directive</a><a class="headerlink" href="#cv-func-id-directive" title="Permalink to this headline">¶</a></h4>
<p>Introduces a function ID that can be used with <code class="docutils literal notranslate"><span class="pre">.cv_loc</span></code>.</p>
<dl class="docutils">
<dt>Syntax:</dt>
<dd><code class="docutils literal notranslate"><span class="pre">.cv_func_id</span></code> <em>FunctionId</em></dd>
</dl>
</div>
<div class="section" id="cv-inline-site-id-directive">
<h4><a class="toc-backref" href="#id23"><code class="docutils literal notranslate"><span class="pre">.cv_inline_site_id</span></code> Directive</a><a class="headerlink" href="#cv-inline-site-id-directive" title="Permalink to this headline">¶</a></h4>
<p>Introduces a function ID that can be used with <code class="docutils literal notranslate"><span class="pre">.cv_loc</span></code>. Includes
<code class="docutils literal notranslate"><span class="pre">inlined</span> <span class="pre">at</span></code> source location information for use in the line table of the
caller, whether the caller is a real function or another inlined call site.</p>
<dl class="docutils">
<dt>Syntax:</dt>
<dd><code class="docutils literal notranslate"><span class="pre">.cv_inline_site_id</span></code> <em>FunctionId</em> <code class="docutils literal notranslate"><span class="pre">within</span></code> <em>Function</em> <code class="docutils literal notranslate"><span class="pre">inlined_at</span></code> <em>FileNumber Line</em> [ <em>Colomn</em> ]</dd>
</dl>
</div>
<div class="section" id="cv-loc-directive">
<h4><a class="toc-backref" href="#id24"><code class="docutils literal notranslate"><span class="pre">.cv_loc</span></code> Directive</a><a class="headerlink" href="#cv-loc-directive" title="Permalink to this headline">¶</a></h4>
<p>The first number is a file number, must have been previously assigned with a
<code class="docutils literal notranslate"><span class="pre">.file</span></code> directive, the second number is the line number and optionally the
third number is a column position (zero if not specified). The remaining
optional items are <code class="docutils literal notranslate"><span class="pre">.loc</span></code> sub-directives.</p>
<dl class="docutils">
<dt>Syntax:</dt>
<dd><code class="docutils literal notranslate"><span class="pre">.cv_loc</span></code> <em>FunctionId FileNumber</em> [ <em>Line</em> ] [ <em>Column</em> ] [ <em>prologue_end</em> ] [ <code class="docutils literal notranslate"><span class="pre">is_stmt</span></code> <em>value</em> ]</dd>
</dl>
</div>
<div class="section" id="cv-linetable-directive">
<h4><a class="toc-backref" href="#id25"><code class="docutils literal notranslate"><span class="pre">.cv_linetable</span></code> Directive</a><a class="headerlink" href="#cv-linetable-directive" title="Permalink to this headline">¶</a></h4>
<dl class="docutils">
<dt>Syntax:</dt>
<dd><code class="docutils literal notranslate"><span class="pre">.cv_linetable</span></code> <em>FunctionId</em> <code class="docutils literal notranslate"><span class="pre">,</span></code> <em>FunctionStart</em> <code class="docutils literal notranslate"><span class="pre">,</span></code> <em>FunctionEnd</em></dd>
</dl>
</div>
<div class="section" id="cv-inline-linetable-directive">
<h4><a class="toc-backref" href="#id26"><code class="docutils literal notranslate"><span class="pre">.cv_inline_linetable</span></code> Directive</a><a class="headerlink" href="#cv-inline-linetable-directive" title="Permalink to this headline">¶</a></h4>
<dl class="docutils">
<dt>Syntax:</dt>
<dd><code class="docutils literal notranslate"><span class="pre">.cv_inline_linetable</span></code> <em>PrimaryFunctionId</em> <code class="docutils literal notranslate"><span class="pre">,</span></code> <em>FileNumber Line FunctionStart FunctionEnd</em></dd>
</dl>
</div>
<div class="section" id="cv-def-range-directive">
<h4><a class="toc-backref" href="#id27"><code class="docutils literal notranslate"><span class="pre">.cv_def_range</span></code> Directive</a><a class="headerlink" href="#cv-def-range-directive" title="Permalink to this headline">¶</a></h4>
<p>The <em>GapStart</em> and <em>GapEnd</em> options may be repeated as needed.</p>
<dl class="docutils">
<dt>Syntax:</dt>
<dd><code class="docutils literal notranslate"><span class="pre">.cv_def_range</span></code> <em>RangeStart RangeEnd</em> [ <em>GapStart GapEnd</em> ] <code class="docutils literal notranslate"><span class="pre">,</span></code> <em>bytes</em></dd>
</dl>
</div>
<div class="section" id="cv-stringtable-directive">
<h4><a class="toc-backref" href="#id28"><code class="docutils literal notranslate"><span class="pre">.cv_stringtable</span></code> Directive</a><a class="headerlink" href="#cv-stringtable-directive" title="Permalink to this headline">¶</a></h4>
</div>
<div class="section" id="cv-filechecksums-directive">
<h4><a class="toc-backref" href="#id29"><code class="docutils literal notranslate"><span class="pre">.cv_filechecksums</span></code> Directive</a><a class="headerlink" href="#cv-filechecksums-directive" title="Permalink to this headline">¶</a></h4>
</div>
<div class="section" id="cv-filechecksumoffset-directive">
<h4><a class="toc-backref" href="#id30"><code class="docutils literal notranslate"><span class="pre">.cv_filechecksumoffset</span></code> Directive</a><a class="headerlink" href="#cv-filechecksumoffset-directive" title="Permalink to this headline">¶</a></h4>
<dl class="docutils">
<dt>Syntax:</dt>
<dd><code class="docutils literal notranslate"><span class="pre">.cv_filechecksumoffset</span></code> <em>FileNumber</em></dd>
</dl>
</div>
<div class="section" id="cv-fpo-data-directive">
<h4><a class="toc-backref" href="#id31"><code class="docutils literal notranslate"><span class="pre">.cv_fpo_data</span></code> Directive</a><a class="headerlink" href="#cv-fpo-data-directive" title="Permalink to this headline">¶</a></h4>
<dl class="docutils">
<dt>Syntax:</dt>
<dd><code class="docutils literal notranslate"><span class="pre">.cv_fpo_data</span></code> <em>procsym</em></dd>
</dl>
</div>
</div>
</div>
<div class="section" id="target-specific-behaviour">
<h2><a class="toc-backref" href="#id32">Target Specific Behaviour</a><a class="headerlink" href="#target-specific-behaviour" title="Permalink to this headline">¶</a></h2>
<div class="section" id="x86">
<h3><a class="toc-backref" href="#id33">X86</a><a class="headerlink" href="#x86" title="Permalink to this headline">¶</a></h3>
<div class="section" id="id3">
<h4><a class="toc-backref" href="#id34">Relocations</a><a class="headerlink" href="#id3" title="Permalink to this headline">¶</a></h4>
<p><code class="docutils literal notranslate"><span class="pre">@ABS8</span></code> can be applied to symbols which appear as immediate operands to
instructions that have an 8-bit immediate form for that operand. It causes
the assembler to use the 8-bit form and an 8-bit relocation (e.g. <code class="docutils literal notranslate"><span class="pre">R_386_8</span></code>
or <code class="docutils literal notranslate"><span class="pre">R_X86_64_8</span></code>) for the symbol.</p>
<p>For example:</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="nf">cmpq</span> <span class="no">$foo@ABS8</span><span class="p">,</span> <span class="nv">%rdi</span>
</pre></div>
</div>
<p>This causes the assembler to select the form of the 64-bit <code class="docutils literal notranslate"><span class="pre">cmpq</span></code> instruction
that takes an 8-bit immediate operand that is sign extended to 64 bits, as
opposed to <code class="docutils literal notranslate"><span class="pre">cmpq</span> <span class="pre">$foo,</span> <span class="pre">%rdi</span></code> which takes a 32-bit immediate operand. This
is also not the same as <code class="docutils literal notranslate"><span class="pre">cmpb</span> <span class="pre">$foo,</span> <span class="pre">%dil</span></code>, which is an 8-bit comparison.</p>
</div>
</div>
<div class="section" id="windows-on-arm">
<h3><a class="toc-backref" href="#id35">Windows on ARM</a><a class="headerlink" href="#windows-on-arm" title="Permalink to this headline">¶</a></h3>
<div class="section" id="stack-probe-emission">
<h4><a class="toc-backref" href="#id36">Stack Probe Emission</a><a class="headerlink" href="#stack-probe-emission" title="Permalink to this headline">¶</a></h4>
<p>The reference implementation (Microsoft Visual Studio 2012) emits stack probes
in the following fashion:</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="nf">movw</span> <span class="no">r4</span><span class="p">,</span> <span class="c">#constant</span>
<span class="nf">bl</span> <span class="no">__chkstk</span>
<span class="nf">sub.w</span> <span class="no">sp</span><span class="p">,</span> <span class="no">sp</span><span class="p">,</span> <span class="no">r4</span>
</pre></div>
</div>
<p>However, this has the limitation of 32 MiB (±16MiB). In order to accommodate
larger binaries, LLVM supports the use of <code class="docutils literal notranslate"><span class="pre">-mcode-model=large</span></code> to allow a 4GiB
range via a slight deviation. It will generate an indirect jump as follows:</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="nf">movw</span> <span class="no">r4</span><span class="p">,</span> <span class="c">#constant</span>
<span class="nf">movw</span> <span class="no">r12</span><span class="p">,</span> <span class="p">:</span><span class="no">lower16</span><span class="p">:</span><span class="no">__chkstk</span>
<span class="nf">movt</span> <span class="no">r12</span><span class="p">,</span> <span class="p">:</span><span class="no">upper16</span><span class="p">:</span><span class="no">__chkstk</span>
<span class="nf">blx</span> <span class="no">r12</span>
<span class="nf">sub.w</span> <span class="no">sp</span><span class="p">,</span> <span class="no">sp</span><span class="p">,</span> <span class="no">r4</span>
</pre></div>
</div>
</div>
<div class="section" id="variable-length-arrays">
<h4><a class="toc-backref" href="#id37">Variable Length Arrays</a><a class="headerlink" href="#variable-length-arrays" title="Permalink to this headline">¶</a></h4>
<p>The reference implementation (Microsoft Visual Studio 2012) does not permit the
emission of Variable Length Arrays (VLAs).</p>
<p>The Windows ARM Itanium ABI extends the base ABI by adding support for emitting
a dynamic stack allocation. When emitting a variable stack allocation, a call
to <code class="docutils literal notranslate"><span class="pre">__chkstk</span></code> is emitted unconditionally to ensure that guard pages are setup
properly. The emission of this stack probe emission is handled similar to the
standard stack probe emission.</p>
<p>The MSVC environment does not emit code for VLAs currently.</p>
</div>
</div>
<div class="section" id="windows-on-arm64">
<h3><a class="toc-backref" href="#id38">Windows on ARM64</a><a class="headerlink" href="#windows-on-arm64" title="Permalink to this headline">¶</a></h3>
<div class="section" id="id4">
<h4><a class="toc-backref" href="#id39">Stack Probe Emission</a><a class="headerlink" href="#id4" title="Permalink to this headline">¶</a></h4>
<p>The reference implementation (Microsoft Visual Studio 2017) emits stack probes
in the following fashion:</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="nf">mov</span> <span class="no">x15</span><span class="p">,</span> <span class="c">#constant</span>
<span class="nf">bl</span> <span class="no">__chkstk</span>
<span class="nf">sub</span> <span class="no">sp</span><span class="p">,</span> <span class="no">sp</span><span class="p">,</span> <span class="no">x15</span><span class="p">,</span> <span class="no">lsl</span> <span class="c">#4</span>
</pre></div>
</div>
<p>However, this has the limitation of 256 MiB (±128MiB). In order to accommodate
larger binaries, LLVM supports the use of <code class="docutils literal notranslate"><span class="pre">-mcode-model=large</span></code> to allow a 8GiB
(±4GiB) range via a slight deviation. It will generate an indirect jump as
follows:</p>
<div class="highlight-gas notranslate"><div class="highlight"><pre><span></span><span class="nf">mov</span> <span class="no">x15</span><span class="p">,</span> <span class="c">#constant</span>
<span class="nf">adrp</span> <span class="no">x16</span><span class="p">,</span> <span class="no">__chkstk</span>
<span class="nf">add</span> <span class="no">x16</span><span class="p">,</span> <span class="no">x16</span><span class="p">,</span> <span class="p">:</span><span class="no">lo12</span><span class="p">:</span><span class="no">__chkstk</span>
<span class="nf">blr</span> <span class="no">x16</span>
<span class="nf">sub</span> <span class="no">sp</span><span class="p">,</span> <span class="no">sp</span><span class="p">,</span> <span class="no">x15</span><span class="p">,</span> <span class="no">lsl</span> <span class="c">#4</span>
</pre></div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
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