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<a name="Emulating-TLS"></a>
<h3 class="section">18.25 Emulating TLS</h3>
<a name="index-Emulated-TLS"></a>

<p>For targets whose psABI does not provide Thread Local Storage via
specific relocations and instruction sequences, an emulation layer is
used.  A set of target hooks allows this emulation layer to be
configured for the requirements of a particular target.  For instance
the psABI may in fact specify TLS support in terms of an emulation
layer.
</p>
<p>The emulation layer works by creating a control object for every TLS
object.  To access the TLS object, a lookup function is provided
which, when given the address of the control object, will return the
address of the current thread&rsquo;s instance of the TLS object.
</p>
<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fGET_005fADDRESS"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_GET_ADDRESS</strong></dt>
<dd><p>Contains the name of the helper function that uses a TLS control
object to locate a TLS instance.  The default causes libgcc&rsquo;s
emulated TLS helper function to be used.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fREGISTER_005fCOMMON"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_REGISTER_COMMON</strong></dt>
<dd><p>Contains the name of the helper function that should be used at
program startup to register TLS objects that are implicitly
initialized to zero.  If this is <code>NULL</code>, all TLS objects will
have explicit initializers.  The default causes libgcc&rsquo;s emulated TLS
registration function to be used.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fSECTION"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_VAR_SECTION</strong></dt>
<dd><p>Contains the name of the section in which TLS control variables should
be placed.  The default of <code>NULL</code> allows these to be placed in
any section.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fTMPL_005fSECTION"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_TMPL_SECTION</strong></dt>
<dd><p>Contains the name of the section in which TLS initializers should be
placed.  The default of <code>NULL</code> allows these to be placed in any
section.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fPREFIX"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_VAR_PREFIX</strong></dt>
<dd><p>Contains the prefix to be prepended to TLS control variable names.
The default of <code>NULL</code> uses a target-specific prefix.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fTMPL_005fPREFIX"></a>Target Hook: <em>const char *</em> <strong>TARGET_EMUTLS_TMPL_PREFIX</strong></dt>
<dd><p>Contains the prefix to be prepended to TLS initializer objects.  The
default of <code>NULL</code> uses a target-specific prefix.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fFIELDS"></a>Target Hook: <em>tree</em> <strong>TARGET_EMUTLS_VAR_FIELDS</strong> <em>(tree <var>type</var>, tree *<var>name</var>)</em></dt>
<dd><p>Specifies a function that generates the FIELD_DECLs for a TLS control
object type.  <var>type</var> is the RECORD_TYPE the fields are for and
<var>name</var> should be filled with the structure tag, if the default of
<code>__emutls_object</code> is unsuitable.  The default creates a type suitable
for libgcc&rsquo;s emulated TLS function.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fINIT"></a>Target Hook: <em>tree</em> <strong>TARGET_EMUTLS_VAR_INIT</strong> <em>(tree <var>var</var>, tree <var>decl</var>, tree <var>tmpl_addr</var>)</em></dt>
<dd><p>Specifies a function that generates the CONSTRUCTOR to initialize a
TLS control object.  <var>var</var> is the TLS control object, <var>decl</var>
is the TLS object and <var>tmpl_addr</var> is the address of the
initializer.  The default initializes libgcc&rsquo;s emulated TLS control object.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fVAR_005fALIGN_005fFIXED"></a>Target Hook: <em>bool</em> <strong>TARGET_EMUTLS_VAR_ALIGN_FIXED</strong></dt>
<dd><p>Specifies whether the alignment of TLS control variable objects is
fixed and should not be increased as some backends may do to optimize
single objects.  The default is false.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fEMUTLS_005fDEBUG_005fFORM_005fTLS_005fADDRESS"></a>Target Hook: <em>bool</em> <strong>TARGET_EMUTLS_DEBUG_FORM_TLS_ADDRESS</strong></dt>
<dd><p>Specifies whether a DWARF <code>DW_OP_form_tls_address</code> location descriptor
may be used to describe emulated TLS control objects.
</p></dd></dl>

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