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<h3 class="section">17.25 Emulating TLS</h3>
<p><a name="index-Emulated-TLS-4851"></a>
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>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's instance of the TLS object.
<div class="defun">
— Target Hook: const char * <b>TARGET_EMUTLS_GET_ADDRESS</b><var><a name="index-TARGET_005fEMUTLS_005fGET_005fADDRESS-4852"></a></var><br>
<blockquote><p>Contains the name of the helper function that uses a TLS control
object to locate a TLS instance. The default causes libgcc's
emulated TLS helper function to be used.
</p></blockquote></div>
<div class="defun">
— Target Hook: const char * <b>TARGET_EMUTLS_REGISTER_COMMON</b><var><a name="index-TARGET_005fEMUTLS_005fREGISTER_005fCOMMON-4853"></a></var><br>
<blockquote><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's emulated TLS
registration function to be used.
</p></blockquote></div>
<div class="defun">
— Target Hook: const char * <b>TARGET_EMUTLS_VAR_SECTION</b><var><a name="index-TARGET_005fEMUTLS_005fVAR_005fSECTION-4854"></a></var><br>
<blockquote><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></blockquote></div>
<div class="defun">
— Target Hook: const char * <b>TARGET_EMUTLS_TMPL_SECTION</b><var><a name="index-TARGET_005fEMUTLS_005fTMPL_005fSECTION-4855"></a></var><br>
<blockquote><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></blockquote></div>
<div class="defun">
— Target Hook: const char * <b>TARGET_EMUTLS_VAR_PREFIX</b><var><a name="index-TARGET_005fEMUTLS_005fVAR_005fPREFIX-4856"></a></var><br>
<blockquote><p>Contains the prefix to be prepended to TLS control variable names.
The default of <code>NULL</code> uses a target-specific prefix.
</p></blockquote></div>
<div class="defun">
— Target Hook: const char * <b>TARGET_EMUTLS_TMPL_PREFIX</b><var><a name="index-TARGET_005fEMUTLS_005fTMPL_005fPREFIX-4857"></a></var><br>
<blockquote><p>Contains the prefix to be prepended to TLS initializer objects. The
default of <code>NULL</code> uses a target-specific prefix.
</p></blockquote></div>
<div class="defun">
— Target Hook: tree <b>TARGET_EMUTLS_VAR_FIELDS</b> (<var>tree type, tree *name</var>)<var><a name="index-TARGET_005fEMUTLS_005fVAR_005fFIELDS-4858"></a></var><br>
<blockquote><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's emulated TLS function.
</p></blockquote></div>
<div class="defun">
— Target Hook: tree <b>TARGET_EMUTLS_VAR_INIT</b> (<var>tree var, tree decl, tree tmpl_addr</var>)<var><a name="index-TARGET_005fEMUTLS_005fVAR_005fINIT-4859"></a></var><br>
<blockquote><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's emulated TLS control object.
</p></blockquote></div>
<div class="defun">
— Target Hook: bool <b>TARGET_EMUTLS_VAR_ALIGN_FIXED</b><var><a name="index-TARGET_005fEMUTLS_005fVAR_005fALIGN_005fFIXED-4860"></a></var><br>
<blockquote><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></blockquote></div>
<div class="defun">
— Target Hook: bool <b>TARGET_EMUTLS_DEBUG_FORM_TLS_ADDRESS</b><var><a name="index-TARGET_005fEMUTLS_005fDEBUG_005fFORM_005fTLS_005fADDRESS-4861"></a></var><br>
<blockquote><p>Specifies whether a DWARF <code>DW_OP_form_tls_address</code> location descriptor
may be used to describe emulated TLS control objects.
</p></blockquote></div>
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