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<a name="Named-Address-Spaces"></a>
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<a name="Adding-support-for-named-address-spaces"></a>
<h3 class="section">17.29 Adding support for named address spaces</h3>
<a name="index-named-address-spaces"></a>

<p>The draft technical report of the ISO/IEC JTC1 S22 WG14 N1275
standards committee, <cite>Programming Languages - C - Extensions to
support embedded processors</cite>, specifies a syntax for embedded
processors to specify alternate address spaces.  You can configure a
GCC port to support section 5.1 of the draft report to add support for
address spaces other than the default address space.  These address
spaces are new keywords that are similar to the <code>volatile</code> and
<code>const</code> type attributes.
</p>
<p>Pointers to named address spaces can have a different size than
pointers to the generic address space.
</p>
<p>For example, the SPU port uses the <code>__ea</code> address space to refer
to memory in the host processor, rather than memory local to the SPU
processor.  Access to memory in the <code>__ea</code> address space involves
issuing DMA operations to move data between the host processor and the
local processor memory address space.  Pointers in the <code>__ea</code>
address space are either 32 bits or 64 bits based on the
<samp>-mea32</samp> or <samp>-mea64</samp> switches (native SPU pointers are
always 32 bits).
</p>
<p>Internally, address spaces are represented as a small integer in the
range 0 to 15 with address space 0 being reserved for the generic
address space.
</p>
<p>To register a named address space qualifier keyword with the C front end,
the target may call the <code>c_register_addr_space</code> routine.  For example,
the SPU port uses the following to declare <code>__ea</code> as the keyword for
named address space #1:
</p><div class="smallexample">
<pre class="smallexample">#define ADDR_SPACE_EA 1
c_register_addr_space (&quot;__ea&quot;, ADDR_SPACE_EA);
</pre></div>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fPOINTER_005fMODE"></a>Target Hook: <em>machine_mode</em> <strong>TARGET_ADDR_SPACE_POINTER_MODE</strong> <em>(addr_space_t <var>address_space</var>)</em></dt>
<dd><p>Define this to return the machine mode to use for pointers to
<var>address_space</var> if the target supports named address spaces.
The default version of this hook returns <code>ptr_mode</code>.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fADDRESS_005fMODE"></a>Target Hook: <em>machine_mode</em> <strong>TARGET_ADDR_SPACE_ADDRESS_MODE</strong> <em>(addr_space_t <var>address_space</var>)</em></dt>
<dd><p>Define this to return the machine mode to use for addresses in
<var>address_space</var> if the target supports named address spaces.
The default version of this hook returns <code>Pmode</code>.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fVALID_005fPOINTER_005fMODE"></a>Target Hook: <em>bool</em> <strong>TARGET_ADDR_SPACE_VALID_POINTER_MODE</strong> <em>(machine_mode <var>mode</var>, addr_space_t <var>as</var>)</em></dt>
<dd><p>Define this to return nonzero if the port can handle pointers
with machine mode <var>mode</var> to address space <var>as</var>.  This target
hook is the same as the <code>TARGET_VALID_POINTER_MODE</code> target hook,
except that it includes explicit named address space support.  The default
version of this hook returns true for the modes returned by either the
<code>TARGET_ADDR_SPACE_POINTER_MODE</code> or <code>TARGET_ADDR_SPACE_ADDRESS_MODE</code>
target hooks for the given address space.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fLEGITIMATE_005fADDRESS_005fP"></a>Target Hook: <em>bool</em> <strong>TARGET_ADDR_SPACE_LEGITIMATE_ADDRESS_P</strong> <em>(machine_mode <var>mode</var>, rtx <var>exp</var>, bool <var>strict</var>, addr_space_t <var>as</var>)</em></dt>
<dd><p>Define this to return true if <var>exp</var> is a valid address for mode
<var>mode</var> in the named address space <var>as</var>.  The <var>strict</var>
parameter says whether strict addressing is in effect after reload has
finished.  This target hook is the same as the
<code>TARGET_LEGITIMATE_ADDRESS_P</code> target hook, except that it includes
explicit named address space support.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fLEGITIMIZE_005fADDRESS"></a>Target Hook: <em>rtx</em> <strong>TARGET_ADDR_SPACE_LEGITIMIZE_ADDRESS</strong> <em>(rtx <var>x</var>, rtx <var>oldx</var>, machine_mode <var>mode</var>, addr_space_t <var>as</var>)</em></dt>
<dd><p>Define this to modify an invalid address <var>x</var> to be a valid address
with mode <var>mode</var> in the named address space <var>as</var>.  This target
hook is the same as the <code>TARGET_LEGITIMIZE_ADDRESS</code> target hook,
except that it includes explicit named address space support.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fSUBSET_005fP"></a>Target Hook: <em>bool</em> <strong>TARGET_ADDR_SPACE_SUBSET_P</strong> <em>(addr_space_t <var>subset</var>, addr_space_t <var>superset</var>)</em></dt>
<dd><p>Define this to return whether the <var>subset</var> named address space is
contained within the <var>superset</var> named address space.  Pointers to
a named address space that is a subset of another named address space
will be converted automatically without a cast if used together in
arithmetic operations.  Pointers to a superset address space can be
converted to pointers to a subset address space via explicit casts.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fZERO_005fADDRESS_005fVALID"></a>Target Hook: <em>bool</em> <strong>TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID</strong> <em>(addr_space_t <var>as</var>)</em></dt>
<dd><p>Define this to modify the default handling of address 0 for the
address space.  Return true if 0 should be considered a valid address.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fCONVERT"></a>Target Hook: <em>rtx</em> <strong>TARGET_ADDR_SPACE_CONVERT</strong> <em>(rtx <var>op</var>, tree <var>from_type</var>, tree <var>to_type</var>)</em></dt>
<dd><p>Define this to convert the pointer expression represented by the RTL
<var>op</var> with type <var>from_type</var> that points to a named address
space to a new pointer expression with type <var>to_type</var> that points
to a different named address space.  When this hook it called, it is
guaranteed that one of the two address spaces is a subset of the other,
as determined by the <code>TARGET_ADDR_SPACE_SUBSET_P</code> target hook.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fDEBUG"></a>Target Hook: <em>int</em> <strong>TARGET_ADDR_SPACE_DEBUG</strong> <em>(addr_space_t <var>as</var>)</em></dt>
<dd><p>Define this to define how the address space is encoded in dwarf.
The result is the value to be used with <code>DW_AT_address_class</code>.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fADDR_005fSPACE_005fDIAGNOSE_005fUSAGE"></a>Target Hook: <em>void</em> <strong>TARGET_ADDR_SPACE_DIAGNOSE_USAGE</strong> <em>(addr_space_t <var>as</var>, location_t <var>loc</var>)</em></dt>
<dd><p>Define this hook if the availability of an address space depends on
command line options and some diagnostics should be printed when the
address space is used.  This hook is called during parsing and allows
to emit a better diagnostic compared to the case where the address space
was not registered with <code>c_register_addr_space</code>.  <var>as</var> is
the address space as registered with <code>c_register_addr_space</code>.
<var>loc</var> is the location of the address space qualifier token.
The default implementation does nothing.
</p></dd></dl>

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