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@ -2384,24 +2384,10 @@ xml:id="dbdoclet.50655240_pgfId-1156194">
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<para>Function pointer</para>
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</entry>
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</row>
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<row revisionflag="changed">
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<entry morerows="3">
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<row>
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<entry morerows="2">
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<para>Binary Floating-Point</para>
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</entry>
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<entry>
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<para>_Float16</para>
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</entry>
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<entry>
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<para>2</para>
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</entry>
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<entry>
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<para>Halfword</para>
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</entry>
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<entry>
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<para>Half-precision float</para>
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</entry>
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</row>
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<row>
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<entry>
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<para>float</para>
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</entry>
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@ -2778,7 +2764,7 @@ xml:id="dbdoclet.50655240_pgfId-1156194">
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<para>Vector of 1 signed quadword.</para>
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</entry>
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</row>
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<row>
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<row revisionflag="deleted">
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<entry>
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<para></para>
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</entry>
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@ -4091,9 +4077,9 @@ xml:id="dbdoclet.50655240_pgfId-1156194">
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If a function contains any prefixed (8-byte) instructions,
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functions should preferably be aligned on at least a 64-byte
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boundary. In ISA 3.1, executing a prefixed instruction that
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crosses a 64-byte boundary will cause a SIGILL that must be
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handled by the kernel. Compilers and assemblers can avoid
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this if functions are aligned on a 64-byte boundary.
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crosses a 64-byte boundary causes an alignment interrupt.
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Compilers and assemblers can avoid this if functions are
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aligned on a 64-byte boundary.
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</para>
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</section>
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</section>
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@ -4142,12 +4128,12 @@ xml:id="dbdoclet.50655240_pgfId-1156194">
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tables is further described in the referenced section. A program may
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contain any combination of the function call protocols in these
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tables.
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<note><para>Note that
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this ABI does not define protocols where the caller does not use
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a TOC pointer, but does preserve r2. It is most efficient when
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such functions are always leaf procedures. It is not forbidden for
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such a function to call another function, but in this case it is
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up to the caller to save and restore r2 around each call.
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<note><para>This ABI does not define protocols where the
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caller does not use a TOC pointer, but does preserve r2. It
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is most efficient when such functions are always leaf
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procedures. It is not forbidden for such a function to call
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another function, but in this case it is up to the caller to
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save and restore r2 around each call.
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</para></note>
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</para>
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<table frame="all" pgwide="1"
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@ -4471,10 +4457,9 @@ xml:id="dbdoclet.50655240_pgfId-1156194">
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linkage table (PLT) stub that saves r2 and replaces the nop
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instruction with a restore of r2. (The save of r2 may be omitted
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from the PLT stub if the R_PPC64_TOCSAVE relocation is used; see
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<xref linkend="dbdoclet.50655241_90220" />.) If the callee requires
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a TOC, the PLT stub also includes code to place the callee's global
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entry point into r12. See <xref linkend="dbdoclet.50655242_82622"
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/> for a full description of PLT stubs.
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<xref linkend="dbdoclet.50655241_90220" />.) See <xref
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linkend="dbdoclet.50655242_82622" /> for a full description
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of PLT stubs.
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</para>
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</section>
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<section xml:id="dbdoclet.50655241_Ext1">
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@ -4487,8 +4472,7 @@ xml:id="dbdoclet.50655240_pgfId-1156194">
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after the bl instruction for the call. Instead, the compiler
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annotates the bl instruction with an R_PPC64_REL24_NOTOC
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relocation. The linker generates a PLT stub that does not include
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a save of r2. If the callee requires a TOC, the PLT stub also
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includes code to place the callee's global entry point into r12.
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a save of r2.
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</para>
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</section>
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<section xml:id="dbdoclet.50655241_Local20">
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@ -4685,19 +4669,23 @@ xml:id="dbdoclet.50655240_pgfId-1156194">
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<entry>
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<para>Nonvolatile<footnote>
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<para>Register r2 is nonvolatile with respect to calls
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between <phrase revisionflag="added">most</phrase> functions
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in the same compilation unit. It is saved and restored by
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code inserted
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by the linker resolving a call to an external function. For
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more information, see <xref linkend="dbdoclet.50655240_51083"
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/> <phrase revisionflag="added"> and <xref
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linkend="dbdoclet.50655241_FnLinkage" /></phrase>.</para>
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between functions in the same compilation
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unit<phrase revisionflag="deleted">. It is saved and
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restored by code inserted by the linker resolving a
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call to an external function.</phrase><phrase
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revisionflag="added">, except under the conditions
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in footnote (b).</phrase> For more information, see
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<xref linkend="dbdoclet.50655240_51083" /> <phrase
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revisionflag="added"> and <xref
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linkend="dbdoclet.50655241_FnLinkage"
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/></phrase>.</para>
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</footnote><phrase revisionflag="added"> or
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Volatile<footnote>
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<para>Register r2 is volatile and available for use in a
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function that does not use a TOC pointer and that does
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not preserve r2. See
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<xref linkend="dbdoclet.50655241_FnLinkage" />.</para>
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not guarantee that it preserves r2. See
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<xref linkend="dbdoclet.50655241_FnLinkage" /> and
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<xref linkend="dbdoclet.50655241_95185" />.</para>
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</footnote></phrase></para>
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</entry>
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<entry>
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@ -5042,11 +5030,7 @@ xml:id="dbdoclet.50655240_pgfId-1156194">
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revisionflag="changed">least-significant halves of those VSX
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registers corresponding</phrase> to the classic floating-point
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registers (that is, vsr0–vsr31), <phrase
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revisionflag="changed">are</phrase> volatile. <phrase
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revisionflag="added">If the most-significant half of such a
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VSX register is a non-volatile floating-point register that is
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not used for a function call, the entire VSX register is
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volatile.</phrase></para>
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revisionflag="changed">are</phrase> volatile.</para>
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<table frame="all" pgwide="1" xml:id="dbdoclet.50655240_83567">
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<title>Floating-Point Register Roles for Binary Floating-Point
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@ -6009,8 +5993,15 @@ xml:id="dbdoclet.50655240_pgfId-1156194">
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<para>Any future type requiring 16-byte alignment (see
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<xref linkend="dbdoclet.50655240_15141" />) or processed in vector
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registers</para>
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<para>For the purpose of determining a qualified floating-point argument,
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_Float128 shall be considered a vector data type. In addition, _Float128
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<para>For the purpose of determining a qualified
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floating-point argument, <phrase
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revisionflag="deleted">_Float128</phrase><phrase
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revisionflag="added">IEEE BINARY 128 QUADRUPLE
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PRECISION</phrase> shall be considered a vector data type. In
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addition, <phrase
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revisionflag="deleted">_Float128</phrase><phrase
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revisionflag="added">IEEE BINARY 128 QUADRUPLE
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PRECISION</phrase>
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is like a vector data type for determining if multiple aggregate members
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are like.</para>
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<para>A homogeneous aggregate can consist of a variety of nested
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@ -6594,7 +6585,7 @@ s6 - 72 (stored)</programlisting>
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Area must be large enough to accommodate all parameters, including
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parameters passed in registers.</para>
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<para revisionflag="added">
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The caller of any function with an ellipsis in its prototype
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The caller of any function with a variable argument list
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must allocate a Parameter Save Area, as described in <xref
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linkend="dbdoclet.50655240_78421" />.
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</para>
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@ -6652,44 +6643,6 @@ s6 - 72 (stored)</programlisting>
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</listitem>
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</itemizedlist>
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</section>
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<section xml:id="dbdoclet.50655240___tailcall"
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revisionflag="added">
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<title>Tail-Call Optimization</title>
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<para>
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When the last action of a function <emphasis>F</emphasis> is
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to perform a function call to a function
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<emphasis>G</emphasis>, and optionally return the value
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returned from <emphasis>G</emphasis>, a compiler may perform a
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<emphasis>tail-call optimization</emphasis> so long as the
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optimization is undetectable by the caller of
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<emphasis>G</emphasis>. The full details of and requirements
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for tail-call optimization will not be described here, but in
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essence <emphasis>F</emphasis> removes its stack frame and
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issues a direct branch to <emphasis>G</emphasis>, which reuses
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the stack space and the saved link register so that
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<emphasis>G</emphasis> eventually returns to the caller of
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<emphasis>F</emphasis>.
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</para>
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<para>
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When the call from <emphasis>F</emphasis> to
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<emphasis>G</emphasis> is not local, and
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<emphasis>F</emphasis> is a TOC-preserving function, tail-call
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optimization is disallowed because <emphasis>F</emphasis> and
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<emphasis>G</emphasis> may have different TOC pointers.
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Tail-call optimization cannot guarantee that the correct TOC
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will be restored when <emphasis>G</emphasis> returns.
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</para>
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<para>
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When the call from <emphasis>F</emphasis> to
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<emphasis>G</emphasis> is local, and <emphasis>F</emphasis> is
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a TOC-preserving function, but <emphasis>G</emphasis> is
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<emphasis>not</emphasis> a TOC-preserving function, then
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tail-call optimization is again disallowed. In this case,
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<emphasis>G</emphasis> may have placed any value into register
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r2, and the correct TOC will not be restored when
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<emphasis>G</emphasis> returns.
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</para>
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</section>
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</section>
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<section xml:id="dbdoclet.50655240___RefHeading___Toc377640591">
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<title>Coding Examples</title>
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@ -8255,19 +8208,30 @@ bctrl</programlisting>
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</tbody>
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</tgroup>
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</table>
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<para>Function calls <phrase revisionflag="added">often</phrase>
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<para revisionflag="deleted">Function calls
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need to be performed in conjunction with
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establishing, maintaining, and restoring addressability through the TOC
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pointer register, r2. When a function is called, the TOC pointer register
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may be modified. <phrase revisionflag="added">In many cases,</phrase>
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<phrase revisionflag="changed">the</phrase> caller must provide a nop
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may be modified. The caller must provide a nop
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after the bl instruction performing a call, if r2 is not known to have
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the same value in the callee. This is generally true for external calls.
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The linker will replace the nop with an r2 restoring instruction if the
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caller and callee use different r2 values<phrase
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revisionflag="changed">.</phrase> The linker leaves it unchanged if they
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caller and callee use different r2 values, The linker leaves it
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unchanged if they
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use the same r2 value. This scheme avoids having a compiler generate an
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overconservative r2 save and restore around every external call.</para>
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overconservative r2 save and restore around every external
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call.</para>
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<para revisionflag="added">
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When a function requires addressability through the TOC
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pointer register, r2, and that function calls another function
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that may not preserve the value of r2, the caller must provide
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a nop after the bl instruction performing the call. The
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linker will replace the nop with an r2-restoring instruction
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if it determines that r2 may be changed as a result of the
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call; otherwise the linker will leave the nop unchanged. See
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<xref linkend="dbdoclet.50655241_FnLinkage" /> for a full
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description of when a nop must be inserted.
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</para>
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<para revisionflag="added">
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There are two cases where the caller need not provide a nop after
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the bl instruction performing a call:
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Bill Schmidt
5 years ago