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<title>GNU Compiler Collection (GCC) Internals: Shrink-wrapping separate components</title>
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<a name="Shrink_002dwrapping-separate-components"></a>
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<p>
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Next: <a href="Stack-Smashing-Protection.html#Stack-Smashing-Protection" accesskey="n" rel="next">Stack Smashing Protection</a>, Previous: <a href="Tail-Calls.html#Tail-Calls" accesskey="p" rel="prev">Tail Calls</a>, Up: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="u" rel="up">Stack and Calling</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Shrink_002dwrapping-separate-components-1"></a>
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<h4 class="subsection">17.9.14 Shrink-wrapping separate components</h4>
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<a name="index-shrink_002dwrapping-separate-components"></a>
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<p>The prologue may perform a variety of target dependent tasks such as
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saving callee-saved registers, saving the return address, aligning the
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stack, creating a stack frame, initializing the PIC register, setting
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up the static chain, etc.
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</p>
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<p>On some targets some of these tasks may be independent of others and
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thus may be shrink-wrapped separately. These independent tasks are
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referred to as components and are handled generically by the target
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independent parts of GCC.
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</p>
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<p>Using the following hooks those prologue or epilogue components can be
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shrink-wrapped separately, so that the initialization (and possibly
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teardown) those components do is not done as frequently on execution
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paths where this would unnecessary.
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</p>
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<p>What exactly those components are is up to the target code; the generic
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code treats them abstractly, as a bit in an <code>sbitmap</code>. These
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<code>sbitmap</code>s are allocated by the <code>shrink_wrap.get_separate_components</code>
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and <code>shrink_wrap.components_for_bb</code> hooks, and deallocated by the
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generic code.
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</p>
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<dl>
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<dt><a name="index-TARGET_005fSHRINK_005fWRAP_005fGET_005fSEPARATE_005fCOMPONENTS"></a>Target Hook: <em>sbitmap</em> <strong>TARGET_SHRINK_WRAP_GET_SEPARATE_COMPONENTS</strong> <em>(void)</em></dt>
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<dd><p>This hook should return an <code>sbitmap</code> with the bits set for those
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components that can be separately shrink-wrapped in the current function.
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Return <code>NULL</code> if the current function should not get any separate
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shrink-wrapping.
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Don’t define this hook if it would always return <code>NULL</code>.
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If it is defined, the other hooks in this group have to be defined as well.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSHRINK_005fWRAP_005fCOMPONENTS_005fFOR_005fBB"></a>Target Hook: <em>sbitmap</em> <strong>TARGET_SHRINK_WRAP_COMPONENTS_FOR_BB</strong> <em>(basic_block)</em></dt>
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<dd><p>This hook should return an <code>sbitmap</code> with the bits set for those
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components where either the prologue component has to be executed before
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the <code>basic_block</code>, or the epilogue component after it, or both.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSHRINK_005fWRAP_005fDISQUALIFY_005fCOMPONENTS"></a>Target Hook: <em>void</em> <strong>TARGET_SHRINK_WRAP_DISQUALIFY_COMPONENTS</strong> <em>(sbitmap <var>components</var>, edge <var>e</var>, sbitmap <var>edge_components</var>, bool <var>is_prologue</var>)</em></dt>
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<dd><p>This hook should clear the bits in the <var>components</var> bitmap for those
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components in <var>edge_components</var> that the target cannot handle on edge
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<var>e</var>, where <var>is_prologue</var> says if this is for a prologue or an
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epilogue instead.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSHRINK_005fWRAP_005fEMIT_005fPROLOGUE_005fCOMPONENTS"></a>Target Hook: <em>void</em> <strong>TARGET_SHRINK_WRAP_EMIT_PROLOGUE_COMPONENTS</strong> <em>(sbitmap)</em></dt>
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<dd><p>Emit prologue insns for the components indicated by the parameter.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSHRINK_005fWRAP_005fEMIT_005fEPILOGUE_005fCOMPONENTS"></a>Target Hook: <em>void</em> <strong>TARGET_SHRINK_WRAP_EMIT_EPILOGUE_COMPONENTS</strong> <em>(sbitmap)</em></dt>
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<dd><p>Emit epilogue insns for the components indicated by the parameter.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSHRINK_005fWRAP_005fSET_005fHANDLED_005fCOMPONENTS"></a>Target Hook: <em>void</em> <strong>TARGET_SHRINK_WRAP_SET_HANDLED_COMPONENTS</strong> <em>(sbitmap)</em></dt>
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<dd><p>Mark the components in the parameter as handled, so that the
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<code>prologue</code> and <code>epilogue</code> named patterns know to ignore those
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components. The target code should not hang on to the <code>sbitmap</code>, it
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will be deleted after this call.
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</p></dd></dl>
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<p>
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Next: <a href="Stack-Smashing-Protection.html#Stack-Smashing-Protection" accesskey="n" rel="next">Stack Smashing Protection</a>, Previous: <a href="Tail-Calls.html#Tail-Calls" accesskey="p" rel="prev">Tail Calls</a>, Up: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="u" rel="up">Stack and Calling</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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