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<head>
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<title>Frame Layout (GNU Compiler Collection (GCC) Internals)</title>
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Stack-and-Calling.html#Stack-and-Calling" rel="up" title="Stack and Calling">
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<link href="Exception-Handling.html#Exception-Handling" rel="next" title="Exception Handling">
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<a name="Frame-Layout"></a>
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<div class="header">
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<p>
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Next: <a href="Exception-Handling.html#Exception-Handling" accesskey="n" rel="next">Exception Handling</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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</div>
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<hr>
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<a name="Basic-Stack-Layout"></a>
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<h4 class="subsection">18.9.1 Basic Stack Layout</h4>
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<a name="index-stack-frame-layout"></a>
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<a name="index-frame-layout"></a>
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<p>Here is the basic stack layout.
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</p>
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<dl>
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<dt><a name="index-STACK_005fGROWS_005fDOWNWARD"></a>Macro: <strong>STACK_GROWS_DOWNWARD</strong></dt>
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<dd><p>Define this macro to be true if pushing a word onto the stack moves the stack
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pointer to a smaller address, and false otherwise.
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</p></dd></dl>
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<dl>
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<dt><a name="index-STACK_005fPUSH_005fCODE"></a>Macro: <strong>STACK_PUSH_CODE</strong></dt>
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<dd><p>This macro defines the operation used when something is pushed
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on the stack. In RTL, a push operation will be
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<code>(set (mem (STACK_PUSH_CODE (reg sp))) …)</code>
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</p>
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<p>The choices are <code>PRE_DEC</code>, <code>POST_DEC</code>, <code>PRE_INC</code>,
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and <code>POST_INC</code>. Which of these is correct depends on
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the stack direction and on whether the stack pointer points
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to the last item on the stack or whether it points to the
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space for the next item on the stack.
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</p>
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<p>The default is <code>PRE_DEC</code> when <code>STACK_GROWS_DOWNWARD</code> is
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true, which is almost always right, and <code>PRE_INC</code> otherwise,
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which is often wrong.
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</p></dd></dl>
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<dl>
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<dt><a name="index-FRAME_005fGROWS_005fDOWNWARD"></a>Macro: <strong>FRAME_GROWS_DOWNWARD</strong></dt>
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<dd><p>Define this macro to nonzero value if the addresses of local variable slots
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are at negative offsets from the frame pointer.
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</p></dd></dl>
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<dl>
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<dt><a name="index-ARGS_005fGROW_005fDOWNWARD"></a>Macro: <strong>ARGS_GROW_DOWNWARD</strong></dt>
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<dd><p>Define this macro if successive arguments to a function occupy decreasing
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addresses on the stack.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSTARTING_005fFRAME_005fOFFSET"></a>Target Hook: <em>HOST_WIDE_INT</em> <strong>TARGET_STARTING_FRAME_OFFSET</strong> <em>(void)</em></dt>
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<dd><p>This hook returns the offset from the frame pointer to the first local
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variable slot to be allocated. If <code>FRAME_GROWS_DOWNWARD</code>, it is the
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offset to <em>end</em> of the first slot allocated, otherwise it is the
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offset to <em>beginning</em> of the first slot allocated. The default
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implementation returns 0.
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</p></dd></dl>
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<dl>
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<dt><a name="index-STACK_005fALIGNMENT_005fNEEDED"></a>Macro: <strong>STACK_ALIGNMENT_NEEDED</strong></dt>
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<dd><p>Define to zero to disable final alignment of the stack during reload.
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The nonzero default for this macro is suitable for most ports.
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</p>
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<p>On ports where <code>TARGET_STARTING_FRAME_OFFSET</code> is nonzero or where there
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is a register save block following the local block that doesn’t require
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alignment to <code>STACK_BOUNDARY</code>, it may be beneficial to disable
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stack alignment and do it in the backend.
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</p></dd></dl>
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<dl>
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<dt><a name="index-STACK_005fPOINTER_005fOFFSET"></a>Macro: <strong>STACK_POINTER_OFFSET</strong></dt>
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<dd><p>Offset from the stack pointer register to the first location at which
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outgoing arguments are placed. If not specified, the default value of
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zero is used. This is the proper value for most machines.
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</p>
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<p>If <code>ARGS_GROW_DOWNWARD</code>, this is the offset to the location above
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the first location at which outgoing arguments are placed.
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</p></dd></dl>
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<dl>
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<dt><a name="index-FIRST_005fPARM_005fOFFSET"></a>Macro: <strong>FIRST_PARM_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
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<dd><p>Offset from the argument pointer register to the first argument’s
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address. On some machines it may depend on the data type of the
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function.
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</p>
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<p>If <code>ARGS_GROW_DOWNWARD</code>, this is the offset to the location above
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the first argument’s address.
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</p></dd></dl>
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<dl>
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<dt><a name="index-STACK_005fDYNAMIC_005fOFFSET"></a>Macro: <strong>STACK_DYNAMIC_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
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<dd><p>Offset from the stack pointer register to an item dynamically allocated
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on the stack, e.g., by <code>alloca</code>.
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</p>
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<p>The default value for this macro is <code>STACK_POINTER_OFFSET</code> plus the
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length of the outgoing arguments. The default is correct for most
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machines. See <samp>function.c</samp> for details.
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</p></dd></dl>
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<dl>
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<dt><a name="index-INITIAL_005fFRAME_005fADDRESS_005fRTX"></a>Macro: <strong>INITIAL_FRAME_ADDRESS_RTX</strong></dt>
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<dd><p>A C expression whose value is RTL representing the address of the initial
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stack frame. This address is passed to <code>RETURN_ADDR_RTX</code> and
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<code>DYNAMIC_CHAIN_ADDRESS</code>. If you don’t define this macro, a reasonable
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default value will be used. Define this macro in order to make frame pointer
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elimination work in the presence of <code>__builtin_frame_address (count)</code> and
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<code>__builtin_return_address (count)</code> for <code>count</code> not equal to zero.
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</p></dd></dl>
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<dl>
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<dt><a name="index-DYNAMIC_005fCHAIN_005fADDRESS"></a>Macro: <strong>DYNAMIC_CHAIN_ADDRESS</strong> <em>(<var>frameaddr</var>)</em></dt>
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<dd><p>A C expression whose value is RTL representing the address in a stack
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frame where the pointer to the caller’s frame is stored. Assume that
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<var>frameaddr</var> is an RTL expression for the address of the stack frame
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itself.
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</p>
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<p>If you don’t define this macro, the default is to return the value
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of <var>frameaddr</var>—that is, the stack frame address is also the
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address of the stack word that points to the previous frame.
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</p></dd></dl>
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<dl>
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<dt><a name="index-SETUP_005fFRAME_005fADDRESSES"></a>Macro: <strong>SETUP_FRAME_ADDRESSES</strong></dt>
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<dd><p>A C expression that produces the machine-specific code to
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setup the stack so that arbitrary frames can be accessed. For example,
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on the SPARC, we must flush all of the register windows to the stack
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before we can access arbitrary stack frames. You will seldom need to
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define this macro. The default is to do nothing.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fBUILTIN_005fSETJMP_005fFRAME_005fVALUE"></a>Target Hook: <em>rtx</em> <strong>TARGET_BUILTIN_SETJMP_FRAME_VALUE</strong> <em>(void)</em></dt>
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<dd><p>This target hook should return an rtx that is used to store
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the address of the current frame into the built in <code>setjmp</code> buffer.
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The default value, <code>virtual_stack_vars_rtx</code>, is correct for most
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machines. One reason you may need to define this target hook is if
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<code>hard_frame_pointer_rtx</code> is the appropriate value on your machine.
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</p></dd></dl>
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<dl>
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<dt><a name="index-FRAME_005fADDR_005fRTX"></a>Macro: <strong>FRAME_ADDR_RTX</strong> <em>(<var>frameaddr</var>)</em></dt>
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<dd><p>A C expression whose value is RTL representing the value of the frame
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address for the current frame. <var>frameaddr</var> is the frame pointer
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of the current frame. This is used for __builtin_frame_address.
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You need only define this macro if the frame address is not the same
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as the frame pointer. Most machines do not need to define it.
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</p></dd></dl>
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<dl>
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<dt><a name="index-RETURN_005fADDR_005fRTX"></a>Macro: <strong>RETURN_ADDR_RTX</strong> <em>(<var>count</var>, <var>frameaddr</var>)</em></dt>
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<dd><p>A C expression whose value is RTL representing the value of the return
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address for the frame <var>count</var> steps up from the current frame, after
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the prologue. <var>frameaddr</var> is the frame pointer of the <var>count</var>
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frame, or the frame pointer of the <var>count</var> - 1 frame if
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<code>RETURN_ADDR_IN_PREVIOUS_FRAME</code> is nonzero.
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</p>
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<p>The value of the expression must always be the correct address when
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<var>count</var> is zero, but may be <code>NULL_RTX</code> if there is no way to
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determine the return address of other frames.
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</p></dd></dl>
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<dl>
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<dt><a name="index-RETURN_005fADDR_005fIN_005fPREVIOUS_005fFRAME"></a>Macro: <strong>RETURN_ADDR_IN_PREVIOUS_FRAME</strong></dt>
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<dd><p>Define this macro to nonzero value if the return address of a particular
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stack frame is accessed from the frame pointer of the previous stack
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frame. The zero default for this macro is suitable for most ports.
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</p></dd></dl>
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<dl>
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<dt><a name="index-INCOMING_005fRETURN_005fADDR_005fRTX"></a>Macro: <strong>INCOMING_RETURN_ADDR_RTX</strong></dt>
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<dd><p>A C expression whose value is RTL representing the location of the
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incoming return address at the beginning of any function, before the
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prologue. This RTL is either a <code>REG</code>, indicating that the return
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value is saved in ‘<samp>REG</samp>’, or a <code>MEM</code> representing a location in
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the stack.
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</p>
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<p>You only need to define this macro if you want to support call frame
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debugging information like that provided by DWARF 2.
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</p>
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<p>If this RTL is a <code>REG</code>, you should also define
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<code>DWARF_FRAME_RETURN_COLUMN</code> to <code>DWARF_FRAME_REGNUM (REGNO)</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-DWARF_005fALT_005fFRAME_005fRETURN_005fCOLUMN"></a>Macro: <strong>DWARF_ALT_FRAME_RETURN_COLUMN</strong></dt>
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<dd><p>A C expression whose value is an integer giving a DWARF 2 column
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number that may be used as an alternative return column. The column
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must not correspond to any gcc hard register (that is, it must not
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be in the range of <code>DWARF_FRAME_REGNUM</code>).
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</p>
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<p>This macro can be useful if <code>DWARF_FRAME_RETURN_COLUMN</code> is set to a
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general register, but an alternative column needs to be used for signal
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frames. Some targets have also used different frame return columns
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over time.
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</p></dd></dl>
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<dl>
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<dt><a name="index-DWARF_005fZERO_005fREG"></a>Macro: <strong>DWARF_ZERO_REG</strong></dt>
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<dd><p>A C expression whose value is an integer giving a DWARF 2 register
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number that is considered to always have the value zero. This should
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only be defined if the target has an architected zero register, and
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someone decided it was a good idea to use that register number to
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terminate the stack backtrace. New ports should avoid this.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fDWARF_005fHANDLE_005fFRAME_005fUNSPEC"></a>Target Hook: <em>void</em> <strong>TARGET_DWARF_HANDLE_FRAME_UNSPEC</strong> <em>(const char *<var>label</var>, rtx <var>pattern</var>, int <var>index</var>)</em></dt>
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<dd><p>This target hook allows the backend to emit frame-related insns that
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contain UNSPECs or UNSPEC_VOLATILEs. The DWARF 2 call frame debugging
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info engine will invoke it on insns of the form
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</p><div class="smallexample">
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<pre class="smallexample">(set (reg) (unspec […] UNSPEC_INDEX))
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</pre></div>
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<p>and
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</p><div class="smallexample">
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<pre class="smallexample">(set (reg) (unspec_volatile […] UNSPECV_INDEX)).
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</pre></div>
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<p>to let the backend emit the call frame instructions. <var>label</var> is
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the CFI label attached to the insn, <var>pattern</var> is the pattern of
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the insn and <var>index</var> is <code>UNSPEC_INDEX</code> or <code>UNSPECV_INDEX</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fDWARF_005fPOLY_005fINDETERMINATE_005fVALUE"></a>Target Hook: <em>unsigned int</em> <strong>TARGET_DWARF_POLY_INDETERMINATE_VALUE</strong> <em>(unsigned int <var>i</var>, unsigned int *<var>factor</var>, int *<var>offset</var>)</em></dt>
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<dd><p>Express the value of <code>poly_int</code> indeterminate <var>i</var> as a DWARF
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expression, with <var>i</var> counting from 1. Return the number of a DWARF
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register <var>R</var> and set ‘<samp>*<var>factor</var></samp>’ and ‘<samp>*<var>offset</var></samp>’ such
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that the value of the indeterminate is:
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</p><div class="smallexample">
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<pre class="smallexample">value_of(<var>R</var>) / <var>factor</var> - <var>offset</var>
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</pre></div>
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<p>A target only needs to define this hook if it sets
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‘<samp>NUM_POLY_INT_COEFFS</samp>’ to a value greater than 1.
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</p></dd></dl>
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<dl>
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<dt><a name="index-INCOMING_005fFRAME_005fSP_005fOFFSET"></a>Macro: <strong>INCOMING_FRAME_SP_OFFSET</strong></dt>
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<dd><p>A C expression whose value is an integer giving the offset, in bytes,
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from the value of the stack pointer register to the top of the stack
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frame at the beginning of any function, before the prologue. The top of
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the frame is defined to be the value of the stack pointer in the
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previous frame, just before the call instruction.
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</p>
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<p>You only need to define this macro if you want to support call frame
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debugging information like that provided by DWARF 2.
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</p></dd></dl>
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<dl>
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<dt><a name="index-DEFAULT_005fINCOMING_005fFRAME_005fSP_005fOFFSET"></a>Macro: <strong>DEFAULT_INCOMING_FRAME_SP_OFFSET</strong></dt>
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<dd><p>Like <code>INCOMING_FRAME_SP_OFFSET</code>, but must be the same for all
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functions of the same ABI, and when using GAS <code>.cfi_*</code> directives
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must also agree with the default CFI GAS emits. Define this macro
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only if <code>INCOMING_FRAME_SP_OFFSET</code> can have different values
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between different functions of the same ABI or when
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<code>INCOMING_FRAME_SP_OFFSET</code> does not agree with GAS default CFI.
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</p></dd></dl>
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<dl>
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<dt><a name="index-ARG_005fPOINTER_005fCFA_005fOFFSET"></a>Macro: <strong>ARG_POINTER_CFA_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
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<dd><p>A C expression whose value is an integer giving the offset, in bytes,
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from the argument pointer to the canonical frame address (cfa). The
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final value should coincide with that calculated by
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<code>INCOMING_FRAME_SP_OFFSET</code>. Which is unfortunately not usable
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during virtual register instantiation.
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</p>
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<p>The default value for this macro is
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<code>FIRST_PARM_OFFSET (fundecl) + crtl->args.pretend_args_size</code>,
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which is correct for most machines; in general, the arguments are found
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immediately before the stack frame. Note that this is not the case on
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some targets that save registers into the caller’s frame, such as SPARC
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and rs6000, and so such targets need to define this macro.
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</p>
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<p>You only need to define this macro if the default is incorrect, and you
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want to support call frame debugging information like that provided by
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DWARF 2.
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</p></dd></dl>
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<dl>
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<dt><a name="index-FRAME_005fPOINTER_005fCFA_005fOFFSET"></a>Macro: <strong>FRAME_POINTER_CFA_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
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<dd><p>If defined, a C expression whose value is an integer giving the offset
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in bytes from the frame pointer to the canonical frame address (cfa).
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The final value should coincide with that calculated by
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<code>INCOMING_FRAME_SP_OFFSET</code>.
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</p>
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<p>Normally the CFA is calculated as an offset from the argument pointer,
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via <code>ARG_POINTER_CFA_OFFSET</code>, but if the argument pointer is
|
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variable due to the ABI, this may not be possible. If this macro is
|
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defined, it implies that the virtual register instantiation should be
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based on the frame pointer instead of the argument pointer. Only one
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of <code>FRAME_POINTER_CFA_OFFSET</code> and <code>ARG_POINTER_CFA_OFFSET</code>
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should be defined.
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</p></dd></dl>
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<dl>
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<dt><a name="index-CFA_005fFRAME_005fBASE_005fOFFSET"></a>Macro: <strong>CFA_FRAME_BASE_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
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<dd><p>If defined, a C expression whose value is an integer giving the offset
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in bytes from the canonical frame address (cfa) to the frame base used
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in DWARF 2 debug information. The default is zero. A different value
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|
may reduce the size of debug information on some ports.
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</p></dd></dl>
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