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<title>GNU Compiler Collection (GCC) Internals: Frame Layout</title>
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<p>
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> &nbsp; [<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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<hr>
<a name="Basic-Stack-Layout"></a>
<h4 class="subsection">17.9.1 Basic Stack Layout</h4>
<a name="index-stack-frame-layout"></a>
<a name="index-frame-layout"></a>
<p>Here is the basic stack layout.
</p>
<dl>
<dt><a name="index-STACK_005fGROWS_005fDOWNWARD"></a>Macro: <strong>STACK_GROWS_DOWNWARD</strong></dt>
<dd><p>Define this macro to be true if pushing a word onto the stack moves the stack
pointer to a smaller address, and false otherwise.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fPUSH_005fCODE"></a>Macro: <strong>STACK_PUSH_CODE</strong></dt>
<dd><p>This macro defines the operation used when something is pushed
on the stack. In RTL, a push operation will be
<code>(set (mem (STACK_PUSH_CODE (reg sp))) &hellip;)</code>
</p>
<p>The choices are <code>PRE_DEC</code>, <code>POST_DEC</code>, <code>PRE_INC</code>,
and <code>POST_INC</code>. Which of these is correct depends on
the stack direction and on whether the stack pointer points
to the last item on the stack or whether it points to the
space for the next item on the stack.
</p>
<p>The default is <code>PRE_DEC</code> when <code>STACK_GROWS_DOWNWARD</code> is
true, which is almost always right, and <code>PRE_INC</code> otherwise,
which is often wrong.
</p></dd></dl>
<dl>
<dt><a name="index-FRAME_005fGROWS_005fDOWNWARD"></a>Macro: <strong>FRAME_GROWS_DOWNWARD</strong></dt>
<dd><p>Define this macro to nonzero value if the addresses of local variable slots
are at negative offsets from the frame pointer.
</p></dd></dl>
<dl>
<dt><a name="index-ARGS_005fGROW_005fDOWNWARD"></a>Macro: <strong>ARGS_GROW_DOWNWARD</strong></dt>
<dd><p>Define this macro if successive arguments to a function occupy decreasing
addresses on the stack.
</p></dd></dl>
<dl>
<dt><a name="index-STARTING_005fFRAME_005fOFFSET"></a>Macro: <strong>STARTING_FRAME_OFFSET</strong></dt>
<dd><p>Offset from the frame pointer to the first local variable slot to be allocated.
</p>
<p>If <code>FRAME_GROWS_DOWNWARD</code>, find the next slot&rsquo;s offset by
subtracting the first slot&rsquo;s length from <code>STARTING_FRAME_OFFSET</code>.
Otherwise, it is found by adding the length of the first slot to the
value <code>STARTING_FRAME_OFFSET</code>.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fALIGNMENT_005fNEEDED"></a>Macro: <strong>STACK_ALIGNMENT_NEEDED</strong></dt>
<dd><p>Define to zero to disable final alignment of the stack during reload.
The nonzero default for this macro is suitable for most ports.
</p>
<p>On ports where <code>STARTING_FRAME_OFFSET</code> is nonzero or where there
is a register save block following the local block that doesn&rsquo;t require
alignment to <code>STACK_BOUNDARY</code>, it may be beneficial to disable
stack alignment and do it in the backend.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fPOINTER_005fOFFSET"></a>Macro: <strong>STACK_POINTER_OFFSET</strong></dt>
<dd><p>Offset from the stack pointer register to the first location at which
outgoing arguments are placed. If not specified, the default value of
zero is used. This is the proper value for most machines.
</p>
<p>If <code>ARGS_GROW_DOWNWARD</code>, this is the offset to the location above
the first location at which outgoing arguments are placed.
</p></dd></dl>
<dl>
<dt><a name="index-FIRST_005fPARM_005fOFFSET"></a>Macro: <strong>FIRST_PARM_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
<dd><p>Offset from the argument pointer register to the first argument&rsquo;s
address. On some machines it may depend on the data type of the
function.
</p>
<p>If <code>ARGS_GROW_DOWNWARD</code>, this is the offset to the location above
the first argument&rsquo;s address.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fDYNAMIC_005fOFFSET"></a>Macro: <strong>STACK_DYNAMIC_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
<dd><p>Offset from the stack pointer register to an item dynamically allocated
on the stack, e.g., by <code>alloca</code>.
</p>
<p>The default value for this macro is <code>STACK_POINTER_OFFSET</code> plus the
length of the outgoing arguments. The default is correct for most
machines. See <samp>function.c</samp> for details.
</p></dd></dl>
<dl>
<dt><a name="index-INITIAL_005fFRAME_005fADDRESS_005fRTX"></a>Macro: <strong>INITIAL_FRAME_ADDRESS_RTX</strong></dt>
<dd><p>A C expression whose value is RTL representing the address of the initial
stack frame. This address is passed to <code>RETURN_ADDR_RTX</code> and
<code>DYNAMIC_CHAIN_ADDRESS</code>. If you don&rsquo;t define this macro, a reasonable
default value will be used. Define this macro in order to make frame pointer
elimination work in the presence of <code>__builtin_frame_address (count)</code> and
<code>__builtin_return_address (count)</code> for <code>count</code> not equal to zero.
</p></dd></dl>
<dl>
<dt><a name="index-DYNAMIC_005fCHAIN_005fADDRESS"></a>Macro: <strong>DYNAMIC_CHAIN_ADDRESS</strong> <em>(<var>frameaddr</var>)</em></dt>
<dd><p>A C expression whose value is RTL representing the address in a stack
frame where the pointer to the caller&rsquo;s frame is stored. Assume that
<var>frameaddr</var> is an RTL expression for the address of the stack frame
itself.
</p>
<p>If you don&rsquo;t define this macro, the default is to return the value
of <var>frameaddr</var>&mdash;that is, the stack frame address is also the
address of the stack word that points to the previous frame.
</p></dd></dl>
<dl>
<dt><a name="index-SETUP_005fFRAME_005fADDRESSES"></a>Macro: <strong>SETUP_FRAME_ADDRESSES</strong></dt>
<dd><p>A C expression that produces the machine-specific code to
setup the stack so that arbitrary frames can be accessed. For example,
on the SPARC, we must flush all of the register windows to the stack
before we can access arbitrary stack frames. You will seldom need to
define this macro. The default is to do nothing.
</p></dd></dl>
<dl>
<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>
<dd><p>This target hook should return an rtx that is used to store
the address of the current frame into the built in <code>setjmp</code> buffer.
The default value, <code>virtual_stack_vars_rtx</code>, is correct for most
machines. One reason you may need to define this target hook is if
<code>hard_frame_pointer_rtx</code> is the appropriate value on your machine.
</p></dd></dl>
<dl>
<dt><a name="index-FRAME_005fADDR_005fRTX"></a>Macro: <strong>FRAME_ADDR_RTX</strong> <em>(<var>frameaddr</var>)</em></dt>
<dd><p>A C expression whose value is RTL representing the value of the frame
address for the current frame. <var>frameaddr</var> is the frame pointer
of the current frame. This is used for __builtin_frame_address.
You need only define this macro if the frame address is not the same
as the frame pointer. Most machines do not need to define it.
</p></dd></dl>
<dl>
<dt><a name="index-RETURN_005fADDR_005fRTX"></a>Macro: <strong>RETURN_ADDR_RTX</strong> <em>(<var>count</var>, <var>frameaddr</var>)</em></dt>
<dd><p>A C expression whose value is RTL representing the value of the return
address for the frame <var>count</var> steps up from the current frame, after
the prologue. <var>frameaddr</var> is the frame pointer of the <var>count</var>
frame, or the frame pointer of the <var>count</var> - 1 frame if
<code>RETURN_ADDR_IN_PREVIOUS_FRAME</code> is nonzero.
</p>
<p>The value of the expression must always be the correct address when
<var>count</var> is zero, but may be <code>NULL_RTX</code> if there is no way to
determine the return address of other frames.
</p></dd></dl>
<dl>
<dt><a name="index-RETURN_005fADDR_005fIN_005fPREVIOUS_005fFRAME"></a>Macro: <strong>RETURN_ADDR_IN_PREVIOUS_FRAME</strong></dt>
<dd><p>Define this macro to nonzero value if the return address of a particular
stack frame is accessed from the frame pointer of the previous stack
frame. The zero default for this macro is suitable for most ports.
</p></dd></dl>
<dl>
<dt><a name="index-INCOMING_005fRETURN_005fADDR_005fRTX"></a>Macro: <strong>INCOMING_RETURN_ADDR_RTX</strong></dt>
<dd><p>A C expression whose value is RTL representing the location of the
incoming return address at the beginning of any function, before the
prologue. This RTL is either a <code>REG</code>, indicating that the return
value is saved in &lsquo;<samp>REG</samp>&rsquo;, or a <code>MEM</code> representing a location in
the stack.
</p>
<p>You only need to define this macro if you want to support call frame
debugging information like that provided by DWARF 2.
</p>
<p>If this RTL is a <code>REG</code>, you should also define
<code>DWARF_FRAME_RETURN_COLUMN</code> to <code>DWARF_FRAME_REGNUM (REGNO)</code>.
</p></dd></dl>
<dl>
<dt><a name="index-DWARF_005fALT_005fFRAME_005fRETURN_005fCOLUMN"></a>Macro: <strong>DWARF_ALT_FRAME_RETURN_COLUMN</strong></dt>
<dd><p>A C expression whose value is an integer giving a DWARF 2 column
number that may be used as an alternative return column. The column
must not correspond to any gcc hard register (that is, it must not
be in the range of <code>DWARF_FRAME_REGNUM</code>).
</p>
<p>This macro can be useful if <code>DWARF_FRAME_RETURN_COLUMN</code> is set to a
general register, but an alternative column needs to be used for signal
frames. Some targets have also used different frame return columns
over time.
</p></dd></dl>
<dl>
<dt><a name="index-DWARF_005fZERO_005fREG"></a>Macro: <strong>DWARF_ZERO_REG</strong></dt>
<dd><p>A C expression whose value is an integer giving a DWARF 2 register
number that is considered to always have the value zero. This should
only be defined if the target has an architected zero register, and
someone decided it was a good idea to use that register number to
terminate the stack backtrace. New ports should avoid this.
</p></dd></dl>
<dl>
<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>
<dd><p>This target hook allows the backend to emit frame-related insns that
contain UNSPECs or UNSPEC_VOLATILEs. The DWARF 2 call frame debugging
info engine will invoke it on insns of the form
</p><div class="smallexample">
<pre class="smallexample">(set (reg) (unspec [&hellip;] UNSPEC_INDEX))
</pre></div>
<p>and
</p><div class="smallexample">
<pre class="smallexample">(set (reg) (unspec_volatile [&hellip;] UNSPECV_INDEX)).
</pre></div>
<p>to let the backend emit the call frame instructions. <var>label</var> is
the CFI label attached to the insn, <var>pattern</var> is the pattern of
the insn and <var>index</var> is <code>UNSPEC_INDEX</code> or <code>UNSPECV_INDEX</code>.
</p></dd></dl>
<dl>
<dt><a name="index-INCOMING_005fFRAME_005fSP_005fOFFSET"></a>Macro: <strong>INCOMING_FRAME_SP_OFFSET</strong></dt>
<dd><p>A C expression whose value is an integer giving the offset, in bytes,
from the value of the stack pointer register to the top of the stack
frame at the beginning of any function, before the prologue. The top of
the frame is defined to be the value of the stack pointer in the
previous frame, just before the call instruction.
</p>
<p>You only need to define this macro if you want to support call frame
debugging information like that provided by DWARF 2.
</p></dd></dl>
<dl>
<dt><a name="index-ARG_005fPOINTER_005fCFA_005fOFFSET"></a>Macro: <strong>ARG_POINTER_CFA_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
<dd><p>A C expression whose value is an integer giving the offset, in bytes,
from the argument pointer to the canonical frame address (cfa). The
final value should coincide with that calculated by
<code>INCOMING_FRAME_SP_OFFSET</code>. Which is unfortunately not usable
during virtual register instantiation.
</p>
<p>The default value for this macro is
<code>FIRST_PARM_OFFSET (fundecl) + crtl-&gt;args.pretend_args_size</code>,
which is correct for most machines; in general, the arguments are found
immediately before the stack frame. Note that this is not the case on
some targets that save registers into the caller&rsquo;s frame, such as SPARC
and rs6000, and so such targets need to define this macro.
</p>
<p>You only need to define this macro if the default is incorrect, and you
want to support call frame debugging information like that provided by
DWARF 2.
</p></dd></dl>
<dl>
<dt><a name="index-FRAME_005fPOINTER_005fCFA_005fOFFSET"></a>Macro: <strong>FRAME_POINTER_CFA_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
<dd><p>If defined, a C expression whose value is an integer giving the offset
in bytes from the frame pointer to the canonical frame address (cfa).
The final value should coincide with that calculated by
<code>INCOMING_FRAME_SP_OFFSET</code>.
</p>
<p>Normally the CFA is calculated as an offset from the argument pointer,
via <code>ARG_POINTER_CFA_OFFSET</code>, but if the argument pointer is
variable due to the ABI, this may not be possible. If this macro is
defined, it implies that the virtual register instantiation should be
based on the frame pointer instead of the argument pointer. Only one
of <code>FRAME_POINTER_CFA_OFFSET</code> and <code>ARG_POINTER_CFA_OFFSET</code>
should be defined.
</p></dd></dl>
<dl>
<dt><a name="index-CFA_005fFRAME_005fBASE_005fOFFSET"></a>Macro: <strong>CFA_FRAME_BASE_OFFSET</strong> <em>(<var>fundecl</var>)</em></dt>
<dd><p>If defined, a C expression whose value is an integer giving the offset
in bytes from the canonical frame address (cfa) to the frame base used
in DWARF 2 debug information. The default is zero. A different value
may reduce the size of debug information on some ports.
</p></dd></dl>
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