279 lines
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279 lines
13 KiB
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<html lang="en">
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<head>
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<title>CFI directives - Using as</title>
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<meta name="description" content="Using as">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Pseudo-Ops.html#Pseudo-Ops" title="Pseudo Ops">
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<link rel="prev" href="Byte.html#Byte" title="Byte">
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<link rel="next" href="Comm.html#Comm" title="Comm">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled ``GNU Free Documentation License''.
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="CFI-directives"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Comm.html#Comm">Comm</a>,
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Previous: <a rel="previous" accesskey="p" href="Byte.html#Byte">Byte</a>,
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Up: <a rel="up" accesskey="u" href="Pseudo-Ops.html#Pseudo-Ops">Pseudo Ops</a>
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<hr>
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</div>
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<h3 class="section">7.10 CFI directives</h3>
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<h4 class="subsection">7.10.1 <code>.cfi_sections </code><var>section_list</var></h4>
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<p><a name="index-g_t_0040code_007bcfi_005fsections_007d-directive-301"></a><code>.cfi_sections</code> may be used to specify whether CFI directives
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should emit <code>.eh_frame</code> section and/or <code>.debug_frame</code> section.
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If <var>section_list</var> is <code>.eh_frame</code>, <code>.eh_frame</code> is emitted,
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if <var>section_list</var> is <code>.debug_frame</code>, <code>.debug_frame</code> is emitted.
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To emit both use <code>.eh_frame, .debug_frame</code>. The default if this
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directive is not used is <code>.cfi_sections .eh_frame</code>.
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<p>On targets that support compact unwinding tables these can be generated
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by specifying <code>.eh_frame_entry</code> instead of <code>.eh_frame</code>.
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<p>Some targets may support an additional name, such as <code>.c6xabi.exidx</code>
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which is used by the target.
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<p>The <code>.cfi_sections</code> directive can be repeated, with the same or different
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arguments, provided that CFI generation has not yet started. Once CFI
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generation has started however the section list is fixed and any attempts to
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redefine it will result in an error.
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<h4 class="subsection">7.10.2 <code>.cfi_startproc [simple]</code></h4>
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<p><a name="index-g_t_0040code_007bcfi_005fstartproc_007d-directive-302"></a><code>.cfi_startproc</code> is used at the beginning of each function that
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should have an entry in <code>.eh_frame</code>. It initializes some internal
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data structures. Don't forget to close the function by
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<code>.cfi_endproc</code>.
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<p>Unless <code>.cfi_startproc</code> is used along with parameter <code>simple</code>
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it also emits some architecture dependent initial CFI instructions.
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<h4 class="subsection">7.10.3 <code>.cfi_endproc</code></h4>
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<p><a name="index-g_t_0040code_007bcfi_005fendproc_007d-directive-303"></a><code>.cfi_endproc</code> is used at the end of a function where it closes its
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unwind entry previously opened by
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<code>.cfi_startproc</code>, and emits it to <code>.eh_frame</code>.
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<h4 class="subsection">7.10.4 <code>.cfi_personality </code><var>encoding</var><code> [, </code><var>exp</var><code>]</code></h4>
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<p><a name="index-g_t_0040code_007bcfi_005fpersonality_007d-directive-304"></a><code>.cfi_personality</code> defines personality routine and its encoding.
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<var>encoding</var> must be a constant determining how the personality
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should be encoded. If it is 255 (<code>DW_EH_PE_omit</code>), second
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argument is not present, otherwise second argument should be
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a constant or a symbol name. When using indirect encodings,
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the symbol provided should be the location where personality
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can be loaded from, not the personality routine itself.
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The default after <code>.cfi_startproc</code> is <code>.cfi_personality 0xff</code>,
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no personality routine.
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<h4 class="subsection">7.10.5 <code>.cfi_personality_id </code><var>id</var></h4>
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<p><a name="index-g_t_0040code_007bcfi_005fpersonality_005fid_007d-directive-305"></a><code>cfi_personality_id</code> defines a personality routine by its index as
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defined in a compact unwinding format.
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Only valid when generating compact EH frames (i.e.
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with <code>.cfi_sections eh_frame_entry</code>.
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<h4 class="subsection">7.10.6 <code>.cfi_fde_data [</code><var>opcode1</var><code> [, ...]]</code></h4>
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<p><a name="index-g_t_0040code_007bcfi_005ffde_005fdata_007d-directive-306"></a><code>cfi_fde_data</code> is used to describe the compact unwind opcodes to be
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used for the current function. These are emitted inline in the
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<code>.eh_frame_entry</code> section if small enough and there is no LSDA, or
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in the <code>.gnu.extab</code> section otherwise.
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Only valid when generating compact EH frames (i.e.
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with <code>.cfi_sections eh_frame_entry</code>.
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<h4 class="subsection">7.10.7 <code>.cfi_lsda </code><var>encoding</var><code> [, </code><var>exp</var><code>]</code></h4>
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<p><code>.cfi_lsda</code> defines LSDA and its encoding.
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<var>encoding</var> must be a constant determining how the LSDA
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should be encoded. If it is 255 (<code>DW_EH_PE_omit</code>), the second
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argument is not present, otherwise the second argument should be a constant
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or a symbol name. The default after <code>.cfi_startproc</code> is <code>.cfi_lsda 0xff</code>,
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meaning that no LSDA is present.
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<h4 class="subsection">7.10.8 <code>.cfi_inline_lsda</code> [<var>align</var>]</h4>
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<p><code>.cfi_inline_lsda</code> marks the start of a LSDA data section and
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switches to the corresponding <code>.gnu.extab</code> section.
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Must be preceded by a CFI block containing a <code>.cfi_lsda</code> directive.
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Only valid when generating compact EH frames (i.e.
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with <code>.cfi_sections eh_frame_entry</code>.
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<p>The table header and unwinding opcodes will be generated at this point,
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so that they are immediately followed by the LSDA data. The symbol
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referenced by the <code>.cfi_lsda</code> directive should still be defined
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in case a fallback FDE based encoding is used. The LSDA data is terminated
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by a section directive.
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<p>The optional <var>align</var> argument specifies the alignment required.
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The alignment is specified as a power of two, as with the
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<code>.p2align</code> directive.
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<h4 class="subsection">7.10.9 <code>.cfi_def_cfa </code><var>register</var><code>, </code><var>offset</var></h4>
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<p><code>.cfi_def_cfa</code> defines a rule for computing CFA as: <i>take
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address from </i><var>register</var><i> and add </i><var>offset</var><i> to it</i>.
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<h4 class="subsection">7.10.10 <code>.cfi_def_cfa_register </code><var>register</var></h4>
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<p><code>.cfi_def_cfa_register</code> modifies a rule for computing CFA. From
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now on <var>register</var> will be used instead of the old one. Offset
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remains the same.
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<h4 class="subsection">7.10.11 <code>.cfi_def_cfa_offset </code><var>offset</var></h4>
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<p><code>.cfi_def_cfa_offset</code> modifies a rule for computing CFA. Register
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remains the same, but <var>offset</var> is new. Note that it is the
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absolute offset that will be added to a defined register to compute
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CFA address.
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<h4 class="subsection">7.10.12 <code>.cfi_adjust_cfa_offset </code><var>offset</var></h4>
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<p>Same as <code>.cfi_def_cfa_offset</code> but <var>offset</var> is a relative
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value that is added/subtracted from the previous offset.
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<h4 class="subsection">7.10.13 <code>.cfi_offset </code><var>register</var><code>, </code><var>offset</var></h4>
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<p>Previous value of <var>register</var> is saved at offset <var>offset</var> from
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CFA.
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<h4 class="subsection">7.10.14 <code>.cfi_val_offset </code><var>register</var><code>, </code><var>offset</var></h4>
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<p>Previous value of <var>register</var> is CFA + <var>offset</var>.
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<h4 class="subsection">7.10.15 <code>.cfi_rel_offset </code><var>register</var><code>, </code><var>offset</var></h4>
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<p>Previous value of <var>register</var> is saved at offset <var>offset</var> from
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the current CFA register. This is transformed to <code>.cfi_offset</code>
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using the known displacement of the CFA register from the CFA.
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This is often easier to use, because the number will match the
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code it's annotating.
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<h4 class="subsection">7.10.16 <code>.cfi_register </code><var>register1</var><code>, </code><var>register2</var></h4>
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<p>Previous value of <var>register1</var> is saved in register <var>register2</var>.
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<h4 class="subsection">7.10.17 <code>.cfi_restore </code><var>register</var></h4>
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<p><code>.cfi_restore</code> says that the rule for <var>register</var> is now the
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same as it was at the beginning of the function, after all initial
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instruction added by <code>.cfi_startproc</code> were executed.
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<h4 class="subsection">7.10.18 <code>.cfi_undefined </code><var>register</var></h4>
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<p>From now on the previous value of <var>register</var> can't be restored anymore.
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<h4 class="subsection">7.10.19 <code>.cfi_same_value </code><var>register</var></h4>
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<p>Current value of <var>register</var> is the same like in the previous frame,
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i.e. no restoration needed.
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<h4 class="subsection">7.10.20 <code>.cfi_remember_state</code> and <code>.cfi_restore_state</code></h4>
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<p><code>.cfi_remember_state</code> pushes the set of rules for every register onto an
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implicit stack, while <code>.cfi_restore_state</code> pops them off the stack and
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places them in the current row. This is useful for situations where you have
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multiple <code>.cfi_*</code> directives that need to be undone due to the control
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flow of the program. For example, we could have something like this (assuming
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the CFA is the value of <code>rbp</code>):
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<pre class="smallexample"> je label
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popq %rbx
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.cfi_restore %rbx
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popq %r12
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.cfi_restore %r12
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popq %rbp
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.cfi_restore %rbp
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.cfi_def_cfa %rsp, 8
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ret
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label:
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/* Do something else */
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</pre>
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<p>Here, we want the <code>.cfi</code> directives to affect only the rows corresponding
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to the instructions before <code>label</code>. This means we'd have to add multiple
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<code>.cfi</code> directives after <code>label</code> to recreate the original save
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locations of the registers, as well as setting the CFA back to the value of
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<code>rbp</code>. This would be clumsy, and result in a larger binary size. Instead,
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we can write:
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<pre class="smallexample"> je label
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popq %rbx
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.cfi_remember_state
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.cfi_restore %rbx
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popq %r12
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.cfi_restore %r12
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popq %rbp
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.cfi_restore %rbp
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.cfi_def_cfa %rsp, 8
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ret
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label:
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.cfi_restore_state
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/* Do something else */
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</pre>
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<p>That way, the rules for the instructions after <code>label</code> will be the same
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as before the first <code>.cfi_restore</code> without having to use multiple
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<code>.cfi</code> directives.
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<h4 class="subsection">7.10.21 <code>.cfi_return_column </code><var>register</var></h4>
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<p>Change return column <var>register</var>, i.e. the return address is either
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directly in <var>register</var> or can be accessed by rules for <var>register</var>.
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<h4 class="subsection">7.10.22 <code>.cfi_signal_frame</code></h4>
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<p>Mark current function as signal trampoline.
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<h4 class="subsection">7.10.23 <code>.cfi_window_save</code></h4>
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<p>SPARC register window has been saved.
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<h4 class="subsection">7.10.24 <code>.cfi_escape</code> <var>expression</var>[, <small class="dots">...</small>]</h4>
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<p>Allows the user to add arbitrary bytes to the unwind info. One
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might use this to add OS-specific CFI opcodes, or generic CFI
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opcodes that GAS does not yet support.
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<h4 class="subsection">7.10.25 <code>.cfi_val_encoded_addr </code><var>register</var><code>, </code><var>encoding</var><code>, </code><var>label</var></h4>
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<p>The current value of <var>register</var> is <var>label</var>. The value of <var>label</var>
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will be encoded in the output file according to <var>encoding</var>; see the
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description of <code>.cfi_personality</code> for details on this encoding.
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<p>The usefulness of equating a register to a fixed label is probably
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limited to the return address register. Here, it can be useful to
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mark a code segment that has only one return address which is reached
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by a direct branch and no copy of the return address exists in memory
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or another register.
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</body></html>
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