314 lines
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314 lines
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<title>Using as: CFI directives</title>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="CFI-directives"></a>
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<div class="header">
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<p>
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Next: <a href="Comm.html#Comm" accesskey="n" rel="next">Comm</a>, Previous: <a href="Byte.html#Byte" accesskey="p" rel="prev">Byte</a>, Up: <a href="Pseudo-Ops.html#Pseudo-Ops" accesskey="u" rel="up">Pseudo Ops</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="CFI-directives-1"></a>
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<h3 class="section">7.10 CFI directives</h3>
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<a name="g_t_002ecfi_005fsections-section_005flist"></a>
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<h4 class="subsection">7.10.1 <code>.cfi_sections <var>section_list</var></code></h4>
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<a name="index-cfi_005fsections-directive"></a>
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<p><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>
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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>
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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>
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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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</p>
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<a name="g_t_002ecfi_005fstartproc-_005bsimple_005d"></a>
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<h4 class="subsection">7.10.2 <code>.cfi_startproc [simple]</code></h4>
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<a name="index-cfi_005fstartproc-directive"></a>
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<p><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>
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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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</p>
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<a name="g_t_002ecfi_005fendproc"></a>
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<h4 class="subsection">7.10.3 <code>.cfi_endproc</code></h4>
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<a name="index-cfi_005fendproc-directive"></a>
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<p><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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</p>
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<a name="g_t_002ecfi_005fpersonality-encoding-_005b_002c-exp_005d"></a>
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<h4 class="subsection">7.10.4 <code>.cfi_personality <var>encoding</var> [, <var>exp</var>]</code></h4>
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<a name="index-cfi_005fpersonality-directive"></a>
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<p><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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</p>
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<a name="g_t_002ecfi_005fpersonality_005fid-id"></a>
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<h4 class="subsection">7.10.5 <code>.cfi_personality_id <var>id</var></code></h4>
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<a name="index-cfi_005fpersonality_005fid-directive"></a>
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<p><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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</p>
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<a name="g_t_002ecfi_005ffde_005fdata-_005bopcode1-_005b_002c-_2026_005d_005d"></a>
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<h4 class="subsection">7.10.6 <code>.cfi_fde_data [<var>opcode1</var> [, …]]</code></h4>
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<a name="index-cfi_005ffde_005fdata-directive"></a>
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<p><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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</p>
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<a name="g_t_002ecfi_005flsda-encoding-_005b_002c-exp_005d"></a>
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<h4 class="subsection">7.10.7 <code>.cfi_lsda <var>encoding</var> [, <var>exp</var>]</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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</p>
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<a name="g_t_002ecfi_005finline_005flsda-_005balign_005d"></a>
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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>
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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>
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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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</p>
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<a name="g_t_002ecfi_005fdef_005fcfa-register_002c-offset"></a>
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<h4 class="subsection">7.10.9 <code>.cfi_def_cfa <var>register</var>, <var>offset</var></code></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 <var>register</var> and add <var>offset</var> to it</i>.
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</p>
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<a name="g_t_002ecfi_005fdef_005fcfa_005fregister-register"></a>
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<h4 class="subsection">7.10.10 <code>.cfi_def_cfa_register <var>register</var></code></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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</p>
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<a name="g_t_002ecfi_005fdef_005fcfa_005foffset-offset"></a>
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<h4 class="subsection">7.10.11 <code>.cfi_def_cfa_offset <var>offset</var></code></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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</p>
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<a name="g_t_002ecfi_005fadjust_005fcfa_005foffset-offset"></a>
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<h4 class="subsection">7.10.12 <code>.cfi_adjust_cfa_offset <var>offset</var></code></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/substracted from the previous offset.
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</p>
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<a name="g_t_002ecfi_005foffset-register_002c-offset"></a>
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<h4 class="subsection">7.10.13 <code>.cfi_offset <var>register</var>, <var>offset</var></code></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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</p>
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<a name="g_t_002ecfi_005frel_005foffset-register_002c-offset"></a>
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<h4 class="subsection">7.10.14 <code>.cfi_rel_offset <var>register</var>, <var>offset</var></code></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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</p>
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<a name="g_t_002ecfi_005fregister-register1_002c-register2"></a>
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<h4 class="subsection">7.10.15 <code>.cfi_register <var>register1</var>, <var>register2</var></code></h4>
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<p>Previous value of <var>register1</var> is saved in register <var>register2</var>.
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</p>
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<a name="g_t_002ecfi_005frestore-register"></a>
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<h4 class="subsection">7.10.16 <code>.cfi_restore <var>register</var></code></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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</p>
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<a name="g_t_002ecfi_005fundefined-register"></a>
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<h4 class="subsection">7.10.17 <code>.cfi_undefined <var>register</var></code></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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</p>
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<a name="g_t_002ecfi_005fsame_005fvalue-register"></a>
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<h4 class="subsection">7.10.18 <code>.cfi_same_value <var>register</var></code></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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</p>
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<a name="g_t_002ecfi_005fremember_005fstate-and-_002ecfi_005frestore_005fstate"></a>
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<h4 class="subsection">7.10.19 <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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</p>
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<div class="smallexample">
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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></div>
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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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</p>
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<div class="smallexample">
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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></div>
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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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</p>
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<a name="g_t_002ecfi_005freturn_005fcolumn-register"></a>
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<h4 class="subsection">7.10.20 <code>.cfi_return_column <var>register</var></code></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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</p>
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<a name="g_t_002ecfi_005fsignal_005fframe"></a>
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<h4 class="subsection">7.10.21 <code>.cfi_signal_frame</code></h4>
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<p>Mark current function as signal trampoline.
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</p>
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<a name="g_t_002ecfi_005fwindow_005fsave"></a>
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<h4 class="subsection">7.10.22 <code>.cfi_window_save</code></h4>
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<p>SPARC register window has been saved.
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</p>
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<a name="g_t_002ecfi_005fescape-expression_005b_002c-_2026_005d"></a>
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<h4 class="subsection">7.10.23 <code>.cfi_escape</code> <var>expression</var>[, …]</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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</p>
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<a name="g_t_002ecfi_005fval_005fencoded_005faddr-register_002c-encoding_002c-label"></a>
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<h4 class="subsection">7.10.24 <code>.cfi_val_encoded_addr <var>register</var>, <var>encoding</var>, <var>label</var></code></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>
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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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Comm.html#Comm" accesskey="n" rel="next">Comm</a>, Previous: <a href="Byte.html#Byte" accesskey="p" rel="prev">Byte</a>, Up: <a href="Pseudo-Ops.html#Pseudo-Ops" accesskey="u" rel="up">Pseudo Ops</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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