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<title>Sections - GNU Compiler Collection (GCC) Internals</title>
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<link rel="up" href="Target-Macros.html#Target-Macros" title="Target Macros">
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<link rel="prev" href="Scheduling.html#Scheduling" title="Scheduling">
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any later version published by the Free Software Foundation; with the
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<a name="Sections"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="PIC.html#PIC">PIC</a>,
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Previous: <a rel="previous" accesskey="p" href="Scheduling.html#Scheduling">Scheduling</a>,
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Up: <a rel="up" accesskey="u" href="Target-Macros.html#Target-Macros">Target Macros</a>
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<hr>
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</div>
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<h3 class="section">17.18 Dividing the Output into Sections (Texts, Data, <small class="dots">...</small>)</h3>
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<!-- the above section title is WAY too long. maybe cut the part between -->
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<!-- the (...)? -mew 10feb93 -->
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<p>An object file is divided into sections containing different types of
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data. In the most common case, there are three sections: the <dfn>text
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section</dfn>, which holds instructions and read-only data; the <dfn>data
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section</dfn>, which holds initialized writable data; and the <dfn>bss
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section</dfn>, which holds uninitialized data. Some systems have other kinds
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of sections.
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<p><samp><span class="file">varasm.c</span></samp> provides several well-known sections, such as
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<code>text_section</code>, <code>data_section</code> and <code>bss_section</code>.
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The normal way of controlling a <var>foo</var><code>_section</code> variable
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is to define the associated <var>FOO</var><code>_SECTION_ASM_OP</code> macro,
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as described below. The macros are only read once, when <samp><span class="file">varasm.c</span></samp>
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initializes itself, so their values must be run-time constants.
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They may however depend on command-line flags.
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<p><em>Note:</em> Some run-time files, such <samp><span class="file">crtstuff.c</span></samp>, also make
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use of the <var>FOO</var><code>_SECTION_ASM_OP</code> macros, and expect them
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to be string literals.
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<p>Some assemblers require a different string to be written every time a
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section is selected. If your assembler falls into this category, you
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should define the <code>TARGET_ASM_INIT_SECTIONS</code> hook and use
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<code>get_unnamed_section</code> to set up the sections.
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<p>You must always create a <code>text_section</code>, either by defining
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<code>TEXT_SECTION_ASM_OP</code> or by initializing <code>text_section</code>
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in <code>TARGET_ASM_INIT_SECTIONS</code>. The same is true of
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<code>data_section</code> and <code>DATA_SECTION_ASM_OP</code>. If you do not
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create a distinct <code>readonly_data_section</code>, the default is to
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reuse <code>text_section</code>.
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<p>All the other <samp><span class="file">varasm.c</span></samp> sections are optional, and are null
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if the target does not provide them.
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<div class="defun">
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— Macro: <b>TEXT_SECTION_ASM_OP</b><var><a name="index-TEXT_005fSECTION_005fASM_005fOP-4514"></a></var><br>
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<blockquote><p>A C expression whose value is a string, including spacing, containing the
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assembler operation that should precede instructions and read-only data.
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Normally <code>"\t.text"</code> is right.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>HOT_TEXT_SECTION_NAME</b><var><a name="index-HOT_005fTEXT_005fSECTION_005fNAME-4515"></a></var><br>
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<blockquote><p>If defined, a C string constant for the name of the section containing most
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frequently executed functions of the program. If not defined, GCC will provide
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a default definition if the target supports named sections.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>UNLIKELY_EXECUTED_TEXT_SECTION_NAME</b><var><a name="index-UNLIKELY_005fEXECUTED_005fTEXT_005fSECTION_005fNAME-4516"></a></var><br>
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<blockquote><p>If defined, a C string constant for the name of the section containing unlikely
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executed functions in the program.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>DATA_SECTION_ASM_OP</b><var><a name="index-DATA_005fSECTION_005fASM_005fOP-4517"></a></var><br>
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<blockquote><p>A C expression whose value is a string, including spacing, containing the
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assembler operation to identify the following data as writable initialized
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data. Normally <code>"\t.data"</code> is right.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>SDATA_SECTION_ASM_OP</b><var><a name="index-SDATA_005fSECTION_005fASM_005fOP-4518"></a></var><br>
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<blockquote><p>If defined, a C expression whose value is a string, including spacing,
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containing the assembler operation to identify the following data as
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initialized, writable small data.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>READONLY_DATA_SECTION_ASM_OP</b><var><a name="index-READONLY_005fDATA_005fSECTION_005fASM_005fOP-4519"></a></var><br>
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<blockquote><p>A C expression whose value is a string, including spacing, containing the
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assembler operation to identify the following data as read-only initialized
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data.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>BSS_SECTION_ASM_OP</b><var><a name="index-BSS_005fSECTION_005fASM_005fOP-4520"></a></var><br>
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<blockquote><p>If defined, a C expression whose value is a string, including spacing,
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containing the assembler operation to identify the following data as
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uninitialized global data. If not defined, and
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<code>ASM_OUTPUT_ALIGNED_BSS</code> not defined,
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uninitialized global data will be output in the data section if
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<samp><span class="option">-fno-common</span></samp> is passed, otherwise <code>ASM_OUTPUT_COMMON</code> will be
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used.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>SBSS_SECTION_ASM_OP</b><var><a name="index-SBSS_005fSECTION_005fASM_005fOP-4521"></a></var><br>
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<blockquote><p>If defined, a C expression whose value is a string, including spacing,
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containing the assembler operation to identify the following data as
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uninitialized, writable small data.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>TLS_COMMON_ASM_OP</b><var><a name="index-TLS_005fCOMMON_005fASM_005fOP-4522"></a></var><br>
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<blockquote><p>If defined, a C expression whose value is a string containing the
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assembler operation to identify the following data as thread-local
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common data. The default is <code>".tls_common"</code>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>TLS_SECTION_ASM_FLAG</b><var><a name="index-TLS_005fSECTION_005fASM_005fFLAG-4523"></a></var><br>
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<blockquote><p>If defined, a C expression whose value is a character constant
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containing the flag used to mark a section as a TLS section. The
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default is <code>'T'</code>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>INIT_SECTION_ASM_OP</b><var><a name="index-INIT_005fSECTION_005fASM_005fOP-4524"></a></var><br>
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<blockquote><p>If defined, a C expression whose value is a string, including spacing,
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containing the assembler operation to identify the following data as
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initialization code. If not defined, GCC will assume such a section does
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not exist. This section has no corresponding <code>init_section</code>
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variable; it is used entirely in runtime code.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>FINI_SECTION_ASM_OP</b><var><a name="index-FINI_005fSECTION_005fASM_005fOP-4525"></a></var><br>
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<blockquote><p>If defined, a C expression whose value is a string, including spacing,
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containing the assembler operation to identify the following data as
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finalization code. If not defined, GCC will assume such a section does
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not exist. This section has no corresponding <code>fini_section</code>
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variable; it is used entirely in runtime code.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>INIT_ARRAY_SECTION_ASM_OP</b><var><a name="index-INIT_005fARRAY_005fSECTION_005fASM_005fOP-4526"></a></var><br>
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<blockquote><p>If defined, a C expression whose value is a string, including spacing,
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containing the assembler operation to identify the following data as
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part of the <code>.init_array</code> (or equivalent) section. If not
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defined, GCC will assume such a section does not exist. Do not define
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both this macro and <code>INIT_SECTION_ASM_OP</code>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>FINI_ARRAY_SECTION_ASM_OP</b><var><a name="index-FINI_005fARRAY_005fSECTION_005fASM_005fOP-4527"></a></var><br>
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<blockquote><p>If defined, a C expression whose value is a string, including spacing,
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containing the assembler operation to identify the following data as
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part of the <code>.fini_array</code> (or equivalent) section. If not
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defined, GCC will assume such a section does not exist. Do not define
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both this macro and <code>FINI_SECTION_ASM_OP</code>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>CRT_CALL_STATIC_FUNCTION</b> (<var>section_op, function</var>)<var><a name="index-CRT_005fCALL_005fSTATIC_005fFUNCTION-4528"></a></var><br>
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<blockquote><p>If defined, an ASM statement that switches to a different section
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via <var>section_op</var>, calls <var>function</var>, and switches back to
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the text section. This is used in <samp><span class="file">crtstuff.c</span></samp> if
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<code>INIT_SECTION_ASM_OP</code> or <code>FINI_SECTION_ASM_OP</code> to calls
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to initialization and finalization functions from the init and fini
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sections. By default, this macro uses a simple function call. Some
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ports need hand-crafted assembly code to avoid dependencies on
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registers initialized in the function prologue or to ensure that
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constant pools don't end up too far way in the text section.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>TARGET_LIBGCC_SDATA_SECTION</b><var><a name="index-TARGET_005fLIBGCC_005fSDATA_005fSECTION-4529"></a></var><br>
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<blockquote><p>If defined, a string which names the section into which small
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variables defined in crtstuff and libgcc should go. This is useful
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when the target has options for optimizing access to small data, and
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you want the crtstuff and libgcc routines to be conservative in what
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they expect of your application yet liberal in what your application
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expects. For example, for targets with a <code>.sdata</code> section (like
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MIPS), you could compile crtstuff with <code>-G 0</code> so that it doesn't
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require small data support from your application, but use this macro
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to put small data into <code>.sdata</code> so that your application can
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access these variables whether it uses small data or not.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>FORCE_CODE_SECTION_ALIGN</b><var><a name="index-FORCE_005fCODE_005fSECTION_005fALIGN-4530"></a></var><br>
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<blockquote><p>If defined, an ASM statement that aligns a code section to some
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arbitrary boundary. This is used to force all fragments of the
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<code>.init</code> and <code>.fini</code> sections to have to same alignment
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and thus prevent the linker from having to add any padding.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>JUMP_TABLES_IN_TEXT_SECTION</b><var><a name="index-JUMP_005fTABLES_005fIN_005fTEXT_005fSECTION-4531"></a></var><br>
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<blockquote><p>Define this macro to be an expression with a nonzero value if jump
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tables (for <code>tablejump</code> insns) should be output in the text
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section, along with the assembler instructions. Otherwise, the
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readonly data section is used.
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<p>This macro is irrelevant if there is no separate readonly data section.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_ASM_INIT_SECTIONS</b> (<var>void</var>)<var><a name="index-TARGET_005fASM_005fINIT_005fSECTIONS-4532"></a></var><br>
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<blockquote><p>Define this hook if you need to do something special to set up the
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<samp><span class="file">varasm.c</span></samp> sections, or if your target has some special sections
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of its own that you need to create.
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<p>GCC calls this hook after processing the command line, but before writing
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any assembly code, and before calling any of the section-returning hooks
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described below.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: int <b>TARGET_ASM_RELOC_RW_MASK</b> (<var>void</var>)<var><a name="index-TARGET_005fASM_005fRELOC_005fRW_005fMASK-4533"></a></var><br>
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<blockquote><p>Return a mask describing how relocations should be treated when
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selecting sections. Bit 1 should be set if global relocations
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should be placed in a read-write section; bit 0 should be set if
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local relocations should be placed in a read-write section.
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<p>The default version of this function returns 3 when <samp><span class="option">-fpic</span></samp>
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is in effect, and 0 otherwise. The hook is typically redefined
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when the target cannot support (some kinds of) dynamic relocations
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in read-only sections even in executables.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: section * <b>TARGET_ASM_SELECT_SECTION</b> (<var>tree exp, int reloc, unsigned HOST_WIDE_INT align</var>)<var><a name="index-TARGET_005fASM_005fSELECT_005fSECTION-4534"></a></var><br>
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<blockquote><p>Return the section into which <var>exp</var> should be placed. You can
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assume that <var>exp</var> is either a <code>VAR_DECL</code> node or a constant of
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some sort. <var>reloc</var> indicates whether the initial value of <var>exp</var>
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requires link-time relocations. Bit 0 is set when variable contains
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local relocations only, while bit 1 is set for global relocations.
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<var>align</var> is the constant alignment in bits.
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<p>The default version of this function takes care of putting read-only
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variables in <code>readonly_data_section</code>.
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<p>See also <var>USE_SELECT_SECTION_FOR_FUNCTIONS</var>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>USE_SELECT_SECTION_FOR_FUNCTIONS</b><var><a name="index-USE_005fSELECT_005fSECTION_005fFOR_005fFUNCTIONS-4535"></a></var><br>
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<blockquote><p>Define this macro if you wish TARGET_ASM_SELECT_SECTION to be called
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for <code>FUNCTION_DECL</code>s as well as for variables and constants.
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<p>In the case of a <code>FUNCTION_DECL</code>, <var>reloc</var> will be zero if the
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function has been determined to be likely to be called, and nonzero if
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it is unlikely to be called.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_ASM_UNIQUE_SECTION</b> (<var>tree decl, int reloc</var>)<var><a name="index-TARGET_005fASM_005fUNIQUE_005fSECTION-4536"></a></var><br>
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<blockquote><p>Build up a unique section name, expressed as a <code>STRING_CST</code> node,
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and assign it to ‘<samp><span class="samp">DECL_SECTION_NAME (</span><var>decl</var><span class="samp">)</span></samp>’.
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As with <code>TARGET_ASM_SELECT_SECTION</code>, <var>reloc</var> indicates whether
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the initial value of <var>exp</var> requires link-time relocations.
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<p>The default version of this function appends the symbol name to the
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ELF section name that would normally be used for the symbol. For
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example, the function <code>foo</code> would be placed in <code>.text.foo</code>.
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Whatever the actual target object format, this is often good enough.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: section * <b>TARGET_ASM_FUNCTION_RODATA_SECTION</b> (<var>tree decl</var>)<var><a name="index-TARGET_005fASM_005fFUNCTION_005fRODATA_005fSECTION-4537"></a></var><br>
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<blockquote><p>Return the readonly data section associated with
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‘<samp><span class="samp">DECL_SECTION_NAME (</span><var>decl</var><span class="samp">)</span></samp>’.
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The default version of this function selects <code>.gnu.linkonce.r.name</code> if
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the function's section is <code>.gnu.linkonce.t.name</code>, <code>.rodata.name</code>
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if function is in <code>.text.name</code>, and the normal readonly-data section
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otherwise.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: const char * <b>TARGET_ASM_MERGEABLE_RODATA_PREFIX</b><var><a name="index-TARGET_005fASM_005fMERGEABLE_005fRODATA_005fPREFIX-4538"></a></var><br>
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<blockquote><p>Usually, the compiler uses the prefix <code>".rodata"</code> to construct
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section names for mergeable constant data. Define this macro to override
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the string if a different section name should be used.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: section * <b>TARGET_ASM_TM_CLONE_TABLE_SECTION</b> (<var>void</var>)<var><a name="index-TARGET_005fASM_005fTM_005fCLONE_005fTABLE_005fSECTION-4539"></a></var><br>
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<blockquote><p>Return the section that should be used for transactional memory clone tables.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: section * <b>TARGET_ASM_SELECT_RTX_SECTION</b> (<var>machine_mode mode, rtx x, unsigned HOST_WIDE_INT align</var>)<var><a name="index-TARGET_005fASM_005fSELECT_005fRTX_005fSECTION-4540"></a></var><br>
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<blockquote><p>Return the section into which a constant <var>x</var>, of mode <var>mode</var>,
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should be placed. You can assume that <var>x</var> is some kind of
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constant in RTL. The argument <var>mode</var> is redundant except in the
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case of a <code>const_int</code> rtx. <var>align</var> is the constant alignment
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in bits.
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<p>The default version of this function takes care of putting symbolic
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constants in <code>flag_pic</code> mode in <code>data_section</code> and everything
|
|
else in <code>readonly_data_section</code>.
|
|
</p></blockquote></div>
|
|
|
|
<div class="defun">
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|
— Target Hook: tree <b>TARGET_MANGLE_DECL_ASSEMBLER_NAME</b> (<var>tree decl, tree id</var>)<var><a name="index-TARGET_005fMANGLE_005fDECL_005fASSEMBLER_005fNAME-4541"></a></var><br>
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|
<blockquote><p>Define this hook if you need to postprocess the assembler name generated
|
|
by target-independent code. The <var>id</var> provided to this hook will be
|
|
the computed name (e.g., the macro <code>DECL_NAME</code> of the <var>decl</var> in C,
|
|
or the mangled name of the <var>decl</var> in C++). The return value of the
|
|
hook is an <code>IDENTIFIER_NODE</code> for the appropriate mangled name on
|
|
your target system. The default implementation of this hook just
|
|
returns the <var>id</var> provided.
|
|
</p></blockquote></div>
|
|
|
|
<div class="defun">
|
|
— Target Hook: void <b>TARGET_ENCODE_SECTION_INFO</b> (<var>tree decl, rtx rtl, int new_decl_p</var>)<var><a name="index-TARGET_005fENCODE_005fSECTION_005fINFO-4542"></a></var><br>
|
|
<blockquote><p>Define this hook if references to a symbol or a constant must be
|
|
treated differently depending on something about the variable or
|
|
function named by the symbol (such as what section it is in).
|
|
|
|
<p>The hook is executed immediately after rtl has been created for
|
|
<var>decl</var>, which may be a variable or function declaration or
|
|
an entry in the constant pool. In either case, <var>rtl</var> is the
|
|
rtl in question. Do <em>not</em> use <code>DECL_RTL (</code><var>decl</var><code>)</code>
|
|
in this hook; that field may not have been initialized yet.
|
|
|
|
<p>In the case of a constant, it is safe to assume that the rtl is
|
|
a <code>mem</code> whose address is a <code>symbol_ref</code>. Most decls
|
|
will also have this form, but that is not guaranteed. Global
|
|
register variables, for instance, will have a <code>reg</code> for their
|
|
rtl. (Normally the right thing to do with such unusual rtl is
|
|
leave it alone.)
|
|
|
|
<p>The <var>new_decl_p</var> argument will be true if this is the first time
|
|
that <code>TARGET_ENCODE_SECTION_INFO</code> has been invoked on this decl. It will
|
|
be false for subsequent invocations, which will happen for duplicate
|
|
declarations. Whether or not anything must be done for the duplicate
|
|
declaration depends on whether the hook examines <code>DECL_ATTRIBUTES</code>.
|
|
<var>new_decl_p</var> is always true when the hook is called for a constant.
|
|
|
|
<p><a name="index-g_t_0040code_007bSYMBOL_005fREF_005fFLAG_007d_002c-in-_0040code_007bTARGET_005fENCODE_005fSECTION_005fINFO_007d-4543"></a>The usual thing for this hook to do is to record flags in the
|
|
<code>symbol_ref</code>, using <code>SYMBOL_REF_FLAG</code> or <code>SYMBOL_REF_FLAGS</code>.
|
|
Historically, the name string was modified if it was necessary to
|
|
encode more than one bit of information, but this practice is now
|
|
discouraged; use <code>SYMBOL_REF_FLAGS</code>.
|
|
|
|
<p>The default definition of this hook, <code>default_encode_section_info</code>
|
|
in <samp><span class="file">varasm.c</span></samp>, sets a number of commonly-useful bits in
|
|
<code>SYMBOL_REF_FLAGS</code>. Check whether the default does what you need
|
|
before overriding it.
|
|
</p></blockquote></div>
|
|
|
|
<div class="defun">
|
|
— Target Hook: const char * <b>TARGET_STRIP_NAME_ENCODING</b> (<var>const char *name</var>)<var><a name="index-TARGET_005fSTRIP_005fNAME_005fENCODING-4544"></a></var><br>
|
|
<blockquote><p>Decode <var>name</var> and return the real name part, sans
|
|
the characters that <code>TARGET_ENCODE_SECTION_INFO</code>
|
|
may have added.
|
|
</p></blockquote></div>
|
|
|
|
<div class="defun">
|
|
— Target Hook: bool <b>TARGET_IN_SMALL_DATA_P</b> (<var>const_tree exp</var>)<var><a name="index-TARGET_005fIN_005fSMALL_005fDATA_005fP-4545"></a></var><br>
|
|
<blockquote><p>Returns true if <var>exp</var> should be placed into a “small data” section.
|
|
The default version of this hook always returns false.
|
|
</p></blockquote></div>
|
|
|
|
<div class="defun">
|
|
— Target Hook: bool <b>TARGET_HAVE_SRODATA_SECTION</b><var><a name="index-TARGET_005fHAVE_005fSRODATA_005fSECTION-4546"></a></var><br>
|
|
<blockquote><p>Contains the value true if the target places read-only
|
|
“small data” into a separate section. The default value is false.
|
|
</p></blockquote></div>
|
|
|
|
<div class="defun">
|
|
— Target Hook: bool <b>TARGET_PROFILE_BEFORE_PROLOGUE</b> (<var>void</var>)<var><a name="index-TARGET_005fPROFILE_005fBEFORE_005fPROLOGUE-4547"></a></var><br>
|
|
<blockquote><p>It returns true if target wants profile code emitted before prologue.
|
|
|
|
<p>The default version of this hook use the target macro
|
|
<code>PROFILE_BEFORE_PROLOGUE</code>.
|
|
</p></blockquote></div>
|
|
|
|
<div class="defun">
|
|
— Target Hook: bool <b>TARGET_BINDS_LOCAL_P</b> (<var>const_tree exp</var>)<var><a name="index-TARGET_005fBINDS_005fLOCAL_005fP-4548"></a></var><br>
|
|
<blockquote><p>Returns true if <var>exp</var> names an object for which name resolution
|
|
rules must resolve to the current “module” (dynamic shared library
|
|
or executable image).
|
|
|
|
<p>The default version of this hook implements the name resolution rules
|
|
for ELF, which has a looser model of global name binding than other
|
|
currently supported object file formats.
|
|
</p></blockquote></div>
|
|
|
|
<div class="defun">
|
|
— Target Hook: bool <b>TARGET_HAVE_TLS</b><var><a name="index-TARGET_005fHAVE_005fTLS-4549"></a></var><br>
|
|
<blockquote><p>Contains the value true if the target supports thread-local storage.
|
|
The default value is false.
|
|
</p></blockquote></div>
|
|
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