Sections - GNU Compiler Collection (GCC) Internals

An object file is divided into sections containing different types of data. In the most common case, there are three sections: the text section, which holds instructions and read-only data; the data section, which holds initialized writable data; and the bss section, which holds uninitialized data. Some systems have other kinds of sections.

varasm.c provides several well-known sections, such as text_section, data_section and bss_section. The normal way of controlling a foo_section variable is to define the associated FOO_SECTION_ASM_OP macro, as described below. The macros are only read once, when varasm.c initializes itself, so their values must be run-time constants. They may however depend on command-line flags.

Note: Some run-time files, such crtstuff.c, also make use of the FOO_SECTION_ASM_OP macros, and expect them to be string literals.

Some assemblers require a different string to be written every time a section is selected. If your assembler falls into this category, you should define the TARGET_ASM_INIT_SECTIONS hook and use get_unnamed_section to set up the sections.

You must always create a text_section, either by defining TEXT_SECTION_ASM_OP or by initializing text_section in TARGET_ASM_INIT_SECTIONS. The same is true of data_section and DATA_SECTION_ASM_OP. If you do not create a distinct readonly_data_section, the default is to reuse text_section.

All the other varasm.c sections are optional, and are null if the target does not provide them.

— Macro: TEXT_SECTION_ASM_OP

A C expression whose value is a string, including spacing, containing the assembler operation that should precede instructions and read-only data. Normally "\t.text" is right.

— Macro: HOT_TEXT_SECTION_NAME

If defined, a C string constant for the name of the section containing most frequently executed functions of the program. If not defined, GCC will provide a default definition if the target supports named sections.

— Macro: UNLIKELY_EXECUTED_TEXT_SECTION_NAME

If defined, a C string constant for the name of the section containing unlikely executed functions in the program.

— Macro: DATA_SECTION_ASM_OP

A C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as writable initialized data. Normally "\t.data" is right.

— Macro: SDATA_SECTION_ASM_OP

If defined, a C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as initialized, writable small data.

— Macro: READONLY_DATA_SECTION_ASM_OP

A C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as read-only initialized data.

— Macro: BSS_SECTION_ASM_OP

If defined, a C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as uninitialized global data. If not defined, and neither ASM_OUTPUT_BSS nor ASM_OUTPUT_ALIGNED_BSS are defined, uninitialized global data will be output in the data section if -fno-common is passed, otherwise ASM_OUTPUT_COMMON will be used.

— Macro: SBSS_SECTION_ASM_OP

If defined, a C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as uninitialized, writable small data.

— Macro: TLS_COMMON_ASM_OP

If defined, a C expression whose value is a string containing the assembler operation to identify the following data as thread-local common data. The default is ".tls_common".

— Macro: TLS_SECTION_ASM_FLAG

If defined, a C expression whose value is a character constant containing the flag used to mark a section as a TLS section. The default is 'T'.

— Macro: INIT_SECTION_ASM_OP

If defined, a C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as initialization code. If not defined, GCC will assume such a section does not exist. This section has no corresponding init_section variable; it is used entirely in runtime code.

— Macro: FINI_SECTION_ASM_OP

If defined, a C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as finalization code. If not defined, GCC will assume such a section does not exist. This section has no corresponding fini_section variable; it is used entirely in runtime code.

— Macro: INIT_ARRAY_SECTION_ASM_OP

If defined, a C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as part of the .init_array (or equivalent) section. If not defined, GCC will assume such a section does not exist. Do not define both this macro and INIT_SECTION_ASM_OP.

— Macro: FINI_ARRAY_SECTION_ASM_OP

If defined, a C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as part of the .fini_array (or equivalent) section. If not defined, GCC will assume such a section does not exist. Do not define both this macro and FINI_SECTION_ASM_OP.

— Macro: CRT_CALL_STATIC_FUNCTION (section_op, function)

If defined, an ASM statement that switches to a different section via section_op, calls function, and switches back to the text section. This is used in crtstuff.c if INIT_SECTION_ASM_OP or FINI_SECTION_ASM_OP to calls to initialization and finalization functions from the init and fini sections. By default, this macro uses a simple function call. Some ports need hand-crafted assembly code to avoid dependencies on registers initialized in the function prologue or to ensure that constant pools don't end up too far way in the text section.

— Macro: TARGET_LIBGCC_SDATA_SECTION

If defined, a string which names the section into which small variables defined in crtstuff and libgcc should go. This is useful when the target has options for optimizing access to small data, and you want the crtstuff and libgcc routines to be conservative in what they expect of your application yet liberal in what your application expects. For example, for targets with a .sdata section (like MIPS), you could compile crtstuff with -G 0 so that it doesn't require small data support from your application, but use this macro to put small data into .sdata so that your application can access these variables whether it uses small data or not.

— Macro: FORCE_CODE_SECTION_ALIGN

If defined, an ASM statement that aligns a code section to some arbitrary boundary. This is used to force all fragments of the .init and .fini sections to have to same alignment and thus prevent the linker from having to add any padding.

— Macro: JUMP_TABLES_IN_TEXT_SECTION

Define this macro to be an expression with a nonzero value if jump tables (for tablejump insns) should be output in the text section, along with the assembler instructions. Otherwise, the readonly data section is used.
This macro is irrelevant if there is no separate readonly data section.

— Target Hook: void TARGET_ASM_INIT_SECTIONS (void)

Define this hook if you need to do something special to set up the varasm.c sections, or if your target has some special sections of its own that you need to create.
GCC calls this hook after processing the command line, but before writing any assembly code, and before calling any of the section-returning hooks described below.

— Target Hook: int TARGET_ASM_RELOC_RW_MASK (void)

Return a mask describing how relocations should be treated when selecting sections. Bit 1 should be set if global relocations should be placed in a read-write section; bit 0 should be set if local relocations should be placed in a read-write section.
The default version of this function returns 3 when -fpic is in effect, and 0 otherwise. The hook is typically redefined when the target cannot support (some kinds of) dynamic relocations in read-only sections even in executables.

— Target Hook: section * TARGET_ASM_SELECT_SECTION (tree exp, int reloc, unsigned HOST_WIDE_INT align)

Return the section into which exp should be placed. You can assume that exp is either a VAR_DECL node or a constant of some sort. reloc indicates whether the initial value of exp requires link-time relocations. Bit 0 is set when variable contains local relocations only, while bit 1 is set for global relocations. align is the constant alignment in bits.
The default version of this function takes care of putting read-only variables in readonly_data_section.
See also USE_SELECT_SECTION_FOR_FUNCTIONS.

— Macro: USE_SELECT_SECTION_FOR_FUNCTIONS

Define this macro if you wish TARGET_ASM_SELECT_SECTION to be called for FUNCTION_DECLs as well as for variables and constants.
In the case of a FUNCTION_DECL, reloc will be zero if the function has been determined to be likely to be called, and nonzero if it is unlikely to be called.

— Target Hook: void TARGET_ASM_UNIQUE_SECTION (tree decl, int reloc)

Build up a unique section name, expressed as a STRING_CST node, and assign it to `DECL_SECTION_NAME (decl)'. As with TARGET_ASM_SELECT_SECTION, reloc indicates whether the initial value of exp requires link-time relocations.
The default version of this function appends the symbol name to the ELF section name that would normally be used for the symbol. For example, the function foo would be placed in .text.foo. Whatever the actual target object format, this is often good enough.

— Target Hook: section * TARGET_ASM_FUNCTION_RODATA_SECTION (tree decl)

Return the readonly data section associated with `DECL_SECTION_NAME (decl)'. The default version of this function selects .gnu.linkonce.r.name if the function's section is .gnu.linkonce.t.name, .rodata.name if function is in .text.name, and the normal readonly-data section otherwise.

— Target Hook: section * TARGET_ASM_SELECT_RTX_SECTION (enum machine_mode mode, rtx x, unsigned HOST_WIDE_INT align)

Return the section into which a constant x, of mode mode, should be placed. You can assume that x is some kind of constant in RTL. The argument mode is redundant except in the case of a const_int rtx. align is the constant alignment in bits.
The default version of this function takes care of putting symbolic constants in flag_pic mode in data_section and everything else in readonly_data_section.

— Target Hook: tree TARGET_MANGLE_DECL_ASSEMBLER_NAME (tree decl, tree id)

Define this hook if you need to postprocess the assembler name generated by target-independent code. The id provided to this hook will be the computed name (e.g., the macro DECL_NAME of the decl in C, or the mangled name of the decl in C++). The return value of the hook is an IDENTIFIER_NODE for the appropriate mangled name on your target system. The default implementation of this hook just returns the id provided.

— Target Hook: void TARGET_ENCODE_SECTION_INFO (tree decl, rtx rtl, int new_decl_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).
The hook is executed immediately after rtl has been created for decl, which may be a variable or function declaration or an entry in the constant pool. In either case, rtl is the rtl in question. Do not use DECL_RTL (decl) in this hook; that field may not have been initialized yet.
In the case of a constant, it is safe to assume that the rtl is a mem whose address is a symbol_ref. Most decls will also have this form, but that is not guaranteed. Global register variables, for instance, will have a reg for their rtl. (Normally the right thing to do with such unusual rtl is leave it alone.)
The new_decl_p argument will be true if this is the first time that TARGET_ENCODE_SECTION_INFO 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 DECL_ATTRIBUTES. new_decl_p is always true when the hook is called for a constant.
The usual thing for this hook to do is to record flags in the symbol_ref, using SYMBOL_REF_FLAG or SYMBOL_REF_FLAGS. 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 SYMBOL_REF_FLAGS.
The default definition of this hook, default_encode_section_info in varasm.c, sets a number of commonly-useful bits in SYMBOL_REF_FLAGS. Check whether the default does what you need before overriding it.

17.19 Dividing the Output into Sections (Texts, Data, ...)