GNU Compiler Collection (GCC) Internals: Sections

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### 17.18 Dividing the Output into Sections (Texts, Data, …)

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 `ASM_OUTPUT_ALIGNED_BSS` not 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_DECL`s 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: const char * TARGET_ASM_MERGEABLE_RODATA_PREFIX

  Usually, the compiler uses the prefix `".rodata"` to construct section names for mergeable constant data. Define this macro to override the string if a different section name should be used.

Target Hook: section * TARGET_ASM_TM_CLONE_TABLE_SECTION (void)

  Return the section that should be used for transactional memory clone tables.

Target Hook: section * TARGET_ASM_SELECT_RTX_SECTION (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.

Target Hook: const char * TARGET_STRIP_NAME_ENCODING (const char *name)

  Decode name and return the real name part, sans the characters that `TARGET_ENCODE_SECTION_INFO` may have added.

Target Hook: bool TARGET_IN_SMALL_DATA_P (const_tree exp)

  Returns true if exp should be placed into a “small data” section. The default version of this hook always returns false.

Target Hook: bool TARGET_HAVE_SRODATA_SECTION

  Contains the value true if the target places read-only “small data” into a separate section. The default value is false.

Target Hook: bool TARGET_PROFILE_BEFORE_PROLOGUE (void)

  It returns true if target wants profile code emitted before prologue.

  The default version of this hook use the target macro `PROFILE_BEFORE_PROLOGUE`.

Target Hook: bool TARGET_BINDS_LOCAL_P (const_tree exp)

  Returns true if exp names an object for which name resolution rules must resolve to the current “module” (dynamic shared library or executable image).

  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.

Target Hook: bool TARGET_HAVE_TLS

  Contains the value true if the target supports thread-local storage. The default value is false.

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