GNU Compiler Collection (GCC) Internals: File Framework

GNU Compiler Collection (GCC) Internals: File Framework

17.20.1 The Overall Framework of an Assembler File

This describes the overall framework of an assembly file.

Target Hook: void TARGET_ASM_FILE_START (void): Output to asm_out_file any text which the assembler expects to find at the beginning of a file. The default behavior is controlled by two flags, documented below. Unless your target’s assembler is quite unusual, if you override the default, you should call default_file_start at some point in your target hook. This lets other target files rely on these variables.

Target Hook: bool TARGET_ASM_FILE_START_APP_OFF

If this flag is true, the text of the macro ASM_APP_OFF will be printed as the very first line in the assembly file, unless -fverbose-asm is in effect. (If that macro has been defined to the empty string, this variable has no effect.) With the normal definition of ASM_APP_OFF, the effect is to notify the GNU assembler that it need not bother stripping comments or extra whitespace from its input. This allows it to work a bit faster.

The default is false. You should not set it to true unless you have verified that your port does not generate any extra whitespace or comments that will cause GAS to issue errors in NO_APP mode.

Target Hook: bool TARGET_ASM_FILE_START_FILE_DIRECTIVE: If this flag is true, output_file_directive will be called for the primary source file, immediately after printing ASM_APP_OFF (if that is enabled). Most ELF assemblers expect this to be done. The default is false.

Target Hook: void TARGET_ASM_FILE_END (void): Output to asm_out_file any text which the assembler expects to find at the end of a file. The default is to output nothing.

Function: void file_end_indicate_exec_stack (): Some systems use a common convention, the ‘.note.GNU-stack’ special section, to indicate whether or not an object file relies on the stack being executable. If your system uses this convention, you should define TARGET_ASM_FILE_END to this function. If you need to do other things in that hook, have your hook function call this function.

Target Hook: void TARGET_ASM_LTO_START (void): Output to asm_out_file any text which the assembler expects to find at the start of an LTO section. The default is to output nothing.

Target Hook: void TARGET_ASM_LTO_END (void): Output to asm_out_file any text which the assembler expects to find at the end of an LTO section. The default is to output nothing.

Target Hook: void TARGET_ASM_CODE_END (void): Output to asm_out_file any text which is needed before emitting unwind info and debug info at the end of a file. Some targets emit here PIC setup thunks that cannot be emitted at the end of file, because they couldn’t have unwind info then. The default is to output nothing.

Macro: ASM_COMMENT_START: A C string constant describing how to begin a comment in the target assembler language. The compiler assumes that the comment will end at the end of the line.

Macro: ASM_APP_ON: A C string constant for text to be output before each asm statement or group of consecutive ones. Normally this is "#APP", which is a comment that has no effect on most assemblers but tells the GNU assembler that it must check the lines that follow for all valid assembler constructs.

Macro: ASM_APP_OFF: A C string constant for text to be output after each asm statement or group of consecutive ones. Normally this is "#NO_APP", which tells the GNU assembler to resume making the time-saving assumptions that are valid for ordinary compiler output.

Macro: ASM_OUTPUT_SOURCE_FILENAME (stream, name)

A C statement to output COFF information or DWARF debugging information which indicates that filename name is the current source file to the stdio stream stream.

This macro need not be defined if the standard form of output for the file format in use is appropriate.

Target Hook: void TARGET_ASM_OUTPUT_SOURCE_FILENAME (FILE *file, const char *name)

Output COFF information or DWARF debugging information which indicates that filename name is the current source file to the stdio stream file.

This target hook need not be defined if the standard form of output for the file format in use is appropriate.

Target Hook: void TARGET_ASM_OUTPUT_IDENT (const char *name): Output a string based on name, suitable for the ‘#ident’ directive, or the equivalent directive or pragma in non-C-family languages. If this hook is not defined, nothing is output for the ‘#ident’ directive.

Macro: OUTPUT_QUOTED_STRING (stream, string): A C statement to output the string string to the stdio stream stream. If you do not call the function output_quoted_string in your config files, GCC will only call it to output filenames to the assembler source. So you can use it to canonicalize the format of the filename using this macro.

Target Hook: void TARGET_ASM_NAMED_SECTION (const char *name, unsigned int flags, tree decl): Output assembly directives to switch to section name. The section should have attributes as specified by flags, which is a bit mask of the SECTION_* flags defined in output.h. If decl is non-NULL, it is the VAR_DECL or FUNCTION_DECL with which this section is associated.

Target Hook: bool TARGET_ASM_ELF_FLAGS_NUMERIC (unsigned int flags, unsigned int *num): This hook can be used to encode ELF section flags for which no letter code has been defined in the assembler. It is called by default_asm_named_section whenever the section flags need to be emitted in the assembler output. If the hook returns true, then the numerical value for ELF section flags should be calculated from flags and saved in *num; the value is printed out instead of the normal sequence of letter codes. If the hook is not defined, or if it returns false, then num is ignored and the traditional letter sequence is emitted.

Target Hook: section * TARGET_ASM_FUNCTION_SECTION (tree decl, enum node_frequency freq, bool startup, bool exit): Return preferred text (sub)section for function decl. Main purpose of this function is to separate cold, normal and hot functions. startup is true when function is known to be used only at startup (from static constructors or it is main()). exit is true when function is known to be used only at exit (from static destructors). Return NULL if function should go to default text section.

Target Hook: void TARGET_ASM_FUNCTION_SWITCHED_TEXT_SECTIONS (FILE *file, tree decl, bool new_is_cold): Used by the target to emit any assembler directives or additional labels needed when a function is partitioned between different sections. Output should be written to file. The function decl is available as decl and the new section is ‘cold’ if new_is_cold is true.

Common Target Hook: bool TARGET_HAVE_NAMED_SECTIONS: This flag is true if the target supports TARGET_ASM_NAMED_SECTION. It must not be modified by command-line option processing.

Target Hook: bool TARGET_HAVE_SWITCHABLE_BSS_SECTIONS: This flag is true if we can create zeroed data by switching to a BSS section and then using ASM_OUTPUT_SKIP to allocate the space. This is true on most ELF targets.

Target Hook: unsigned int TARGET_SECTION_TYPE_FLAGS (tree decl, const char *name, int reloc)

Choose a set of section attributes for use by TARGET_ASM_NAMED_SECTION based on a variable or function decl, a section name, and whether or not the declaration’s initializer may contain runtime relocations. decl may be null, in which case read-write data should be assumed.

The default version of this function handles choosing code vs data, read-only vs read-write data, and flag_pic. You should only need to override this if your target has special flags that might be set via __attribute__.

Target Hook: int TARGET_ASM_RECORD_GCC_SWITCHES (print_switch_type type, const char *text)

Provides the target with the ability to record the gcc command line switches that have been passed to the compiler, and options that are enabled. The type argument specifies what is being recorded. It can take the following values:

SWITCH_TYPE_PASSED: text is a command line switch that has been set by the user.
SWITCH_TYPE_ENABLED: text is an option which has been enabled. This might be as a direct result of a command line switch, or because it is enabled by default or because it has been enabled as a side effect of a different command line switch. For example, the -O2 switch enables various different individual optimization passes.
SWITCH_TYPE_DESCRIPTIVE: text is either NULL or some descriptive text which should be ignored. If text is NULL then it is being used to warn the target hook that either recording is starting or ending. The first time type is SWITCH_TYPE_DESCRIPTIVE and text is NULL, the warning is for start up and the second time the warning is for wind down. This feature is to allow the target hook to make any necessary preparations before it starts to record switches and to perform any necessary tidying up after it has finished recording switches.
SWITCH_TYPE_LINE_START: This option can be ignored by this target hook.
SWITCH_TYPE_LINE_END: This option can be ignored by this target hook.

The hook’s return value must be zero. Other return values may be supported in the future.

By default this hook is set to NULL, but an example implementation is provided for ELF based targets. Called elf_record_gcc_switches, it records the switches as ASCII text inside a new, string mergeable section in the assembler output file. The name of the new section is provided by the TARGET_ASM_RECORD_GCC_SWITCHES_SECTION target hook.

Target Hook: const char * TARGET_ASM_RECORD_GCC_SWITCHES_SECTION: This is the name of the section that will be created by the example ELF implementation of the TARGET_ASM_RECORD_GCC_SWITCHES target hook.