GNU Compiler Collection (GCC) Internals: Instruction Output

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#### 17.21.7 Output of Assembler Instructions

This describes assembler instruction output.

Macro: REGISTER_NAMES

  A C initializer containing the assembler’s names for the machine registers, each one as a C string constant. This is what translates register numbers in the compiler into assembler language.

Macro: ADDITIONAL_REGISTER_NAMES

  If defined, a C initializer for an array of structures containing a name and a register number. This macro defines additional names for hard registers, thus allowing the `asm` option in declarations to refer to registers using alternate names.

Macro: OVERLAPPING_REGISTER_NAMES

  If defined, a C initializer for an array of structures containing a name, a register number and a count of the number of consecutive machine registers the name overlaps. This macro defines additional names for hard registers, thus allowing the `asm` option in declarations to refer to registers using alternate names. Unlike `ADDITIONAL_REGISTER_NAMES`, this macro should be used when the register name implies multiple underlying registers.

  This macro should be used when it is important that a clobber in an `asm` statement clobbers all the underlying values implied by the register name. For example, on ARM, clobbering the double-precision VFP register “d0” implies clobbering both single-precision registers “s0” and “s1”.

Macro: ASM_OUTPUT_OPCODE (stream, ptr)

  Define this macro if you are using an unusual assembler that requires different names for the machine instructions.

  The definition is a C statement or statements which output an assembler instruction opcode to the stdio stream stream. The macro-operand ptr is a variable of type `char *` which points to the opcode name in its “internal” form—the form that is written in the machine description. The definition should output the opcode name to stream, performing any translation you desire, and increment the variable ptr to point at the end of the opcode so that it will not be output twice.

  In fact, your macro definition may process less than the entire opcode name, or more than the opcode name; but if you want to process text that includes ‘`%`’-sequences to substitute operands, you must take care of the substitution yourself. Just be sure to increment ptr over whatever text should not be output normally.

  If you need to look at the operand values, they can be found as the elements of `recog_data.operand`.

  If the macro definition does nothing, the instruction is output in the usual way.

Macro: FINAL_PRESCAN_INSN (insn, opvec, noperands)

  If defined, a C statement to be executed just prior to the output of assembler code for insn, to modify the extracted operands so they will be output differently.

  Here the argument opvec is the vector containing the operands extracted from insn, and noperands is the number of elements of the vector which contain meaningful data for this insn. The contents of this vector are what will be used to convert the insn template into assembler code, so you can change the assembler output by changing the contents of the vector.

  This macro is useful when various assembler syntaxes share a single file of instruction patterns; by defining this macro differently, you can cause a large class of instructions to be output differently (such as with rearranged operands). Naturally, variations in assembler syntax affecting individual insn patterns ought to be handled by writing conditional output routines in those patterns.

  If this macro is not defined, it is equivalent to a null statement.

Target Hook: void TARGET_ASM_FINAL_POSTSCAN_INSN (FILE *file, rtx insn, rtx *opvec, int noperands)

  If defined, this target hook is a function which is executed just after the output of assembler code for insn, to change the mode of the assembler if necessary.

  Here the argument opvec is the vector containing the operands extracted from insn, and noperands is the number of elements of the vector which contain meaningful data for this insn. The contents of this vector are what was used to convert the insn template into assembler code, so you can change the assembler mode by checking the contents of the vector.

Macro: PRINT_OPERAND (stream, x, code)

  A C compound statement to output to stdio stream stream the assembler syntax for an instruction operand x. x is an RTL expression.

  code is a value that can be used to specify one of several ways of printing the operand. It is used when identical operands must be printed differently depending on the context. code comes from the ‘`%`’ specification that was used to request printing of the operand. If the specification was just ‘`%digit`’ then code is 0; if the specification was ‘`%ltr digit`’ then code is the ASCII code for ltr.

  If x is a register, this macro should print the register’s name. The names can be found in an array `reg_names` whose type is `char *[]`. `reg_names` is initialized from `REGISTER_NAMES`.

  When the machine description has a specification ‘`%punct`’ (a ‘`%`’ followed by a punctuation character), this macro is called with a null pointer for x and the punctuation character for code.

Macro: PRINT_OPERAND_PUNCT_VALID_P (code)

  A C expression which evaluates to true if code is a valid punctuation character for use in the `PRINT_OPERAND` macro. If `PRINT_OPERAND_PUNCT_VALID_P` is not defined, it means that no punctuation characters (except for the standard one, ‘`%`’) are used in this way.

Macro: PRINT_OPERAND_ADDRESS (stream, x)

  A C compound statement to output to stdio stream stream the assembler syntax for an instruction operand that is a memory reference whose address is x. x is an RTL expression.

  On some machines, the syntax for a symbolic address depends on the section that the address refers to. On these machines, define the hook `TARGET_ENCODE_SECTION_INFO` to store the information into the `symbol_ref`, and then check for it here. See Assembler Format.

Macro: DBR_OUTPUT_SEQEND (file)

  A C statement, to be executed after all slot-filler instructions have been output. If necessary, call `dbr_sequence_length` to determine the number of slots filled in a sequence (zero if not currently outputting a sequence), to decide how many no-ops to output, or whatever.

  Don’t define this macro if it has nothing to do, but it is helpful in reading assembly output if the extent of the delay sequence is made explicit (e.g. with white space).

Note that output routines for instructions with delay slots must be prepared to deal with not being output as part of a sequence (i.e. when the scheduling pass is not run, or when no slot fillers could be found.) The variable `final_sequence` is null when not processing a sequence, otherwise it contains the `sequence` rtx being output.

Macro: REGISTER_PREFIX
Macro: LOCAL_LABEL_PREFIX
Macro: USER_LABEL_PREFIX
Macro: IMMEDIATE_PREFIX

  If defined, C string expressions to be used for the ‘`%R`’, ‘`%L`’, ‘`%U`’, and ‘`%I`’ options of `asm_fprintf` (see `final.c`). These are useful when a single `md` file must support multiple assembler formats. In that case, the various `tm.h` files can define these macros differently.

Macro: ASM_FPRINTF_EXTENSIONS (file, argptr, format)

  If defined this macro should expand to a series of `case` statements which will be parsed inside the `switch` statement of the `asm_fprintf` function. This allows targets to define extra printf formats which may useful when generating their assembler statements. Note that uppercase letters are reserved for future generic extensions to asm_fprintf, and so are not available to target specific code. The output file is given by the parameter file. The varargs input pointer is argptr and the rest of the format string, starting the character after the one that is being switched upon, is pointed to by format.

Macro: ASSEMBLER_DIALECT

  If your target supports multiple dialects of assembler language (such as different opcodes), define this macro as a C expression that gives the numeric index of the assembler language dialect to use, with zero as the first variant.

  If this macro is defined, you may use constructs of the form

      ‘{option0|option1|option2…}’

  in the output templates of patterns (see Output Template) or in the first argument of `asm_fprintf`. This construct outputs ‘`option0`’, ‘`option1`’, ‘`option2`’, etc., if the value of `ASSEMBLER_DIALECT` is zero, one, two, etc. Any special characters within these strings retain their usual meaning. If there are fewer alternatives within the braces than the value of `ASSEMBLER_DIALECT`, the construct outputs nothing. If it’s needed to print curly braces or ‘`|`’ character in assembler output directly, ‘`%{`’, ‘`%}`’ and ‘`%|`’ can be used.

  If you do not define this macro, the characters ‘`{`’, ‘`|`’ and ‘`}`’ do not have any special meaning when used in templates or operands to `asm_fprintf`.

  Define the macros `REGISTER_PREFIX`, `LOCAL_LABEL_PREFIX`, `USER_LABEL_PREFIX` and `IMMEDIATE_PREFIX` if you can express the variations in assembler language syntax with that mechanism. Define `ASSEMBLER_DIALECT` and use the ‘`{option0|option1}`’ syntax if the syntax variant are larger and involve such things as different opcodes or operand order.

Macro: ASM_OUTPUT_REG_PUSH (stream, regno)

  A C expression to output to stream some assembler code which will push hard register number regno onto the stack. The code need not be optimal, since this macro is used only when profiling.

Macro: ASM_OUTPUT_REG_POP (stream, regno)

  A C expression to output to stream some assembler code which will pop hard register number regno off of the stack. The code need not be optimal, since this macro is used only when profiling.

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