Machine Code Insertions

Package Machine_Code provides machine code support as described in the Ada 95 Reference Manual in two separate forms:

• Machine code statements, consisting of qualified expressions that fit the requirements of RM section 13.8.
• An intrinsic callable procedure, providing an alternative mechanism of including machine instructions in a subprogram.

The two features are similar, and both closely related to the mechanism provided by the asm instruction in the GNU C compiler. Full understanding and use of the facilities in this package requires understanding the asm instruction as described in Using and Porting GNU CC by Richard Stallman. Calls to the function Asm and the procedure Asm have identical semantic restrictions and effects as described below. Both are provided so that the procedure call can be used as a statement, and the function call can be used to form a code_statement.

The first example given in the GNU CC documentation is the C asm instruction:

        asm ("fsinx %1 %0" : "=f" (result) : "f" (angle));
     

The equivalent can be written for GNAT as:

     Asm ("fsinx %1 %0",
          My_Float'Asm_Output ("=f", result),
          My_Float'Asm_Input  ("f",  angle));
     

The first argument to Asm is the assembler template, and is identical to what is used in GNU CC. This string must be a static expression. The second argument is the output operand list. It is either a single Asm_Output attribute reference, or a list of such references enclosed in parentheses (technically an array aggregate of such references).

The Asm_Output attribute denotes a function that takes two parameters. The first is a string, the second is the name of a variable of the type designated by the attribute prefix. The first (string) argument is required to be a static expression and designates the constraint for the parameter (e.g. what kind of register is required). The second argument is the variable to be updated with the result. The possible values for constraint are the same as those used in the RTL, and are dependent on the configuration file used to build the GCC back end. If there are no output operands, then this argument may either be omitted, or explicitly given as No_Output_Operands.

The second argument of my_float'Asm_Output functions as though it were an out parameter, which is a little curious, but all names have the form of expressions, so there is no syntactic irregularity, even though normally functions would not be permitted out parameters. The third argument is the list of input operands. It is either a single Asm_Input attribute reference, or a list of such references enclosed in parentheses (technically an array aggregate of such references).

The Asm_Input attribute denotes a function that takes two parameters. The first is a string, the second is an expression of the type designated by the prefix. The first (string) argument is required to be a static expression, and is the constraint for the parameter, (e.g. what kind of register is required). The second argument is the value to be used as the input argument. The possible values for the constant are the same as those used in the RTL, and are dependent on the configuration file used to built the GCC back end.

If there are no input operands, this argument may either be omitted, or explicitly given as No_Input_Operands. The fourth argument, not present in the above example, is a list of register names, called the clobber argument. This argument, if given, must be a static string expression, and is a space or comma separated list of names of registers that must be considered destroyed as a result of the Asm call. If this argument is the null string (the default value), then the code generator assumes that no additional registers are destroyed.

The fifth argument, not present in the above example, called the volatile argument, is by default False. It can be set to the literal value True to indicate to the code generator that all optimizations with respect to the instruction specified should be suppressed, and that in particular, for an instruction that has outputs, the instruction will still be generated, even if none of the outputs are used. See the full description in the GCC manual for further details.

The Asm subprograms may be used in two ways. First the procedure forms can be used anywhere a procedure call would be valid, and correspond to what the RM calls "intrinsic" routines. Such calls can be used to intersperse machine instructions with other Ada statements. Second, the function forms, which return a dummy value of the limited private type Asm_Insn, can be used in code statements, and indeed this is the only context where such calls are allowed. Code statements appear as aggregates of the form:

     Asm_Insn'(Asm (...));
     Asm_Insn'(Asm_Volatile (...));
     

In accordance with RM rules, such code statements are allowed only within subprograms whose entire body consists of such statements. It is not permissible to intermix such statements with other Ada statements.

Typically the form using intrinsic procedure calls is more convenient and more flexible. The code statement form is provided to meet the RM suggestion that such a facility should be made available. The following is the exact syntax of the call to Asm (of course if named notation is used, the arguments may be given in arbitrary order, following the normal rules for use of positional and named arguments)

     ASM_CALL ::= Asm (
                      [Template =>] static_string_EXPRESSION
                    [,[Outputs  =>] OUTPUT_OPERAND_LIST      ]
                    [,[Inputs   =>] INPUT_OPERAND_LIST       ]
                    [,[Clobber  =>] static_string_EXPRESSION ]
                    [,[Volatile =>] static_boolean_EXPRESSION] )
     OUTPUT_OPERAND_LIST ::=
       No_Output_Operands
     | OUTPUT_OPERAND_ATTRIBUTE
     | (OUTPUT_OPERAND_ATTRIBUTE {,OUTPUT_OPERAND_ATTRIBUTE})
     OUTPUT_OPERAND_ATTRIBUTE ::=
       SUBTYPE_MARK'Asm_Output (static_string_EXPRESSION, NAME)
     INPUT_OPERAND_LIST ::=
       No_Input_Operands
     | INPUT_OPERAND_ATTRIBUTE
     | (INPUT_OPERAND_ATTRIBUTE {,INPUT_OPERAND_ATTRIBUTE})
     INPUT_OPERAND_ATTRIBUTE ::=
       SUBTYPE_MARK'Asm_Input (static_string_EXPRESSION, EXPRESSION)