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6. DEC Alpha Backend

This chapter documents the Backend for the DEC Alpha processor family.

6.1 Additional options for this version

This backend provides the following additional options:


Place identical floating point constants at the same memory location. This can reduce program size and increase compilation time.


By default constant data will be placed in the code section (and therefore is accessable with faster pc-relative addressing modes). Using this option it will be placed in the data section. Note that on operating systems with memory protection this option will disable write-protection of constant data.


Do not replace certain builtin functions by inline code. This may be useful if you use this code generator with another frontend than vbcc. stdarg.h will not work with -no-builtins.


Generate stabs debug infos (if -g is specified) rather than DWARF2 which is the default. Consider this obsolete.

6.2 ABI

This backend supports the following registers:

The current version generates assembly output for use with the GNU assembler. The generated code should work on systems with 21064, 21066, 21164 and higher Alpha CPUs.

The registers $0-$8, $16-$28, $f0, $f1 and $f10-$f30 are used as scratch registers (i.e. they can be destroyed in function calls), all other registers are preserved. Of course $31 and $f31 cannot be used.

The first 6 function arguments which have arithmetic or pointer types are passed in registers $16/$f16 through $21/$f21.

Integers and pointers are returned in $0, floats and doubles in $f0. All other types are returned by passing the function the address of the result as a hidden argument - so when you call such a function without a proper declaration in scope you can expect a crash.

The elementary data types are represented like:

    type        size in bits        alignment in bytes 

    char                8                       1 
    short              16                       2 
    int                32                       4 
    long               64                       8
    long long          64                       8 
    all pointers       64                       8 
    float              32                       4 
    double             64                       8 

6.3 Predefined Macros

This backend defines the following macros:


6.4 Stdarg

A possible <stdarg.h> could look like this:

    typedef struct {
      char *regbase;
      char *membase;
      int arg;
    } va_list;

    char *__va_start(void);
    int __va_fixargs(void);

    #define va_start(vl,dummy) \

    #define va_end(vl) (vl.regbase=vl.membase=0)

    #define __va_size(type) ((sizeof(type)+7)/8*8)
    #define va_arg(vl,type) \
     ( \
      ((__typeof(type)&15)<=10&&++vl.arg<=6) ? \
       ( \
        ((__typeof(type)&15)>=6&&(__typeof(type)&15)<=8) ? \
          (vl.regbase+=16,*(type *)(vl.regbase-8)) \
        : \
          (vl.regbase+=16,*(type *)(vl.regbase-16)) \
       ) \
      : \
       (vl.membase+=__va_size(type),*(type *)(vl.membase-__va_size(type))) \

6.5 Known problems

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