Magazyn Exec 5/CD_Magazyn_EXEC_nr_5.iso/Programy/Programowanie/vbcc07e.lzx/vbcc/doc/vbccm68k.doc

Text File | 2001-02-12 | 14.1 KB | 337 lines

vbcc - C compiler (c) in 1995-2000 by Volker Barthelmann


INTRODUCTION

    vbcc is a free portable and retargetable ANSI C compiler.
    It is clearly split into a target independant and a target dependant
    part and supports emulating datatypes of the target machine on any
    other machine so that it is possible to e.g. make a crosscompiler for
    a 64bit machine on a 32bit machine.
    This document only deals with the target dependant parts of the
    Motorola m68k/Coldfire version.


LEGAL

    vbcc is (c) in 1995-2000 by Volker Barthelmann.
    This is a development snapshot which must not be distributed.
    Also, commercial usage is forbidden.


ADDITIONAL OPTIONS FOR THIS VERSION

    -cpu=n      Generate code for cpu n (e.g. -cpu=68020), default: 68000

    -fpu=n      Generate code for fpu n (e.g. -fpu=68881), default: 0

    -sd         Use small data model (see below).

    -sc         Use small code model (see below).

    -prof       Insert code for profiling (not really usable yet).

    -const-in-data

                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.
                This could e.g. be useful if you want to use small data and
                small code, but your code gets too big with all the constant
                data.
                Note that on operating systems with memory protection this
                option will disable write-protection of constant data.

    -use-framepointer

                By default automatic variables are addressed through a7
                instead of a5. This generates slightly better code, because
                the function entry and exit overhead is reduced and a5 can be
                used as register variable etc.
                However this may be a bit confusing when debugging and you
                can force vbcc to use a5 as a fixed framepointer.

    -no-peephole

                The intermediate code does not contain any of the 68k
                addressing modes, so if they are to be used an extra pass
                over the intermediate code is necessary to recognize certain
                patterns that can be expressed using a 68k addressing mode.
                By default vbcc tries to use some 68k addressing modes.
                Currently (ax)+ and subsets of (displ,ax,dy*skal) are used.
                However not all cases where those addressing modes could be
                used are recognized.
                With this option you can prevent vbcc from searching for
                possible addressing modes. This may simplify debugging.

    -no-delayed-popping

                By default arguments of function calls are not always popped
                from the stack immediately after the call, so that the
                arguments of several calls may be popped at once.
                With this option vbcc can be forced to pop them after every
                function call.
                This may simplify debugging and very slightly reduce the
                stack size needed by the compiled program.

    -gas        Create output suitable for the GNU assembler. This is
                mainly useful to create code for other operating systems.

    -no-fp-return

                Do not return floats and doubles in floating-point registers
                even if code for an fpu is generated. This is mainly useful
                to create code for other operating systems.

    -no-mreg-return

                Do not use multiple registers to return types that do not
                fit into a single register. This is mainly for backwards
                compatibility.

    -hunkdebug  When creating debug-output (-g option) create Amiga debug
                hunks rather than DWARF2. This does not work with -gas.

    -no-intz    When generating code for FPU do quick&dirty conversions
                from floating-point to integer. The code may be somewhat
                faster but will not correctly round to zero.
                Only use it if you know what you are doing.

    -d2scratch  obsolete

    -noa4       obsolete


SOME INTERNALS

    The current version generates assembler output for use with the PhxAss
    assembler (c) by Frank Wille. Most peephole optimizations are done by the
    assembler so vbcc only does some that the assembler cannot make.
    The generated executables will probably only work with OS2.0 or higher.

    With -gas assembler output suitable for the GNU assembler is generated
    (the version must understand the Motorola syntax - some old ones do not).
    The output is only slightly modified from the PhxAss-output and will
    therefore result in worse code on gas. However this code generator should
    be usable on most operating systems on 68k machines that way.

    The register names are:

         a0,  a1,  a2,  a3,  a4,  a5,  a6,  a7
         d0,  d1,  d2,  d3,  d4,  d5,  d6,  d7
        fp0, fp1, fp2, fp3, fp4, fp5, fp6, fp7

    The registers d0, d1, a0, a1, fp0 and fp1  are used as scratch registers
    (i.e. they can be destroyed in function calls), all other registers are
    preserved.

    All scalar types up to 4 bytes are returned in register d0 like
    common on the Amiga (although I think pointers should better be returned
    in a0). If compiled for an fpu, floating point values are returned in
    fp0 unless -no-fpreturn is specified.
    Types which are 8, 12 or 16 bytes large will be returned in several
    registers (d0/d1/a0/a1) unless -no-mreg-return is specified.
    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.

    You must not link objects together that have been compiled with
    different settings!

    vbcc uses d0-d7 and a0-a6 for temporary results and register variables
    (a4 is used as small data pointer if -sd is used). a5 can be
    used as frame pointer for automatic variables (but this is not necessary -
    they can be accessed through a7, too). At the moment all local
    variables are addressed via (dist,ax), so a function may have only ~32k
    of local variables if code for <=68000 is generated.

    The elementary data types are represented like:

    type        size in bits        alignment in bytes

    char                8                       1
    short              16                       2
    int                32                       2
    long               32                       2
    all pointers       32                       2
    float(fpu)         32                       2       see below
    double(fpu)        64                       2       see below

    Although it would be better to have all 32bit+ types aligned to 4 bytes
    I chose 2 bytes to be compatible with the Amiga system structures which
    unfortunately have longwords aligned to 4n+2-addresses.

    The amiga68k code generator at the moment only works on systems
    that store floats and doubles in a similar way (IEEE) like the Amiga.


SMALL DATA

    vbcc can access static data in two ways. By default all such data will
    be accessed with full 32bit addresses (large data model).
    However there is a second way. You can set up an address register (a4)
    to point into your data segment and then address data with a 16bit
    offset through this register.
    The advantages of the small data model are that your program will
    usually be smaller (because the 16bit offsets use less space and no
    relocation information is needed) and faster.
    The disadvantages are that one address register cannot be used by the
    compiler and that you can use it only if all your static data occupies
    less than 64kb. Also you may not mix object modules and libraries that
    have been compiled with different data models (you can call functions
    compiled with large data model from object files compiled with small
    data model, but not vice versa and only functions can be called that
    way - other data cannot be accessed) and you probably have to use
    PhxLnk then.

    If you use small data together with functions which are called from
    functions which have not been compiled with vbcc or with the small data
    model then you _must_ define those functions with the __saveds attribute
    or call geta4() as the first statement in your function (do not use
    automatic initializations prior to the call to geta4).
    Note that you must not call geta4() through a function pointer!


SMALL CODE

    If you use the small code model calls to external functions (i.e. from
    libraries or other object files) are done with 16bit offsets over
    the program counter rather than with absolute 32bit addresses.
    The advantage is slightly smaller and faster code.
    The disadvantages are that all the code (including library functions)
    must be small enough and that you may have to use PhxLnk. However
    you can link objects/libraries together if they have been compiled
    with different code models.


CPUs

    At the moment the values of -cpu=n have those effects:

    n<68000     - code for the Coldfire family is generated

    n>=68000    - code for the 68k family is generated

    n>=68020:   - 32bit multiplication/division/modulo is done with the
                  mul?.l, div?.l and div?l.l instructions
                - tst.l ax is used
                - extb.l dx is used
                - 16/32bit offsets are used in certain addressing modes
                - link.l is used
                - addressing modes with scaling are used
                - (dx) is used if no address register is available (not yet)

    n==68040    - 8bit constants are not copied in data registers


FPUs

    At the moment the values of -fpu=n have those effects:

    n>68000:    - floating point calculations are done using the fpu
    n=68040:
    n=68060:    - instructions that have to be emulated on these fpus
                  will not be used; at the moment this only includes
                  the fintrz instruction in case of the 040


MATH

    Integer math hopefully works without problems on all cpus. Long multiply
    on cpus <68020 uses inline routines. This may increase code size a bit,
    but it should be significantly faster, because function call overhead
    is not necessary and the compiler can use the registers which already
    contain the sources and/or need the result. However if anyone really
    wants an option for using library routines for multiply, this can easily
    be implemented. Long division and modulo is handled by calls to library
    functions. At the moment standard library calls with parameter passing
    via stack are used. This is rather slow, but division takes quite some
    time anyway.
    (mult/div/mod with constant powers of two are replaced by corresponding
     bitwise operations (div/mod only if the other operand is unsigned), but
     sums of powers of two not yet).

    If no FPU is specified floating point math is done using the C=
    math libraries. 32 bit IEEE format is used for float and 64 bit IEEE
    for double.

    Floating point math is done with the FPU if one is specified (see above).
    Floating point values are kept in registers then and therefore may
    have extended precision sometimes, which is not ANSI compliant (but
    will usually cause no harm). When floating point values are stored in
    memory they use the same IEEE formats as without FPU.
    Float or double return values are passed in fp0.

    Note that you must not link object files together if they were not
    compiled with the same -fpu settings and that you have to link with
    the proper math library (see vclib.doc).


TARGET-SPECIFIC VARIABLE ATTRIBUTES

    The m68k-backend offers the following variable attributes:

    __saveds:    Load the pointer to the small data segment at
                 function-entry. Applicable only to functions.

    __chip:      Place variable in chip-memory. Only applicable on
                 AmigaOS to variables with static storage-duration.

    __far:       Do not place this variable in the small-data segment
                 in small-data-mode. No effect in large-data-mode.
                 Only applicable to variables with static storage-
                 duration.

    __near:      Currently ignored.

    __interrupt: This is used to declare interrupt-handlers. The
                 function using this attribute will save all registers
                 it destroys (including scratch-registers) and return
                 with rfe rather than rts.

    __section(<string-literal>):
                 Places the variable/function in a section named
                 according to the argument.


STACK

    If the -stack-check option is used, every function-prologue will
    call the function __stack_check with the stacksize needed by this
    function on the stack. This function has to consider its own
    stacksize and must restore all registers.

    If the compiler is able to calculate the maximum stack-size of a
    function including all callees, it will add a comment in the
    generated assembly-output (subject to change to labels).


STDARG

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

    typedef unsigned char *va_list;

    #define va_start(ap, lastarg) ((ap) = (va_list)(&lastarg + 1))
    #define va_arg(ap, type) ((ap) += \
      (sizeof(type)<sizeof(int)?sizeof(int):sizeof(type)), ((type *)(ap))[-1])
    #define va_end(ap) ((ap) = 0L)


KNOWN PROBLEMS

    - Converting floating point values to unsigned integers is not correct
      if the value is >LONG_MAX and code for an FPU is generated.

    - The extended precision of the FPU registers can cause problems if
      a program depends on the exact precision. Most programs will not
      have trouble with that, but programs which do exact comparisons
      with floating point types (e.g. to try to calculate the number
      of significant bits) may not work as expected (especially if the
      optimizer was turned on).


Volker Barthelmann                                      [e-mail]