10. VideoCore IV Backend

This chapter documents the Backend for the VideoCore IV processor family.

The backend is in a very early stage, it is not complete, and it can not yet be considered useful!

Also note that it is based on freely available, inofficial, and possibly incorrect information on the target processor.

10.1 Additional options for this version

This backend provides the following additional options:

‘-short-double’

Use native 32bit floating point for double and long double. This is much more efficient, but not ISO C conforming.

‘-one-section’

Put all code and data in the same section (.text).

‘-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 reduce the stack size needed by the compiled program.

‘-no-peephole’

Disable most backend peephole optimizations. Just for testing.

‘-noext-regs’

Do not use registers r16-r23. Just for testing.

‘-cond-limit=<n>’

Set the limit (in number of intermediate code instructions) for the length of code-sequences considered for conditional execution (default: 2).

10.2 ABI

This backend supports the following registers:

• - r0 through r31 for the general purpose registers

Additionally, the register pairs r0/r1 r2/r3, r4/r5, r6/r7, r8/r9, r10/r11, r12/r13, r14/r15, r16/r17, r18/r19, r20/r21, r22/r23 are available.

r14, r15, r24-r31 are currently reserved by the backend.

The current version generates assembly output for use with ‘vasm’.

The registers r0-r5 and r14-r15 are used as scratch registers (i.e. they can be destroyed in function calls), all other registers are preserved. r25 is the stack-pointer.

The first 6 function arguments which have integer, float32 or pointer types are passed in registers r0 through r5. All other arguments are passed on the stack.

Integers, float32 and pointers are returned in r0. 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               32                       4 
    long long          64                       8    not yet supported
    all pointers       32                       4 
    float              32                       4 
    double             64 (32)                  4 
    long double        64 (32)                  4
    

10.3 Target-specific variable-attributes

The vidcore-backend offers the following variable-attributes:

__section("name","attr")

Place this function/object in section "name" with attributes "attr".

10.4 Target-specific pragmas

The vidcore-backend offers the following #pragmas:

none at the moment...

10.5 Predefined Macros

This backend defines the following macros:

__VIDEOCORE__

__SHORT_DOUBLE__ (if -short-double is used)

10.6 Stdarg

stdarg-implementation is not yet fully working. One restriction is that when calling a varargs function, the prototype must be in scope (this is ISO C conforming). Another one is that the stdarg-macros only work as long as all fixed arguments are passed in registers.

This will be fixed in the future.

10.7 Known problems

• - no support for long long
• - no support for 64bit floating point
• - stdarg problems mentioned above
• - suboptimal code quality
• - ...

This document was generated by vb on January 3, 2015 using texi2html 1.82.