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5. PowerPC Backend

This chapter documents the Backend for the PowerPC processor family.


5.1 Additional options for this version

This backend provides the following additional options:

-merge-constants

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

-const-in-data

By default constant data will be placed in the .rodata section. 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.

-fsub-zero

Use fsub to load a floating-point-register with zero. This is faster but requires all registers to always contain valid values (i.e. no NaNs etc.) which may not be the case depending on startup-code, libraries etc.

-amiga-align

Do not require any alignments greater than 2 bytes. This is needed when accessing Amiga system-structures, but can cause a performance penalty.

-elf

Do not prefix symbols with ’_’. Prefix labels with ’.’.

-poweropen

Generate code for the PowerOpen ABI like used in AIX. This does not work correctly yet.

-sc

Generate code for the modified PowerOpen ABI used in the StormC compiler (aka WarpOS ABI).

-no-regnames

Do not use register names but only numbers in the assembly output. This is necessary to avoid name-conflicts when using ‘-elf’.

-setccs

The V.4 ABI requires signalling (in a bit of the condition code register) when arguments to varargs-functions are passed in floating-point registers. vbcc usually does not make use of this and therefore does not set that bit by default. This may lead to problems when linking objects compiled by vbcc to objects/libraries created by other compilers and calling varargs-functions with floating-point arguments. ‘-setccs’ will fix this problem.

-no-peephole

Do not perform several peephole optimizations. Currently includes:

-use-lmw

Use lmw/stmw-instructions. This can significantly reduce code-size. However these instructions may be slower on certain PPCs.

-madd

Use the fmadd/fmsub instructions for combining multiplication with addition/subtraction in one instruction. As these instructions do not round between the operations, they have increased precision over separate addition and multiplication.

While this usually does no harm, it is not ISO conforming and therefore not the default behaviour.

-eabi

Use the PowerPC Embedded ABI (eabi).

-sd

Place all objects in small data-sections.

-gas

Create code suitable for the GNU assembler.

-no-align-args

Do not align function arguments on the stack stricter than 4 bytes. Default with ‘-poweropen’.

-use-commons

Use real common symbols instead of bss symbols for non-initialized external variables.

-baserel32os4

Use 32bit base-relative addressing as used by AmigaOS 4.

-baserel32mos

Use 32bit base-relative addressing as used by MorphOS.


5.2 ABI

This backend supports the following registers:

Additionally, the register pairs r3/r4, r5/r6, r7/r8, r9/r10, r14/r15, r16/r17, r18/r19, r20/r21, r22/r23, r24/r25, r26/r27, r28/r29 and r30/r31 are available.

r0, r11, r12, f0, f12 and f13 are reserved by the backend.

The current version generates assembly output for use with ‘vasmppc’ or the GNU assembler. The generated code should work on 32bit systems based on a PowerPC CPU using the V.4 ABI or the PowerPC Embedded ABI (eabi).

The registers r0, r3-r12, f0-f13 and cr0-cr1 are used as scratch registers (i.e. they can be destroyed in function calls), all other registers are preserved. r1 is the stack-pointer and r13 is the small-data-pointer if small-data-mode is used.

The first 8 function arguments which have integer or pointer types are passed in registers r3 through r10 and the first 8 floating-point arguments are passed in registers f1 through f8. All other arguments are passed on the stack.

Integers and pointers are returned in r3 (and r4 for long long), floats and doubles in f1. 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 (-amiga-align)

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

5.3 Target-specific variable-attributes

The PPC-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.

__saveall

Make sure all registers are saved by this function. On lower optimization levels, all volatile registers will be saved additionally. On higher levels, only the ones that may be destroyed, are saved.

__interrupt

Return with en rfi-instruction rather than blr.

__section("name","attr")

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


5.4 Target-specific pragmas

The PPC-backend offers the following #pragmas:

#pragma amiga-align

Set alignment like -amiga-alignment option.

#pragma natural-align

Align every type to its own size.

#pragma default-align

Set alignment according to command-line options.


5.5 Predefined Macros

This backend defines the following macros:

__PPC__

5.6 Stack

If the ‘-stack-check’ option is used, every function-prologue will call the function __stack_check with the stacksize needed by this function in register r12. This function has to consider its own stacksize and must restore all registers.


5.7 Stdarg

A possible <stdarg.h> for V.4 ABI could look like this:

 
typedef struct {
  int gpr;
  int fpr;
  char *regbase;
  char *membase;
} va_list;

char *__va_start(void);
char *__va_regbase(void);
int __va_fixedgpr(void);
int __va_fixedfpr(void);

#define va_start(vl,dummy) \
  ( \
    vl.gpr=__va_fixedgpr(), \
    vl.fpr=__va_fixedfpr(), \
    vl.regbase=__va_regbase(), \
    vl.membase=__va_start() \
  )

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

#define va_copy(new,old) ((new)=(old))

#define __va_align(type) (__alignof(type)>=4?__alignof(type):4)

#define __va_do_align(vl,type) ((vl).membase=(char *)((((unsigned int)((vl).membase))+__va_align(type)-1)/__va_align(type)*__va_align(type)))

#define __va_mem(vl,type) (__va_do_align((vl),type),(vl).membase+=sizeof(type),((type*)((vl).membase))[-1])

#define va_arg(vl,type) \
 ( \
  (__typeof(type)&127)>10? \
    __va_mem((vl),type) \
  : \
   ( \
    (((__typeof(type)&127)>=6&&(__typeof(type)&127)<=8)) ? \
     ( \
      ++(vl).fpr<=8 ? \
        ((type*)((vl).regbase+32))[(vl).fpr-1] \
      : \
        __va_mem((vl),type) \
     ) \
    : \
     ( \
      ++(vl).gpr<=8 ? \
        ((type*)((vl).regbase+0))[(vl).gpr-1] \
      : \
        __va_mem((vl),type) \
     ) \
   ) \
 )

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

 
typedef unsigned char *va_list;

#define __va_align(type) (4)

#define __va_do_align(vl,type) ((vl)=(char *)((((unsigned int)(vl))+__va_align(type)-1)/__va_align(type)*__va_align(type)))

#define __va_mem(vl,type) (__va_do_align((vl),type),(vl)+=sizeof(type),((type*)(vl))[-1])

#define va_start(ap, lastarg) ((ap)=(va_list)(&lastarg+1))
 
#define va_arg(vl,type) __va_mem(vl,type)
 
#define va_end(vl) ((vl)=0)

#define va_copy(new,old) ((new)=(old))


5.8 Known problems


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