From: GAMMELJL AT SLU DOT EDU
Date: Mon, 31 Aug 1998 18:04:28 -0500 (CDT)
Subject: passing arguments in registers
To: djgpp AT delorie DOT com
Message-id: <01J19GJU8Q9U94JXNE@SLU.EDU>
Organization: SAINT LOUIS UNIVERSITY  St. Louis, MO
MIME-version: 1.0
Precedence: bulk

The following C/C++ code when compiled:
                gxx example9.cc -S -O2
results in the .s file below it.  It is plain that the arguments
of zadd are passed on the stack.  Eli Zareteskii has told me that
passing them in registers requires
                 __attribute__((regparm=2))
and that the information about this is in the GCC docs.  Please:
someone expert in searching these docs tell me exactly where this
attribute statement goes in the C++ code below and exactly what the
correct syntax is.

____________________The C/C++ source code_______________________________

extern zadd(int m,int n); ------>one wishes to place the attribute
int i,j,k;                       statement in this subroutine either
                                 here or below.
main()
 {i=2;           ------> adds 2 and 3.
  j=3;
  k=zadd(i,j);
  return 0;
 }

zadd(int m,int n)   --------> one wishes to place the attriute statement
  {return m+n;}               in this subroutine

____________________The .s file produced_________________________________

	.file	"example9.cc"
gcc2_compiled.:
___gnu_compiled_cplusplus:
.text
	.align 2
.globl _main
_main:
	pushl %ebp
	movl %esp,%ebp
	call ___main
	movl $2,_i
	movl $3,_j
        pushl $3     -------------> here 2 and 3 are pushed to stack
	pushl $2
        call _zadd__Fii  ----------> zadd is called
        movl %eax,_k     -----------> result returned in %eax
	xorl %eax,%eax
	leave
	ret
	.align 2
.globl _zadd__Fii
_zadd__Fii:
        pushl %ebp         ------> standard entry to assembly subroutine 
	movl %esp,%ebp
        movl 8(%ebp),%eax  ---------> arguments taken from stack
        addl 12(%ebp),%eax            added, and return in %eax
        leave               --------> standard exit from assembly
        ret                           routine
.globl _i
.data
	.align 2
_i:
	.space 4
.globl _j
	.align 2
_j:
	.space 4
.globl _k
	.align 2
_k:
	.space 4