Short code to determine compiler's

[mcdonald at uxe.cso.uiuc.edu]

>Some students here had to determine the number of registers (data 
>and address, we use 680x0's) the C compiler uses.  A friend and
>I wrote the following code to show to some students having trouble.
>It is very short and simple, but it seems to work.  The only logical
>next step is to post it to comp.lang.c and have it torn apart!



>#include <stdio.h>

>main()
>{
>    {
>        int count1; /* Base of stackframe */

>        register x1, x2, x3, x4, x5, x6, x7, x8, x9, x10,
>                 x11, x12, x13, x14, x15, x16, x17, x18, x19;
>        {
>            int count2;
>            printf("Number of data registers = %d\n", 19 - abs(&count1 - &count2) + 1);
>        }
>    }

(code for int * deleted)

Oh my!!!! What on earth makes you think that this will give the
number of register variables actually used? If tried on Microsoft C 5.1
it gives 9 and 9. But, actually Microsoft C uses at most 2 register
variables. Just look at the assembler output of the compiler:

(header removed)
_DATA      SEGMENT
$SG180	DB	'Number of data registers = %d',  0aH,  00H
$SG202	DB	'Number of address registers = %d',  0aH,  00H
_DATA      ENDS
_TEXT      SEGMENT
	ASSUME	CS: _TEXT
; Line 16
	PUBLIC	_main
_main	PROC NEAR
	push	bp
	mov	bp,sp
	mov	ax,42
	call	__chkstk
; Line 17
;	count1 = -20   (These are the offsets from bp of the local variables)
;	x1 = -14
;	x2 = -18
;	x3 = -24
;	x4 = -28
;	x5 = -32
;	x6 = -36
;	x7 = -40
;	x8 = -4
;	x9 = -8
;	x10 = -12
;	x11 = -16
;	x12 = -22
;	x13 = -26
;	x14 = -30
;	x15 = -34
;	x16 = -38
;	x17 = -2
;	x18 = -6
;	x19 = -10
; Line 22
;	count2 = -42
; Line 25
	mov	ax,11
	push	ax
	call	_abs
	add	sp,2
	sub	ax,20
	neg	ax
	push	ax
	mov	ax,OFFSET DGROUP:$SG180
	push	ax
	call	_printf
	add	sp,4

Nothing in the C language says where variables have to go - putting
an & in front of two different variables (not in an array or struct)
and subtracting the resulting pointers
is a meaningless exercise. Somebody might
assign variables random addresses!!!!! Somebody (i.e. Microsoft) DID!!!!!

If the original writer has "students", that seems to imply that he
is a "teacher". I would hope that anyone teaching C would know this.

In fact, the really interesting question is, in legal C, is it
even POSSIBLE to write a program to see how many registers are used?

Doug McDonald