Re: Re: Modifies vs. Replaces

[brender at gemgrp dot zko dot dec dot com (Ron 603-884-2088)]

Michael Eager wrote:
>If Compaq's compilers generate code which has lifetime holes for
>global variables, could you provide a reasonable example?  Perhaps
>with both the source and assembly code for the example?

Following is a compilation listing that should illustrate lifetime holes
for global variables.

Lines begining with ">>>" are annotations that I added to help explain
what is going on in the code.

I believe everything shown is consistent with what Jim Denhert described
(and what I was trying to describe, perhaps less well).

The listing is 132-columns wide...

                                Source Listing                  27-Mar-2001 16:26:20  Compaq C++ X6.3-221-48B3R         Page 1
                                                                27-Mar-2001 16:17:12  global-holes.c

	      1 
	      2 int i, j, k, l;
	      3 
	      4 extern void init(void);
	      5 extern void doprint(void);
	      6 
	      7 main(void) {
	      8 
	      9     init();
	     10 
	     11     k = i + 1;
	     12     j = k/2;
	     13     j = i + k;
	     14     l = k * 5;
	     15    
	     16     doprint();
	     17 }
	     18 

                                Machine Code Listing            27-Mar-2001 16:26:20  Compaq C++ X6.3-221-48B3R         Page 2
                                main                            27-Mar-2001 16:17:12  global-holes.c

				.globl  main
				.ent 	main
	     0000	main:
27BB0000     0000		ldah	gp, main				# gp, (r27)
23BD0000     0004		lda	gp, main				# gp, (gp)
	     0008	L$2:
23DEFFF0     0008		lda	sp, -16(sp)
B75E0000     000C		stq	r26, (sp)
				.mask 0x04000000,-16
				.fmask 0x00000000,0
				.frame  $sp, 16, $26
				.prologue 1
>>>
>>> All prologue up to here
>>>
A77D0000     0010		ldq	r27, init__Xv				# r27, (gp)				    # 000009
6B5B4000     0014		jsr	r26, init__Xv				# r26, (r27)
>>>
>>> Load the address of init from the GOT and call it
>>>
27BA0000     0018		ldah	gp, main				# gp, (r26)
23BD0000     001C		lda	gp, main				# gp, (gp)
>>>
>>> Restablish local got pointer
>>>
A41D0000     0020		ldq	r0, i					# r0, (gp)				    # 000011
A4FD0000     0024		ldq	r7, j					# r7, (gp)				    # 000013
A4DD0000     0028		ldq	r6, l					# r6, (gp)				    # 000014
A47D0000     002C		ldq	r3, k					# r3, (gp)				    # 000011
>>>
>>> Load the addresses of i, j, k, and l respectively from the GOT
>>>
A77D0000     0030		ldq	r27, doprint__Xv			# r27, (gp)				    # 000016
>>>
>>> Load the address of doprint from the GOT
>>>
A0200000     0034		ldl	r1, i					# r1, (r0)				    # 000011
>>>
>>> Load value of i into r1
>>>
40203002     0038		addl	r1, 1, r2
>>>
>>> Add 1 to the value of i and store it in k
>>> At this point r2 is *the* location of k; the global location is irrelevant for a while
>>>
40220001     003C		addl	r1, r2, r1									    # 000013
>>>
>>> Add the value of i (from r1) to the value of k (from r2) and store result in j (using r1 for a new purpose)
>>> At this point r1 is *the* location of j; the global location is irrelevant for a while
>>>
>>> Note that the first assignment to j (line 12) is dead and optimized away
>>> Also, the copy of i previously in r1 is no longer of interest
>>>
40420045     0040		s4addl	r2, r2, r5									    # 000014
>>>
>>> Multiply the value of k (from r2) by 5 (using scale by 2 and add of itself) and store result in l (in r5)
>>> At this point r5 is *the* location of l; the global location is irrelevant
>>>
B0430000     0044		stl	r2, k					# r2, (r3)				    # 000011
B0270000     0048		stl	r1, j					# r1, (r7)				    # 000013
B0A60000     004C		stl	r5, l					# r5, (r6)				    # 000014
>>>
>>> Copy the values of k, j, and l back to their global locations
>>>
6B5B4000     0050		jsr	r26, doprint__Xv			# r26, (r27)				    # 000016
>>>
>>> Call doprint
>>>
27BA0000     0054		ldah	gp, main				# gp, (r26)
A75E0000     0058		ldq	r26, (sp)									    # 000017
23DE0010     005C		lda	sp, 16(sp)
47FF0400     0060		clr	r0
23BD0000     0064		lda	gp, main				# gp, (gp)				    # 000016
6BFA8001     0068		ret	(r26)										    # 000017
>>>
>>> Restore gp pointer for main, tear down the stack and return
>>>
				.end 	main

Routine Size: 108 bytes,    Routine Base: .text + 0000

				.comm	i 4
				.comm	j 4
				.comm	k 4
				.comm	l 4