RE: gcc loop unrolling problem ??

["Konark Goel, Noida" <konarkg at noida dot hcltech dot com>]

Actually -O3 generated code looks better than the code generated by -O3 and
- funroll-all-loops condition combined. it is better optimized, so i think
it both the conditions should generate the code generated by -O3 as it is
more optimized.
Also why is the loop being unrolled only 8 times ? even after changing
switch values it unrolls to 8 times only...? is there something special
about this no. 8 ?
 
thanks,
Kon ??
"The answer is out there....,
  Its the question that drives us...."


-----Original Message-----
From: Jan Hubicka [mailto:hubicka@ucw.cz]
Sent: Thursday, February 12, 2004 4:14 PM
To: Konark Goel, Noida
Cc: gcc@gcc.gnu.org
Subject: Re: gcc loop unrolling problem ??


> 
> I compiled a gcc testcase with only -O3 option and also with -O3 and
> -funroll-all-loops options combined.... For the first case (-O3 option) it
> is generating quite a correct assembly...but in case of -funroll-all-loops
> it is generating quite a no. of calls, which actually is not needed. I
guess
> there's some problem in it. Also the no. of calls being generated is
> dependent upon the minimum and maximum values in the switch cases !! 
> 
> the testcase is this : 
> 
> /* TestCase starts here */
> extern int getch();
> extern int class();
> 
> int
> token()
> {
>     int state = 1;
> 
>     while (1) {
> 	int c=0;
> 	c = getch();
> 	switch (state) {
> 	case 1: break;
> 	case 4: break;
> 	case 5: break;
> 	case 6: 
>             {
> 	        switch (class(c)) {
> 	        default: break;
> 	        }
> 	    } break;
> 	case 7:	break;
> 	}
>     }
> }
> /* TestCase ends.....*/
> 
> 
> ;Assembly for -O3 -funroll-all-loops case ....
> 
> 	.file	"990801-1.c"
> 	.text
> 	.p2align 2,,3
> .globl token
> 	.type	token,@function
> token:
> 	pushl	%ebp
> 	movl	%esp, %ebp
> 	subl	$8, %esp
> 	.p2align 2,,3
> .L16:
> 	call	getch
> 	call	getch
> 	call	getch
> 	call	getch
> 	call	getch
> 	call	getch
> 	call	getch
> 	call	getch
> 	jmp	.L16
> .Lfe1:
> 	.size	token,.Lfe1-token
> 	.ident	"GCC: (GNU) 3.2.2 20030222 (Red Hat Linux 3.2.2-5)"
> 
> 
> ;assembly for only -O3 condition...
> 
> .file	"990801-1.c"
> 	.text
> 	.p2align 2,,3
> .globl token
> 	.type	token,@function
> token:
> 	pushl	%ebp
> 	movl	%esp, %ebp
> 	subl	$8, %esp
> 	.p2align 2,,3
> .L16:
> 	call	getch
> 	jmp	.L16
> .Lfe1:
> 	.size	token,.Lfe1-token
> 	.ident	"GCC: (GNU) 3.2.2 20030222 (Red Hat Linux 3.2.2-5)"
> 
> ;assembly ends..
> 
> 
> as is clear it creates a strange assembly with -O3 -funroll-all-loops
> option......Is this a BUG ??

I think it is just result of unrolling+constant propagation of "state",
so it looks like valid transformation to me.  Why do you think it is
strange?

Honza
> 
> hoping to get a quick response..
> 
> thanks,
> Kon ??
> "The answer is out there....,
>   Its the question that drives us...."