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List:       linux-kernel
Subject:    Re: goto's ???
From:       Mofeed Shahin <shahin () labf ! org>
Date:       1999-05-25 13:49:55
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Thank you for taking the time to explain. The gcc -S option is a great
debugging tool, which I have just added to my debugging methods, Thank
you. 

I new there was a reason for the code being written in such a way. I am
glad I asked the question, as know I can actually write faster code.

Cheers Mof.

On Tue, 25 May 1999, Richard B. Johnson wrote:

> Compile questionable code with the -S option, with output to a file.
> Check the assembly. Usually, the gotos are important to maintain
> straight-through code for the normal case. If you put a return in
> the middle of the code, it doesn't return. It jumps to somewhere
> where the saved registers are restored, the stack leveled, and then
> a return. In other words, there is only one exit path anyway. So
> the designer designed in the exit path so the compiler is less likely
> to muck with the straight through code.
> 
> Given:
> 
> int foo(int bar)
> {
>     if (bar == 1)
>         goto end;
>     if (bar == 2)
>         goto end;
>     printf("%d\n", bar);
>    end:;
>    return 0;
> }
> 
> You get:
> 
> foo:
> 	pushl %ebp
> 	movl %esp,%ebp
> 	cmpl $1,8(%ebp)		; if (bar == 1)
> 	jne .L2			; Not a 1, try next
> 	jmp .L3			; This is the goto
> 	.align 4
> .L2:
> 	cmpl $2,8(%ebp)		; if (bar == 2)
> 	jne .L4                 ; Not a 2
> 	jmp .L3			; This is the goto
> 	.align 4
> .L4:
> 	movl 8(%ebp),%eax	; This is where we print it.
> 	pushl %eax
> 	pushl $.LC0
> 	call printf
> 	addl $8,%esp
> .L3:				; Common exit
> 	xorl %eax,%eax
> 	jmp .L1
> 	.align 4
> .L1:
> 	movl %ebp,%esp
> 	popl %ebp
> 	ret
> 
> Now, you may say; "Nobody writes code like that!". True, it's a demo,
> one would probably write such trivial code as:
> 
> int foo(int bar)
> {
>     if ((bar != 1) && (bar != 2))
>         printf("%d\n", bar);
>    return 0;
> }
> 
> How, guess what this produces?!!
> 
> foo:
> 	pushl %ebp
> 	movl %esp,%ebp
> 	cmpl $1,8(%ebp)		 ; (foo != 1)
> 	je .L2			 ; The goto
> 	cmpl $2,8(%ebp)		 ; (foo != 2)
> 	je .L2                   ; Another goto
> 	movl 8(%ebp),%eax        ; Now we print it.
> 	pushl %eax
> 	pushl $.LC0
> 	call printf
> 	addl $8,%esp
> .L2:
> 	xorl %eax,%eax
> 	jmp .L1
> 	.align 4
> .L1:
> 	movl %ebp,%esp
> 	popl %ebp
> 	ret
> .Lfe1:
> 
> 
> By checking what the compiler produces, you can see if it might be
> best to put a goto in your code even though it might bring up shreeks
> of horror from some.
> 
> You will find that just ordering something a bit differently, while
> maintaining the same logic, may produce a better straight-through path.
> For instance everybody loves 'switches'. They make coding clear and
> straight forward, however, unless the 'case' is sequential, better
> code may be obtained by using 'if() else if()'.
> 
> Cheers,
> Dick Johnson
>                  ***** FILE SYSTEM WAS MODIFIED *****
> Penguin : Linux version 2.2.6 on an i686 machine (400.59 BogoMips).
> Warning : It's hard to remain at the trailing edge of technology.
> 


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