[sdcc-devel] Re: [sdcc - Open Discussion] Does sdcc produce correct code?

[Johan K. <joh...@id...>]

This was an interesting case.

This needs some explanation: the first loop "for (i=0xff; --i; ) { body }"
was translated into "i=0xff; while (--i) { body }" wich isn't really a loop
but a simple ifx and a goto (as is ofcourse the outer while loop). So the
preheader of the second loop "for (j=0xff; --j; ) { body }" was moved out
side the first loop. This really confused addToExitsMarkDepth(), which
refused to increase the loop Depth and therefore limited the live range of
the loop variables to where the body begins (unless ofcourse the var was
used inside the body but only up untill that point).

Anyway, I did something in SDCCloop.c:addToExitsMarkDepth():1.16 that makes
sense to me.

Johan

P.S. and I disabled the stupid fix for bug #467035, so it's there again for
all to see


----- Original Message -----
From: <no...@so...>
To: <no...@so...>
Sent: Wednesday, October 10, 2001 3:22 AM
Subject: [sdcc - Open Discussion] Does sdcc produce correct code?


>
> Read and respond to this message at:
> http://sourceforge.net/forum/message.php?msg_id=246354
> By: sault
>
> I am having serious problems. Just started using sdcc with an 8051 as the
target.
> Works sometimes, not others. I have found a piece of code C code for which
the
> compiler does not appear to produce correct assembly - creating an
infinite
> loop. Both sdcc 2.2.1 and 2.3.0 do the same - they use the r3 register for
both
> the inner and outer loop or a delay. P2 lines are connected to 8 LEDs so I
should
> see binary counting.
>
>
> #include <at89x52.h>
> #include <stdio.h>
> int
> main()
> {
>         unsigned char i, j, c = 0;
>
>         while (1) {
>                 P2 = c++;
>
>                 for (i = 0xff; --i; ) {
>                         for (j = 0xff; --j; );
>                 }
>         }
> }
>
>
>
>
> ;       -----------------------------------------
> ;        function main
> ;       -----------------------------------------
> _main:
>         ar2 = 0x02
>         ar3 = 0x03
>         ar4 = 0x04
>         ar5 = 0x05
>         ar6 = 0x06
>         ar7 = 0x07
>         ar0 = 0x00
>         ar1 = 0x01
>
>         mov     r2,#0x00
> 00102$:
>         mov     _P2,r2
>         inc     r2
>         mov     r3,#0xFF
> 00107$:
>         dec     r3
>         mov     a,r3
>         jz  00102$
> 00117$:
>         mov     r3,#0xFF
> 00104$:
>         dec     r3
>         mov     a,r3
>         jz  00107$
> 00118$:
>         sjmp 00104$
> 00110$:
>         ret
>
> ;-----
>
> The problem goes away if the time wasting loop is written as:
>
> for (i = 0xff; i; i--) {
> for (j = 0xff; j; j--);
> }
>
> because it then uses two different registers for outer and inner loops.
The
> compiler does even stranger things  if written as:
>
> for (i = 0xff; i--; ) {
> for (j = 0xff; j--; );
> }
>
> by putting in useless assembly expressions.
>
> Am I expecting too much of this compiler? How should one write C code to
work
> with this compiler?
>
> Saul.
>
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