Bug using -O2 with cross compiler

[Ed Hepler]

I wrote:

> I believe that I have found a bug when using GCC as a cross compiler
> to target the MC68000 from a Linux platform.
> 
> I am using gcc-2.95.2
> 
> The gcc command is:
> 
>   m68k-coff-gcc -m68000 -g -ggdb -O2 -c blink_led.c
> 
> The code fragment being compiled is:
> 
> #define DELAY 5000000
> 
> void
> delay() {
>   int i;
> 
>   for(i= 0; i < DELAY; i++);
> }
> 
> 
> The produced code looks like:
> 
> 00000408 <delay>:
>  408:   4e56 0000       linkw %fp,#0
>  40c:   203c 004c 4b3f  movel #4999999,%d0
>  412:   51c8 fffe       dbf %d0,412 <delay+0xa>
>  416:   4240            clrw %d0
>  418:   5380            subql #1,%d0
>  41a:   64f6            bccs 412 <delay+0xa>
>  41c:   4e5e            unlk %fp
>  41e:   4e75            rts
> 
> 
> The problem(s):
> 
> 1. The dbcc instruction only acts upon the low order 16 bits of the
>    specified register.  So, loading an initial value into D0 which
>    extends into the upper half of the register is a problem...
> 
> 2. I'm not quite sure why the compiler generates the instructions at
>    locations 416, 418, and 41a...  In fact, don't these instructions
>    put this into an infinite loop?  
> 
>    416: clears D0
>    418: subtracts 1 from D0 (leaving 0xffffffff in D0, no carry out)
>    41a: branches on carry clear (always) back to the beginning of the
>         original loop...

Whoops... There is no carry, but a borrow, so this doesn't result in
an infinite loop...

>    I suspect that these extra instructions were supposed to handle the
>    upper 16 bits of the register???  but are maybe somehow misguided???

But I still don't understand why the code was emitted...
 
> The code produced with -O or -O1 seems to be OK...

Problem 1 remains...

Thanks,

Ed Hepler


Dr. Edward L. Hepler
  President,                     Adjunct Professor, 
  VLSI Concepts, Inc.            Villanova University Graduate Courses:
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      Architecture, Design,          ECE-8445 Advanced Computer Architecture
      and CAD                        ECE-8460 VLSI Design
email: hepler@vlsi-concepts.com  or  elh@ece.vill.edu
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