Re: c/8639: simple integer arithmetic expression broken

["Christian Ehrhardt" <ehrhardt at mathematik dot uni-ulm dot de>]

On Tue, Nov 19, 2002 at 02:42:14PM -0000, steveJepsen@netscape.net wrote:
> >Number:         8639
> >Category:       c
> >Synopsis:       simple integer arithmetic expression broken

This is:
a) real
b) present in 3.2, 3.2.1 and 3.3
c) present at least in C and C++
d) A regression from 2.95.2
d) arch independant (!)
e) A show stopper (IMHO)

Someone please mark this as confirmed, priority high. It should probably
also be reclassified as middle end.

> >Confidential:   no
> >Severity:       critical
> >Priority:       medium
> >Responsible:    unassigned
> >State:          open
> >Class:          wrong-code
> >Submitter-Id:   net
> >Arrival-Date:   Tue Nov 19 06:46:02 PST 2002
> >Closed-Date:
> >Last-Modified:
> >Originator:     Steve Jepsen
> >Release:        unknown-1.0
> >Organization:
> >Environment:
> gcc version 3.2 20020903 (Red Hat Linux 8.0 3.2-7)
> >Description:
> the expression:
> 
>    bug_result = (80 - 4 * intvar) / 20;
> 
> with intvar == 1 returns 4 (should return 3)

I did a few experiments with expression of the form (A-B*intvar)/C with
A, B and C beeing constant. In these expressions the Bug only seems to
happen if C divides A and B divides C. According to the assembler output
this is rewritten as A/C - intvar/(C/B) which is NOT the same.
A/C - (intvar-1+C/B)/(C/B) would be the same as far as I can see.

A minimal example is this:

int main ()
{
	int x = 1;
	int res = (80 - 4*x)/20;
	return res;
}

Compiled with gcc t.c this gives:
elbereth$ gcc -Wall t.c
elbereth$ a.out
elbereth$ echo $? 
4
elbereth$ 

Here's the asm output on sparc which shows that the expression
is rewritten as (20 - x/5).

main:
        !#PROLOGUE# 0
        save    %sp, -120, %sp
        !#PROLOGUE# 1
        mov     1, %o0
        st      %o0, [%fp-20]
        ld      [%fp-20], %o0
        sll     %o0, 2, %o1
        mov     81, %o0
        sub     %o0, %o1, %o0
        mov     20, %o1
        call    .div, 0
         nop
        st      %o0, [%fp-24]
        ld      [%fp-24], %o0
        mov     %o0, %i0
        nop
        ret
        restore


   regards    Christian