Re: Help with integer division overflow issue

[Paul Ripke <stix%stix.id.au@localhost>]

On Thu, Jan 11, 2024 at 06:39:41PM -0800, Jason Thorpe wrote:
> 
> > On Jan 9, 2024, at 7:22?AM, Jason Thorpe <thorpej%me.com@localhost> wrote:
> > 
> >> On Jan 9, 2024, at 7:14?AM, Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost> wrote:
> >> 
> >> milan-% cc -o test test.c
> >> milan-% ./test
> >> milan-% 
> >> ---
> >> 
> >> I.e. at least looks no exception.
> > 
> > Yes, same with TME emulating a Sun3/160.
> 
> Ok, can you try this one?
> 
> #include <limits.h>
> #include <stdlib.h>
> #include <stdio.h>
> 
> int divisor = -1;
> 
> int
> main(int argc, char *argv[])
> {
> int val = INT_MIN;
> int ccr = 0;
> 
> __asm __volatile("divsl divisor,%1" : "=r" (val) : "0" (val));
> __asm __volatile("move %%ccr,%0" : "=r" (ccr));
> 
> printf("val = %d (0x%08x)\n", val, val);
> printf("ccr = 0x%02x\n", ccr);
> 
> return 0;
> }
> 
> On TME, the result is:
> 
> sun3-160$ ./a.out
> val = -2147483648 (0x80000000)
> ccr = 0x02
> sun3-160$ 
> 
> The V flag is set?but I?m a little surprised that the N flag is not set!

Although that seems legal from the docs I have?

...

DIVS, DIVSL

... Overflow may be detected and set before the instruction completes. If the in-
struction detects an overflow, it sets the overflow condition code, and the oper-
ands are unaffected.

...

N - Set if the quotient is negative; cleared otherwise; undefined if overflow or divide
by zero occurs.
Z - Set if the quotient is zero; cleared otherwise; undefined if overflow or divide by
zero occurs.
V - Set if division overflow occurs; undefined if divide by zero occurs; cleared oth-
erwise.

-- 
Paul Ripke
"Great minds discuss ideas, average minds discuss events, small minds
 discuss people."
-- Disputed: Often attributed to Eleanor Roosevelt. 1948.