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Re: Bad choices by expand_mult_highpart


Richard Sandiford wrote:

>        * expmed.c (choose_mult_variant, expand_mult_const): New, split from...
>        (expand_mult): ...here.
>        (extract_high_half): New, split out from expand_mult_highpart.
>        (expand_highpart_optab): Likewise.  Don't clobber target prematurely.
>        (expand_highpart): Evaluate the cost of a shift/add sequence,
>        then see if any of the specialized optabs are cheaper.

This somehow yields incorrect arithmetic results on s390x; the test case:

long var = 1998;

int main (void)
{
  printf ("%ld %ld\n", var / 60, var % 60);
  return 0;
}

now returns
95 -3702
instead of
33 18

I haven't quite understood why yet (in fact, I haven't quite understood the
whole division-by-multiplication logic in detail yet), but reverting your
patch makes the problem go away ...

Any idea?

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  weigand@informatik.uni-erlangen.de


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