| Message ID | 3615860.z70LMau7zY@polaris |
|---|---|
| State | New |
| Headers | show |
On Tue, May 27, 2014 at 10:29 PM, Eric Botcazou <ebotcazou@adacore.com> wrote: > This is an old bug in div_and_round_double for ROUND_DIV_EXPR: when the code > detects that it needs to adjust the quotient, it needs to decide whether it > increases or decreases it by 1. This only depends on the expected sign of the > quotient, but the test reads: > > if (*hquo < 0) > > So if the quotient is 0, the code will always bump it, thus yielding 1, even > if the expected quotient is negative. This causes the attached Ada testcase > to fail at -O on all active branches... except for mainline, because of the > correct implementation of the same test in wi::div_round: > > if (wi::neg_p (x, sgn) != wi::neg_p (y, sgn)) > return quotient - 1; > else > return quotient + 1; > > It turns out that div_and_round_double has a predicate quo_neg that implements > the same test as the wide-int one and ought to be used here. > > Tested on x86_64-suse-linux (of course this probably doesn't mean anything on > the mainline, but it was also test with 4.7 and 4.9 compilers) and applied on > the mainline as obvious. I suppose you also install on branches? Thanks, Richard. > > 2014-05-27 Eric Botcazou <ebotcazou@adacore.com> > > * double-int.c (div_and_round_double) <ROUND_DIV_EXPR>: Use the proper > predicate to detect a negative quotient. > > > 2014-05-27 Eric Botcazou <ebotcazou@adacore.com> > > * gnat.dg/overflow_fixed.adb: New test. > > > -- > Eric Botcazou
> I suppose you also install on branches?
No plan to do so since this isn't a regression, unless you insist. :-)
On Wed, May 28, 2014 at 12:17 PM, Eric Botcazou <ebotcazou@adacore.com> wrote: >> I suppose you also install on branches? > > No plan to do so since this isn't a regression, unless you insist. :-) Well, a wrong-code bug plus a very obvious fix certainly qualifies. Richard. > -- > Eric Botcazou
> Well, a wrong-code bug plus a very obvious fix certainly qualifies.
Fine with me, onto which branch(es) do you want me to put it?
On Wed, May 28, 2014 at 12:23 PM, Eric Botcazou <ebotcazou@adacore.com> wrote: >> Well, a wrong-code bug plus a very obvious fix certainly qualifies. > > Fine with me, onto which branch(es) do you want me to put it? Where you have tested it already, no need to spend extra cycles. Richard. > -- > Eric Botcazou
Index: double-int.c =================================================================== --- double-int.c (revision 210911) +++ double-int.c (working copy) @@ -588,7 +588,7 @@ div_and_round_double (unsigned code, int == (unsigned HOST_WIDE_INT) htwice) && (labs_den <= ltwice))) { - if (*hquo < 0) + if (quo_neg) /* quo = quo - 1; */ add_double (*lquo, *hquo, (HOST_WIDE_INT) -1, (HOST_WIDE_INT) -1, lquo, hquo);