From gnat@frii.com

Until you do it in separate steps, 'time % -60' is never negative.
This smells like a spurious NV conversion. Somewhere.

Nat

From [Unknown Contact. See original ticket]

Nathan Torkington writes​:

perl -le 'print time % -60'
perl -le '$x = time; print $x % -60'
perl -le '$x = time; $y = $x % -60; print $y'

Until you do it in separate steps, 'time % -60' is never negative.
This smells like a spurious NV conversion. Somewhere.

Is not it that time may take an argument, and % may preceed an identifier?

Ilya

From [Unknown Contact. See original ticket]

Ilya Zakharevich writes​:

perl -le 'print time % -60'
perl -le '$x = time; print $x % -60'
perl -le '$x = time; $y = $x % -60; print $y'

Is not it that time may take an argument, and % may preceed an identifier?

I don't think so. The second case, which doesn't give an argument to
time(), prints the same thing as the first case.

Nat

From [Unknown Contact. See original ticket]

Nathan Torkington <gnat@​frii.com> wrote

perl -le 'print time % -60'
perl -le '$x = time; print $x % -60'
perl -le '$x = time; $y = $x % -60; print $y'

Until you do it in separate steps, 'time % -60' is never negative.
This smells like a spurious NV conversion. Somewhere.

Nope. It's a false optimisation. The last time it showed up, the
example was something like

perl -le 'print time; print time*1000'

I don't recall the details, but basically Perl is saying something like
"I know time() returns an IV, so I know the operation (* or %) is
on integers, so I'll force `use integer'". So you're getting the
altered semantics that you get from `use integer'.

I also don't recall why this didn't get fixed - whether it was just
lack of tuits, lack of understanding of the relevant code (that's my
excuse), or a desire to put performance above correct semantics.

Mike Guy

From @gsar

On Sat, 23 Oct 1999 16​:24​:17 BST, "M.J.T. Guy" wrote​:

Nathan Torkington <gnat@​frii.com> wrote

perl -le 'print time % -60'
perl -le '$x = time; print $x % -60'
perl -le '$x = time; $y = $x % -60; print $y'

Until you do it in separate steps, 'time % -60' is never negative.
This smells like a spurious NV conversion. Somewhere.

Nope. It's a false optimisation. The last time it showed up, the
example was something like

perl -le 'print time; print time*1000'

I don't recall the details, but basically Perl is saying something like
"I know time() returns an IV, so I know the operation (* or %) is
on integers, so I'll force `use integer'". So you're getting the
altered semantics that you get from `use integer'.

I also don't recall why this didn't get fixed - whether it was just
lack of tuits, lack of understanding of the relevant code (that's my
excuse), or a desire to put performance above correct semantics.

AFAIK this particular bug in modulus has never come up before, or I
would have fixed it. :-) Note that the "consistent C modulus"
behavior based on C<use integer> was something that got added in
5.004. Pity the testsuite isn't good enough to have caught this. :-(

Sarathy
gsar@​ActiveState.com

-----------------------------------8<-----------------------------------
Change 4428 by gsar@auger on 1999/10/23 20:28:56

	fix accidental C modulo semantics on integer-valued operations
	(e.g. caused C<length("abc") % -10> to return 3 rather than -7)

Affected files ...

... //depot/perl/op.c#205 edit
... //depot/perl/t/op/int.t#6 edit

Differences ...

==== //depot/perl/op.c#205 (text) ====
Index: perl/op.c
--- perl/op.c.~1~	Sat Oct 23 13:29:02 1999
+++ perl/op.c	Sat Oct 23 13:29:02 1999
@@ -2112,8 +2112,12 @@
 	return o;
 
     if (!(PL_hints & HINT_INTEGER)) {
-	if (type == OP_DIVIDE || !(o->op_flags & OPf_KIDS))
+	if (type == OP_MODULO
+	    || type == OP_DIVIDE
+	    || !(o->op_flags & OPf_KIDS))
+	{
 	    return o;
+	}
 
 	for (curop = ((UNOP*)o)->op_first; curop; curop = curop->op_sibling) {
 	    if (curop->op_type == OP_CONST) {

==== //depot/perl/t/op/int.t#6 (xtext) ====
Index: perl/t/op/int.t
--- perl/t/op/int.t.~1~	Sat Oct 23 13:29:02 1999
+++ perl/t/op/int.t	Sat Oct 23 13:29:02 1999
@@ -1,9 +1,7 @@
 #!./perl
 
-# $RCSfile: int.t,v $$Revision: 4.1 $$Date: 92/08/07 18:28:00 $
-
-print "1..4\n";
+print "1..6\n";
 
 # compile time evaluation
 
 if (int(1.234) == 1) {print "ok 1\n";} else {print "not ok 1\n";}
@@ -15,3 +13,12 @@
 $x = 1.234;
 if (int($x) == 1) {print "ok 3\n";} else {print "not ok 3\n";}
 if (int(-$x) == -1) {print "ok 4\n";} else {print "not ok 4\n";}
+
+$x = length("abc") % -10;
+print $x == -7 ? "ok 5\n" : "# expected -7, got $x\nnot ok 5\n";
+
+{
+    use integer;
+    $x = length("abc") % -10;
+    print $x == 3 ? "ok 6\n" : "# expected 3, got $x\nnot ok 6\n";
+}
End of Patch.

From @gisle

Gurusamy Sarathy <gsar@​ActiveState.com> writes​:

+{
+ use integer;
+ $x = length("abc") % -10;
+ print $x == 3 ? "ok 6\n" : "# expected 3, got $x\nnot ok 6\n";
+}
End of Patch.

Is this safe?

Under 'use integer' you get the %-operator as implemented by C and C
allow more than one result to be correct for negative operands.
Perhaps something like is safer​:

use integer;
$x = length("abc") % -10;
$y = 3 % -10;
print $x == $y ? "ok 6\n" : "# expected $y, got $x\nnot ok 6\n";

Regards,
Gisle

From @gsar

On 24 Oct 1999 12​:25​:01 +0200, Gisle Aas wrote​:

Under 'use integer' you get the %-operator as implemented by C and C
allow more than one result to be correct for negative operands.
Perhaps something like is safer​:

use integer;
$x = length("abc") % -10;
$y = 3 % -10;
print $x == $y ? "ok 6\n" : "# expected $y, got $x\nnot ok 6\n";

Good point, but I think we could do somewhat better.

Sarathy
gsar@​ActiveState.com

-----------------------------------8<-----------------------------------
Change 4441 by gsar@auger on 1999/10/24 14:33:11

	test in change#4428 needs strict interpretation of C modulus

Affected files ...

... //depot/perl/t/op/int.t#8 edit

Differences ...

==== //depot/perl/t/op/int.t#8 (xtext) ====
Index: perl/t/op/int.t
--- perl/t/op/int.t.~1~	Sun Oct 24 07:33:15 1999
+++ perl/t/op/int.t	Sun Oct 24 07:33:15 1999
@@ -25,5 +25,6 @@
 {
     use integer;
     $x = length("abc") % -10;
-    print $x == 3 ? "ok 6\n" : "# expected 3, got $x\nnot ok 6\n";
+    $y = (3/-10)*-10;
+    print $x+$y == 3 && abs($x) < 10 ? "ok 6\n" : "not ok 6\n";
 }
End of Patch.