[Python-Dev] gcc 4.2 exposes signed integer overflows (original) (raw)
Tim Peters tim.peters at gmail.com
Mon Aug 28 04:32:52 CEST 2006
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[Anthony Baxter]
Regardless of whether we consider gcc's behaviour to be correct or not,
It is correct, but more to the point it's, umm, /there/ ;-)
I do agree we need a fix for this in 2.5 final. That should also be backported to release24-maint for the 2.4.4 release, and maybe release23-maint, as Barry recently started talking about cutting a 2.3.6.
Can I nominate Tim, with his terrifying knowledge of C compiler esoterica, as the person to pick the best fix?
It's a bitch. Changing to
if (y == -1 && x < 0 && (unsigned long)x == -(unsigned long)x)is the obvious fix, but violates our "no warnings" policy: the MS compiler warns about applying unary minus to an unsigned operand -- it "looks insane" to /their/ compiler writers ;-). Elegant patch below -- LOL.
Would be nice if someone verified it worked on a box where it matters. Would also be nice if people checked to see whether their compiler(s) warn about something else now.
IIndex: Objects/intobject.c
--- Objects/intobject.c (revision 51618) +++ Objects/intobject.c (working copy) @@ -564,8 +564,14 @@ "integer division or modulo by zero"); return DIVMOD_ERROR; }
/* (-sys.maxint-1)/-1 is the only overflow case. */if (y == -1 && x < 0 && x == -x)
/* (-sys.maxint-1)/-1 is the only overflow case. x is the most* negative long iff x < 0 and, on a 2's-complement box, x == -x.* However, -x is undefined (by C) if x /is/ the most negative long* (it's a signed overflow case), and some compilers care. So we cast* x to unsigned long first. However, then other compilers warn about* applying unary minus to an unsigned operand. Hence the weird "0-".*/if (y == -1 && x < 0 && (unsigned long)x == 0-(unsigned long)x) return DIVMOD_OVERFLOW; xdivy = x / y; xmody = x - xdivy * y;
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