From: Jeffrey A Law <law@cygnus.com>
To: kenner@vlsi1.ultra.nyu.edu (Richard Kenner)
Cc: gcc@gcc.gnu.org
Subject: Re: Pathalogical divides
Date: Sat, 23 Sep 2000 13:38:00 -0000 [thread overview]
Message-ID: <810.969669639@upchuck> (raw)
In-Reply-To: <10009211926.AA26210@vlsi1.ultra.nyu.edu>
In message < 10009211926.AA26210@vlsi1.ultra.nyu.edu >you write:
> Consider the following program on x86:
>
> int rem (int a, int b) { return a % b; }
>
> int
> main ()
> {
> printf ("%d\n", rem (0x80000000, -1));;
> }
>
> When run, rather than producing zero, as expected, it gets a SIGFPE.
> This is because the division of the largest negative integer by negative on
> e
> results in an overflow.
>
> So the first question is whether this is valid C behavior.
Not valid C behavior.
> Next, compile the above with -O3 on an x86 and notice that GCC gets a
> SIGFPE when constant-folding.
>
> Finally, consider:
>
> int
> foo (int a, int b)
> {
> return (a - ((a == 0x80000000 && b == -1) ? 0 : a % b)) / b;
> }
>
> This program when passed "normal" arguments does not get an overflow.
> But GCC pulls the conditional out of the subtraction and division and
> causes the compiler to run into the SIGFPE above.
>
> I think the compiler crash needs to be fixed. We can do it either by
> protecting the integer part of simplify_binary_operation against SIGFPE
> just like the FP or explicitly testing for this case just like we
> check for divide by zero.
We need to fix the compiler crash.
jeff
next prev parent reply other threads:[~2000-09-23 13:38 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2000-09-21 12:12 Richard Kenner
2000-09-22 1:45 ` Andreas Schwab
2000-09-23 13:38 ` Jeffrey A Law [this message]
2000-09-25 6:53 ` Alexandre Oliva
2000-09-24 22:53 ` Geoff Keating
2000-09-25 2:35 Robert Dewar
2000-09-24 23:29 ` Geoff Keating
2000-09-25 2:43 Richard Kenner
2000-09-25 14:27 ` Geoff Keating
2000-09-26 4:37 ` Jamie Lokier
2000-09-25 5:19 Robert Dewar
2000-09-25 6:57 Robert Dewar
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