From: Aldy Hernandez <aldyh@redhat.com>
To: Richard Biener <richard.guenther@gmail.com>
Cc: GCC Mailing List <gcc@gcc.gnu.org>
Subject: Re: Can gcc.dg/torture/pr67828.c be an infinite loop?
Date: Fri, 24 Sep 2021 10:59:34 +0200 [thread overview]
Message-ID: <3d8e40f3-20e8-f265-85d8-d667aa65052a@redhat.com> (raw)
In-Reply-To: <CAFiYyc3oKQENp7O7arPniTpSgSn1UPe7Y2LY04V-S5uhm_Ck=A@mail.gmail.com>
On 9/24/21 10:08 AM, Richard Biener wrote:
> On Fri, Sep 24, 2021 at 10:04 AM Aldy Hernandez via Gcc <gcc@gcc.gnu.org> wrote:
>>
>> Hi folks.
>>
>> My upcoming threading improvements turn the test below into an infinite
>> runtime loop:
>>
>> int a, b;
>> short c;
>>
>> int
>> main ()
>> {
>> int j, d = 1;
>> for (; c >= 0; c++)
>> {
>> BODY:
>> a = d;
>> d = 0;
>> if (b)
>> {
>> xprintf (0);
>> if (j)
>> xprintf (0);
>> }
>> }
>> xprintf (d);
>> exit (0);
>> }
>>
>> On the false edge out of if(b) we thread directly to BODY, eliding the
>> loop conditional, because we know that c>=0 because it could never overflow.
>>
>> Since B is globally initialized to 0, this has the effect of turning the
>> test into an infinite loop.
>>
>> Is this correct, or did I miss something?
>
> Yes, 'c' will wrap to negative SHORT_MIN and terminate the loop via
> the c>=0 test.
Huh, so SHORT_MAX + 1 = SHORT_MIN? I thought that was an overflow, and
therefore undefined.
Aldy
>
> Mind c++ is really (short)(((int)c)++) and signed integer truncation
> is implementation
> defined.
>
> Richard.
>
>> Aldy
>>
>
next prev parent reply other threads:[~2021-09-24 8:59 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-24 8:03 Aldy Hernandez
2021-09-24 8:08 ` Richard Biener
2021-09-24 8:59 ` Aldy Hernandez [this message]
2021-09-24 11:30 ` David Brown
2021-09-24 9:29 ` Andrew Pinski
2021-09-24 9:35 ` Aldy Hernandez
2021-09-24 9:38 ` Andrew Pinski
2021-09-24 10:49 ` Aldy Hernandez
2021-09-24 11:33 ` David Brown
2021-09-24 11:08 ` Richard Earnshaw
2021-09-24 11:37 ` David Brown
2021-09-24 11:41 ` Aldy Hernandez
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