| From: | Xi Wang <xi(dot)wang(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Greg Stark <stark(at)mit(dot)edu>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: [PATCH 0/3] Work around icc miscompilation |
| Date: | 2013-02-23 17:39:21 |
| Message-ID: | CAKU6vyZ9NnVdB46xtxWFYWHg2Syt+59fsA7+aCeXfTsP_uki6Q@mail.gmail.com |
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| Lists: | pgsql-hackers |
It depends on the context. In the patches, `a' is known to be
non-negative, so `INT_MAX - a' cannot overflow. If you ignore the
context and talk about the general case, then it can.
- xi
On Sat, Feb 23, 2013 at 12:25 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Greg Stark <stark(at)mit(dot)edu> writes:
>> He's changing things to do
>
>> if (INT_MAX - a > b)
>> PG_THROW ("a+b would overflow")
>> else
>> x=a+b;
>
>> Why would a smarter compiler be licensed to conclude that it can
>> optimize away anything? "INT_MAX-a > b" is always well defined.
>
> Really? Can't "INT_MAX - a" overflow?
>
> regards, tom lane
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