Path: csiph.com!3.us.feeder.erje.net!feeder.erje.net!news.linkpendium.com!news.linkpendium.com!news.iecc.com!.POSTED.news.iecc.com!nerds-end
From: Hans-Peter Diettrich <DrDiettrich1@netscape.net>
Newsgroups: comp.compilers
Subject: Re: Bounds checking, Optimization techniques and undefined behavior
Date: Mon, 6 May 2019 10:15:23 +0200
Organization: Compilers Central
Lines: 32
Sender: news@iecc.com
Approved: comp.compilers@iecc.com
Message-ID: <19-05-034@comp.compilers>
References: <19-04-021@comp.compilers> <19-04-023@comp.compilers> <19-04-037@comp.compilers> <19-04-039@comp.compilers> <19-04-042@comp.compilers> <19-04-044@comp.compilers> <19-04-047@comp.compilers> <19-05-004@comp.compilers> <19-05-006@comp.compilers> <19-05-016@comp.compilers> <19-05-020@comp.compilers> <19-05-024@comp.compilers> <19-05-025@comp.compilers> <19-05-028@comp.compilers> <19-05-029@comp.compilers>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Injection-Info: gal.iecc.com; posting-host="news.iecc.com:2001:470:1f07:1126:0:676f:7373:6970"; logging-data="33450"; mail-complaints-to="abuse@iecc.com"
Keywords: errors, debug
Posted-Date: 06 May 2019 10:37:49 EDT
X-submission-address: compilers@iecc.com
X-moderator-address: compilers-request@iecc.com
X-FAQ-and-archives: http://compilers.iecc.com
Xref: csiph.com comp.compilers:2269

Am 05.05.2019 um 20:44 schrieb Hans-Peter Diettrich:
> Am 05.05.2019 um 12:14 schrieb Bart:
>
>> But how do they get there? Take this:
>>
>>     int A[10], *p;
>>     p = &A[3];
>>
>> You intend p to refer to the 4-element slice A[3..6], but how does the
>> language know that? How can it stop code from writing to p[5]?
>
> Not pointers are bad, but pointer arithmetic is. It should be allowed
> only with objects of known bounds.
>
> DoDi
> [In this case the bounds look known to me. -John]

The bounds are known for the array, so that the pointer is guaranteed
valid by compile time check. But p[x] or p+x can not be guaranteed valid
without considerable runtime and bounds storage overhead. Also simple
p++ operation or the like requires an update of the bounds information.
That's what I consider pointer arithmetic, not above &A[3].

I learned to love the Pascal indexing instead of pointers, because a
loop like
   for i := 1 to 10 do sum := sum + A[i];
can be optimized safely by the Pascal/Delphi compiler into pointer and
auto increment, so that no speed penalty exists vs. explicit pointer
usage. But in a C for loop the index variable can be changed in code, so
that even above code would execute slower with bounds checks.

DoDi