RE: GCC optimizes integer overflow: bug or feature?

[Dave Korn]

On 20 December 2006 02:40, Mike Stump wrote:

> On Dec 19, 2006, at 6:33 PM, Dave Korn wrote:
>> On 20 December 2006 02:28, Andrew Pinski wrote:
>> 
>>>> Paul Brook wrote:
>>>>>> Compiler can optimize it any way it wants,
>>>>>> as long as result is the same as unoptimized one.
>>>>> 
>>>>> We have an option for that. It's called -O0.
>>>>> 
>>>>> Pretty much all optimization will change the behavior of your
>>>>> program.
>>>> 
>>>> Now that's a bit TOO strong a statement, critical optimizations like
>>>> register allocation and instruction scheduling will generally not change
>>>> the behavior of the program (though the basic decision to put something
>>>> in a register will, and *surely* no one suggests avoiding this critical
>>>> optimization).
>>> 
>>> Actually they will with multi threaded program, since you can have a case
>>> where it works and now it is broken because one thread has speed up so
>>> much it writes to a variable which had a copy on another thread's stack.
>> 
>> Why isn't that just a buggy program with wilful disregard for the use of
>> correct synchronisation techniques?
> 
> It is that, as well as a program that features a result that is
> different from unoptimized code.

  Well, I for one wish to complain.  All the empty for loops that I use for
precision timing have been getting faster and faster over the past few years.
It used to take several minutes for i to count from one to a hundred million.
Now it seems to take no time at all!

    cheers,
      DaveK
-- 
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