[gambit-list] Re: configure must use -fwrapv as a gcc option for building gambit

Bradley Lucier lucier at math.purdue.edu
Fri Jun 3 16:03:13 EDT 2005


On Jun 3, 2005, at 3:58 PM, feeley at iro.umontreal.ca wrote:

> Quoting Bradley Lucier <lucier at math.purdue.edu>:
>
>> Here's the documentation for -fwrapv in gcc-4.0 in
>>
>> http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Code-Gen- 
>> Options.html#Code-
>> Gen-Options
>>
>> -fwrapv
>>      This option instructs the compiler to assume that signed  
>> arithmetic
>> overflow of addition, subtraction and multiplication wraps around  
>> using
>> twos-complement representation. This flag enables some optimizations
>> and disables other. This option is enabled by default for the Java
>> front-end, as required by the Java language specification.
>>
>>
>> Since we assume this of all processors, we need to enable this flag.
>>
>> Brad
>>
>
> You mean by default gcc will not wrap?  This sounds bogus because it  
> would
> cause major backward compatibility problems.  What happens if you don't
> specify the flag?

By default, signed integer overflow is undefined in C, so if it occurs  
in a C program, the program is undefined, so gcc can assume that it  
doesn't occur and can enable some optimizations that assume it never  
occurs.  You don't want this to happen,

Brad




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