changing "configure" to default to "gcc -g -O2 -fwrapv ..."

[Brooks Moses]

Robert Dewar wrote:
> Ian Lance Taylor wrote:
>> The new option -fstrict-overflow tells gcc that it can assume the
>> strict signed overflow semantics prescribed by the language standard.
>> This option is enabled by default at -O2 and higher.  Using
>> -fno-strict-overflow will tell gcc that it can not assume that signed
>> overflow is undefined behaviour.  The general effect of using this
>> option will be that signed overflow will become implementation
>> defined.  This will disable a number of generally harmless
>> optimizations, but will not have the same effect as -fwrapv.
> 
> Can you share the implementation definition (implementation defined
> generally means that the implementation must define what it does).
> This seems awfully vague.

I believe that what Ian means here is that the effect of a signed 
overflow will be to produce some arbitrary but well-defined result, with 
"well-defined" here meaning that the program will behave in a manner 
that's consistent with every subsequent access of it in the source code 
seeing the same value (until it's redefined, of course).  The actual 
value that is seen should, however, be considered entirely arbitrary.

This is distinguished from the -fstrict-overflow, which can produce 
results that are inconsistent, such as cases where "n" is negative but 
the"n>0" path of an "if" statement is taken.

An example of that is "if (m>0) {n=m+1; if (n>0) {...}}".  If the 
-fstrict-overflow option is specified, then the compiler can act as if 
m+1 never overflows, and optimize away the second check even if the 
addition is actually implemented with wrap in the case of overflow. 
Supply m=0xEFFFFFFF to the resulting program, and the "..." gets 
executed with a negative n.

All that -fno-strict-overflow does is prevent that sort of optimization; 
it doesn't make any guarantees whatsoever about what value n will 
actually have in the resulting code, just that program is guaranteed to 
act as if the "n>0" and the "..." both see the same value.

- Brooks