[PATCH][4.3] Deprecate -ftrapv

[Ross Ridge]

Robert Dewar write: 
>Usually there are ways of telling what is going on at a sufficiently
>low level, but in any case, code using the conditional jump instruction
>(jo/jno) is hugely better than what we do now (and it is often faster
>to usea jo than into).

Ross Ridge wrote: 
>My point is that using INTO or some other processor's overlow mechanism
>that requires operating system support wouldn't necessarily be better for
>Ada, even it performs better (or uses less space) than the alternatives.
>Having the program crash with a vague exception would meet the
>requirements of -ftrapv, but not Ada.

Robert Dewar write: 
>But, once again, using the processor specific JO instruction will be
>much better for Ada than double length arithmetic, using JO does not
>involve a "program crash with a vague exception".

*sigh*  The possibility of using GCC's -ftrapv support to implement
overlow exceptions in Ada was mentioned in this thread.  There's no
requirement that -ftrapv do anything other than crash when overflow
occurs.  A -ftrapv that did everything you've said you wanted, performed
faster and caught 100% of overflows 100% reliabily wouldn't necessarily be
better for Ada.  On the 32-bit IA-32 platform, either the JO instruction
or a INTO instruction could legitimately be used to provide a more
optimal implementation of -ftrapv.  Even the JO instruction could do
nothing more than jump to abort().

					Ross Ridge