[PATCH][4.3] Deprecate -ftrapv

[Robert Dewar]

Florian Weimer wrote:

> Are we talking about the Ada semantics or GNAT without -gnato?  In the
> latter case, the documentation disagrees with you:
> 
>      The reason that we distinguish overflow checking from other kinds
>      of range constraint checking is that a failure of an overflow
>      check can generate an incorrect value, but cannot cause erroneous
>      behavior. This is unlike the situation with a constraint check on
>      an array subscript, where failure to perform the check can result
>      in random memory description, or the range check on a case
>      statement, where failure to perform the check can cause a wild
>      jump.
> 
> And to avoid erroneous execution on integer overflow, I think you have
> to specify the equivalent -fwrapv (otherwise, the middle end might do
> nasty things to your range checks).

The documentation needs minor adjustment, I will remove the word
erroneous, and rephrase it as:

> The reason that we distinguish overflow checking from other kinds of
> constraint checking is that a failure of an overflow check, unlike
> for example the failure of a range check, can result in an incorrect
> value, but cannot cause random memory destruction (like an out of range
> subscript), or a wild jump (from an out of range case value). Overflow
> checking is also quite expensive in time and space, since in general it
> requires the use of double length arithmetic.