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Re: [PATCH][4.3] Deprecate -ftrapv


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.


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