On Fri, Nov 02, 2018 at 10:56:31AM +0000, David Laight wrote:

From: Paul E. McKenney

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Sent: 01 November 2018 17:02

...

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And there is a push to define C++ signed arithmetic as 2s complement,
but there are still 1s complement systems with C compilers.  Just not
C++ compilers.  Legacy...

Hmmm... I've used C compilers for DSPs where signed integer arithmetic
used the 'data registers' and would saturate, unsigned used the 'address
registers' and wrapped.
That was deliberate because it is much better to clip analogue values.

There are no C++ compilers for those DSPs, correct?  (Some of the
C++ standards commmittee members believe that they have fully checked,
but they might well have missed something.)

Then there was the annoying cobol run time that didn't update the
result variable if the result wouldn't fit.
Took a while to notice that the sum of a list of values was even wrong!
That would be perfectly valid for C - if unexpected.

Heh!  COBOL and FORTRAN also helped fund my first pass through university.

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But for us using -fno-strict-overflow which actually defines signed
overflow

I wonder how much real code 'strict-overflow' gets rid of?
IIRC gcc silently turns loops like:
	int i; for (i = 1; i != 0; i *= 2) ...
into infinite ones.
Which is never what is required.

The usual response is something like this:

	for (i = 1; i < n; i++)

where the compiler has no idea what range of values "n" might take on.
Can't say that I am convinced by that example, but at least we do have
-fno-strict-overflow.

							Thanx, Paul