C++ Logo


Advanced search

[ub] P1407R0: Tell Programmers About Signed Integer Overflow Behavior

From: Marc Glisse <marc.glisse_at_[hidden]>
Date: Sat, 26 Jan 2019 09:44:50 +0100 (CET)

just a couple points missing from the paper:

1) with g++-7 -O2 -Wall, the motivating example on the left produces:

<source>: In function 'int32_t add_100_without_wrap(int32_t)':
<source>:8:3: warning: assuming signed overflow does not occur when assuming that (X + c) < X is always false [-Wstrict-overflow]
    if (ret < a)

However, we removed the warning from gcc-8 because it was too noisy and
impossible to work around when the optimization is what you actually want.

2) At least with gcc, -ftrapv doesn't really work. You need
-fsanitize=signed-integer-overflow -fsanitize-undefined-trap-on-error for
something roughly equivalent to what -ftrapv is supposed to do.

Now my opinion: you have the wrong target. Compilers that have a -fwrapv
option (or -ftrapv or ubsan or ...) already indirectly describe the
default behavior as undefined (and the standard already describes it as
undefined), so it is already documented. Adding a sentence or 2 in the
standard and on pages that nobody reads won't help. It seems that you want
to talk either to teachers, so they warn their students more about the
properties of signed overflow, or to compiler writers, to convince them to
change the default to -fwrapv or -ftrapv (I hope they don't) or add more

Marc Glisse

Received on 2019-01-26 09:54:48