[rust-dev] No range integer type? Saftey beyond memory?

Robert O'Callahan robert at ocallahan.org
Sun Apr 28 15:03:39 PDT 2013


On Sat, Apr 27, 2013 at 4:23 AM, Graydon Hoare <graydon at mozilla.com> wrote:

> I think it has to be opt-in, yeah. Sadly. I mean, I wish we were living in
> the world of hardware garbage collection and tagged memory words too, and
> hensel codes had won out over floating point, and all our languages had
> well defined total functional subsets with industrial strength provers
> attached to them that we were legally obliged to use.
>

I don't understand the relationship between those features and integer
overflow checking. There are very strong reasons why those features haven't
developed, and none of those reasons apply to integer overflow checking.


>  Definitely a Milestone 2 consideration ;)
>>
>
> Maybe. At least consideration. I think any variant would have to be
> additive (backwards compatible) simply because in languages like this, it's
> _really_ not what people assume as the default.
>

In my experience the default assumption is that integer overflow doesn't
happen.

Just for fun I did "grep -F ' + '" and "grep -F ' - '" in some
mozilla-central graphics, DOM and layout code:
gfx/thebes/*.cpp
gfx/cairo/cairo/src/*.c
content/base/src/*.cpp
content/html/content/src/*.cpp
layout/generic/*.cpp
and skimmed the results. I might have missed something, but I found only
three occurrences of addition/subtraction operators where overflow seemed
expected: two in hash functions in cairo-cache.c, and one in a hash
function in cairo-misc.c.

Based on that data and previous experience with this code, I'm certain that
those occurrences are vastly outweighed by handwritten code that tries to
detect/avoid integer overflows, and also by arithmetic operations that are
still vulnerable to overflow bugs in spite of those checks.

Furthermore, I wonder how many average C/C++ programmers know that overflow
of unsigned values is defined but overflow of signed values is not. I
expect that most people's assumptions are plain incorrect.

So I contend that integer overflow checking is more likely to prevent
unexpected behavior than to cause it, especially when it really matters: in
shipped code that's being attacked. Once in a while someone writing a hash
function or similar will trip over, but it will be easy for them to detect
and correct their mistake.

We've already got in trouble for / trapping divide-by-zero by a
> branch/fault rather than a signal; it hits inner loops and costs us
> performance. Really.
>

Was this some unsafe-language benchmark shootout? Even if those are
important due to some "Rust is slow, clinical tests prove it" bogo-PR
effect, I assume you would disable overflow checking along with
array-bounds checks in unsafe Rust code.

Rob
-- 
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