On Apr 17, 2013, at 8:33 PM, Adam Schnitzer <[email protected]> wrote:
> Jordan, Thank you very much for the feedback, I have a few comments.
>
> (1) Despite being on our list, this is probably better suited to a compiler
> warning.
>
>
> I agree, this warning might be better as a compiler warning. I chose to
> implement this checker as a mainly to learn a bit about the analyzer. This
> one was on the list and seemed like a good place to get started.
Sorry for having an under-specified checker in the list!
>
> (2) Despite being on our list, "unsigned" isn't actually the interesting
> thing to check.
>
>
> When I was reading the checker suggestion, I interpreted the purpose to be a
> more conservative version of a check for unary '+', which, arguably, is often
> dead code. For example, I have seen structures like this fairly commonly:
>
> int array[] = {
> -3,
> -2,
> -1,
> +1
> };
>
> Where the '+' is used for alignment, which we wouldn't want to warn about.
> However, if that array was changed to unsigned, it might be a legitimate
> warning. I thought the assumption was there's at least a decent chance a
> unary '+' on unsigned is dead code. The place where I most commonly it pop up
> was legitimate:
>
> char a = 'A';
> cout << a << " "; // print A
> cout << +a; // prints numerical value of 'A'
>
This is in line with what Jordan had mentioned. If we are writing a
checker/warning that warns on redundant operations (or operations that have no
effect), we would not warn in this case as there will be a promotion.
It should be possible to write a check/warning that finds cases where the unary
plus has no effect by examining the AST. It could be a candidate for a compiler
warning, since the check could be fast and does not require path-sensitive
program exploration. Generally, compiler warnings are better because they reach
more users. If you are interested, you could reach out to the clang community
and see if there is an interest in such a warning. You could also write it as a
checker first, see what is the false positive rate and rewrite this as a
compiler warning is it seems useful.
> But I hadn't considered the checker was intended to target idempotent or
> erroneous promotions. If that is the intent, then it seems challenging to
> decide whether an expression is dead code, or to "force a load", as you put
> it.
>
>
> (3) Macros and templates make this tricky.
>
> I thought the that this might have been the reason why the checker was listed
> as a potential checker, rather than a compiler warning. It does seem like a
> fairly "noisy" warning. I have run it through some student code.
> Unfortunately all warnings it produced were false positives, with the
> exception of one situation similar to the one above.
>
If you are interested in writing the warning, you could look at your results
and see if the suggested changes would get rid of the false positives.
>
> At this point, I'd be fine with throwing this checker out, as its utility
> does seem quite limited. If anyone has any ideas on how this checker can be
> improved to be more useful, I would be interested to hear.
>
> On an unrelated note, do you have any recommendations for what might be a
> approachable second checker?
I think the i++ checker that you've proposed originally would be good. You
could also productize the StreamChecker, which would be path-sensitive and not
too difficult. Note sure if anyone else is working on that..
Jordan, Anton, what do you think?
>
> Adam
>
>
>
>
>
> On Apr 12, 2013, at 23:53 , Adam Schnitzer <[email protected]> wrote:
>
>> This patch is an implementation of the proposed
>> "different.UnaryPlusWithUnsigned", which I implemented
>> as "alpha.deadcode.UnaryPlusWithUnsigned".
>>
>> It is implemented as a simple AST checker. However, it seems that unary '+'
>> is often removed from the AST
>> as dead code. So some of the basic test cases don't work.
>>
>> This is my first (real) patch, so any feedback or criticism is appreciated.
>>
>> Adam Schnitzer
>> <UnaryPlusChecker.patch>
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