> On 22 Sep 2025, at 13:54, David Malcolm via Gcc <[email protected]> wrote:
>
> On Mon, 2025-09-22 at 09:29 +0300, Yair Lenga via Gcc wrote:
>> Hi,
>>
>> I've inherited an old code base of "C" code, which was maintained by
>> relatively inexperience team. One of the common pattern that I've
>> seen was:
>>
>> for (int i=0 ; i < strlen(s) ; i++) { ... }
>>
>> Which has O(n^2) performance, given O(n) performance on s. Acceptable
>> on
>> strings up to 400-500 characters, but painful when large strings -
>> 4k.
>>
>> My question: Is there a path to add new performance-related
>> diagnostics for
>> those cases, in general, any function with expected O(n) in the
>> condition/step of a loop (for ; while ; do ... while) should trigger
>> this
>> warning. The most common pattern that I've seen - I believe other
>> pattern
>> also exists - with strchr, strstr, ...
>>
>> Ideally, this will come with an annotation that can be placed on
>> function
>> to say "I'm expensive, should not be called in a tight loop"
>>
>> [[GNU:expensive]] const int count_something(const char *x) ;
>>
>> which will result in a performance warning on
>> for (int i=0 ; i<count_something(s) ; i++)
>>
>> Looking for feedback, and suggestion on how to get something like
>> that
>> implement - is this something that will be useful ?
>
> Ignoring the attribute idea for now, I think it's relatively easy to
> implement detection of the special-case of
>
> for (int i=0 ; i < strlen(s) ; i++) { ... }
>
> as a gimple pass: we already have loop analysis code which can turn a
> function's CFG into a tree of nested loops, you then iterate through
> the relevant gimple stmts looking for a gimple_call to
> __builtin_strlen, and then issue a warning if you see one.
>
> That might in itself be useful to you (and maybe others?), and might
> serve as a prototype from which a more general warning could be built.
>
> As others have noted, the iterator variable should be size_t, not int,
> but that's a different issue (does it lead to an infinite loop with
> wrapround if you have a string longer than 2^32 chars?)
Perhaps style-checking / linting type actions are better suited to static
analysis than to compiler warnings, per se.
Sort of rule-of-thumb
- the compiler should be emitting warnings/errors if one does something
that is not correct standards-wise.
- style-checking/linting/analysis can be much wider and help catch things
that are legal (sandards-wise) but not sensible (coding-wise)?
0.02GBP, only,
Iain
