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?)
Hope this is constructive
Dave
