Or I should say without un-namespaced macros (GEM5_ prefixed), since GEM5_ONCE itself would be a macro.
Gabe On Mon, Jul 26, 2021 at 11:01 PM Gabe Black <[email protected]> wrote: > No, not that I'm aware of. It would just be to make it feasible to > implement the warn_once functionality without using macros. With c++20, I > can more or less get it to work with some minor template syntax, > warn<once>("xyz"), but that relies on the source location (file, line, > column which may be iffy) to be unique, which is defeated by, for instance, > putting multiple warn_once-s in a macro which then all look like they came > from the location of the macro in the source. > > Gabe > > On Mon, Jul 26, 2021 at 9:01 PM Steve Reinhardt <[email protected]> wrote: > >> Hi Gabe, >> >> Is there a use case for GEM5_ONCE() other than warn_once()? >> >> Thanks, >> >> Steve >> >> >> On Mon, Jul 26, 2021 at 6:06 PM Gabe Black via gem5-dev < >> [email protected]> wrote: >> >>> Hi folks. I'm continuing to research how to turn warn, panic, etc, into >>> regular functions instead of macros, and one particularly sticky problem is >>> how to ensure something like warn_once only happens once. >>> >>> Right now, the macro warn_once expands to something like this: >>> >>> do { >>> static bool once = false; >>> if (!once) { >>> warn(...); >>> once = true; >>> } >>> } while (0) >>> >>> So, a static bool is declared which guards the warn. The problem with >>> this is that it requires declaring a static bool at the call sight, which >>> as you can imagine is hard to do without macros. >>> >>> As far as how it *might* be done, if we can extract the location of the >>> call (file name, source line, column offset), then we could possibly use a >>> template holding a static bool. >>> >>> template <${source location info}> >>> warn_once(...) >>> { >>> static bool once = false; >>> .... >>> } >>> >>> There are a few problems with this approach. First, the source location >>> would have to be broken down into individual primitive types, like an int >>> for the line number, and individual characters for the file name string, >>> since you can't use a struct as a non-type template parameter until c++20. >>> This *might* be possible using fancy template tricks, but it would be a bit >>> ugly and may gum up the compiler, slowing builds. >>> >>> Second, if the column information is not unique (I think the standard is >>> not very specific about what it maps to), then the "once" will apply to >>> more than one thing. This would be particularly true if a macro whose >>> contents all share the same source location had multiple warn_once-s in it. >>> >>> I did a check with grep, and warn_once shows up in all of gem5 about 80 >>> times, so while it's used, it's not used extensively. >>> >>> What I would like to propose is that instead of having warn_once(...), >>> we add a new macro called GEM5_ONCE which would be defined something like >>> the following: >>> >>> #define GEM5_ONCE(statement) do { \ >>> static [[maybe_unused]] bool _once = ([](){ statement; }(), true); \ >>> while (0) >>> >>> Then when you want to warn once (or anything else once), you'd write it >>> like this: >>> >>> GEM5_ONCE(warn("blah blah")); >>> >>> This is *slightly* more verbose, but warn_once is only used 80 times in >>> the whole code base. Also the macro is namespaced now, which is a nice >>> improvement. >>> >>> Gabe >>> _______________________________________________ >>> gem5-dev mailing list -- [email protected] >>> To unsubscribe send an email to [email protected] >>> %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s >> >>
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