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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