On Oct 27, 2011, at 2:52 AM, Douglas Gregor wrote: > On Oct 27, 2011, at 2:45 AM, John McCall wrote: >> On Oct 27, 2011, at 2:33 AM, Douglas Gregor wrote: >>> The results of this optimization are fairly dramatic. On a small >>> application that brings in 14 non-trivial modules, this takes modules >>> from being > 3x slower than a "perfect" PCH file down to 30% slower >>> for a full rebuild. A partial rebuild (where the PCH file or modules >>> can be re-used) is down to 7% slower. Making the PCH file just a >>> little imperfect (e.g., adding two smallish modules used by a bunch of >>> .m files that aren't in the PCH file) tips the scales in favor of the >>> modules approach, with 24% faster partial rebuilds. >> >> >> You've added an extra generally-not-taken branch to the lexing >> of every single identifier-like token; > > Untrue! The out-of-date bit for identifiers feeds into NeedsHandleIdentifier. > So, in the no-PCH/no-modules case, we only pay the cost of checking this bit > for those identifiers that already have something else interesting going on > (e.g., they are poisoned, or need to be macro expanded, or anything else > HandleIdentifier does). In the PCH/modules case, we pay the cost of checking > that bit once for each known identifier that is uttered after a module import.
Ah, okay, so we're checking multiple bits at once and then breaking them out? That is sensible enough. John. _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
