>> Improving incremental mode >> ========================== >> >> Another area that could use work is the incremental compilation logic in the >> driver -- that is, reducing the number of times a file is rebuilt when it >> "doesn't need to be" -- and some of the documentation and driver-level >> counters mentioned above provide insight into that (eg. >> https://github.com/apple/swift/blob/master/docs/CompilerPerformance.md#driver-diagnosis >> >> <https://github.com/apple/swift/blob/master/docs/CompilerPerformance.md#driver-diagnosis>). >> Incremental compilation is based on approximating the "true" dependency >> graph and sometimes this approximation is too coarse; but changing this will >> be a large amount of work, and the nature of a substantial change to that is >> still subject to a lot of analysis and design. In the meantime there may >> also simpler bugs lurking in the dependency-analysis logic, within the >> current dependency approximation; I'd be happy to help anyone who wants to >> spend time bug-hunting in this area understand what they're looking at. > > Has any thought been put into taking advantage of llbuild for dependency > graphs? Can some performance improvements come from that?
Long term I think there's general interest for leveraging it, in place of the miniature build system inside the swift driver; but the discovery/analysis or dependencies and the scheduling/executing of jobs are somewhat separate tasks, and llbuild only does the second. The mapping from a set of swift declarations-and-files into a dependency graph that is a safe (but tight) approximation of the "true" dependencies of the compiler is the hard part. That is, the material that goes into a .swiftdeps file is the hard part; and unfortunately that's a thing build systems delegate to compilers to figure out, since it involves tracing the compiler's internal activities, as it runs each job. -Graydon
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