> [This is a GitHub copy of: > https://mail.openjdk.java.net/pipermail/compiler-dev/2020-March/014389.html ] > > Currently, (com.sun.tools.javac.code.)Symbol/s have a long field > "flags_field", which holds various one-bit information ("Flags") about the > given Symbol. > > We currently have around 64 such Flags, which means we are out of bits in the > long field, and adding new flags is not easy. > > We could change the "flags_field" to be a Set over enums, but this would > increase the memory footprint notably, and would also slow-down access to > flags. Currently, flags operations in javac are very fast and very common, so > this is probably too much burden. There are also flags to which we need to > access as bit masks, e.g. due to writing to classfile. > > My proposal here is to use an intermediate solution, until we find a better > solution, or until a better solution is possible (like due to Valhalla). The > idea is as follows: > -the current long-based Flags are split into 4 groups: > --"flat" Flags, long based, work exactly as before. > --enum-based TypeSymbolFlags, MethodSymbolFlags and VarSymbolFlags, which are > only applicable to TypeSymbols, MethodSymbols and VarSymbols, respectively, > and are checked using methods like Symbol.isFlagSet and set/clear using > methods Symbol.setFlag and Symbol.clearFlag. So these flags are mostly > encapsulated, even though physically they are currently stored in the > flags_field as well. > > There are ~~37~~ 40 "flat" flags and ~~16~~ 17 TypeSymbolFlags (methods and > vars have less flags), 57 in total. This gives us at least 7 new flags before > we run out of long bits in flags_field again - but even if we do, there are > several easy mitigation strategies we could use, like: > -create a new int/long field on TypeSymbols for the TypeSymbolFlags (probably > preferable) > -split TypeSymbolFlags into Class/Package/MethodSymbolFlags > > The positives of this solution include: > -safe(r) access to the flags - the access to the extra/symbol-kind-specific > flags is mostly encapsulated, and hence mostly safe > -improves the abstractions at least for some flags > -reasonably complex patch (attempts to encapsulate all the flags were > troublesome in previous experiments) > -the performances appears to be acceptable. > > The negative that I see is that the code includes several incompatible types > of flags now. These cannot be easily confused by accident (as the type system > will complain), but still we need to be aware of them when writing code. > > Some more alternatives: > -add a new field to Symbol, like long extraFlags, and continue with bit masks > as we did so far using this new field. The drawback is that it is more error > prone (it would be difficult to ensure only the new/extra flags would be > stored and read from extraFlags). > -have a new field, and "flat" and non-"flat" enum-based encapsulated Flags, > but don't separate Type/Method/Var flags. Probably something to consider, > even though I am a bit reluctant to add new fields without trying to avoid it > . > > Any feedback is welcome!
Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision: Removing merge tags are noted on the review - thanks! ------------- Changes: - all: https://git.openjdk.java.net/jdk/pull/2316/files - new: https://git.openjdk.java.net/jdk/pull/2316/files/a9c3ff6e..27e37860 Webrevs: - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2316&range=01 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2316&range=00-01 Stats: 7 lines in 1 file changed: 0 ins; 7 del; 0 mod Patch: https://git.openjdk.java.net/jdk/pull/2316.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/2316/head:pull/2316 PR: https://git.openjdk.java.net/jdk/pull/2316
