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Support lld on pure wasm backend #2
Description
Currently we avoid the need for any linked on wasm32-unknown-unknown, by combining all Rust code into a single compilation unit. This allows us to work around the immaturity of the lld story for wasm, but at the cost of compile times. This issue tracks the status of the lld readiness and rustc's ability to use it.
Activity
alexcrichton commentedon Jan 10, 2018
Awhile back I filed https://bugs.llvm.org/show_bug.cgi?id=35369 with at least one cause of "we can't use lld vanilla upstream". There's a whole lot more issues to solve here, but that's at least one concrete actionable one :)
pepyakin commentedon Jan 23, 2018
JFYI Experimental LLD support has been landed in Emscripten
WebAssembly/binaryen#1346
emscripten-core/emscripten#6056
alexcrichton commentedon Jan 30, 2018
I've tested on top of the LLVM 6 upgrade (plus a bit farther on LLVM for a few more patches) with LLD master and everything looks to be working beautifully! Once https://reviews.llvm.org/D42511 lands we won't even need wasm-gc any more, yay!
alexcrichton commentedon Jan 30, 2018
Ok I've now completed a test run of our entire test suite using LLD to link wasm binaries together. Specifically I was using llvm-mirror/lld@95a9b67 with https://reviews.llvm.org/D42511 applied manually (but that probably wasn't necessary), llvm-mirror/llvm@d14b8f6, and alexcrichton/rust@106657e.
I believe the sequence of steps here are now:
I'd hope we can just switch the defaults overnight and not worry about maintaining the old "LTO everything" link mode. There'll proably be bugs but likely nothing that's too hard to backport!
alexcrichton commentedon Jan 31, 2018
Ok I got a little excited and I've now implemented all of this -- alexcrichton/rust@cbf406f
Once LLVM 6 lands I'll send that as a PR and we should be able to switch to LLD pronto.
alexcrichton commentedon Feb 10, 2018
PR for this
alexcrichton commentedon Mar 4, 2018
Landed!
alexcrichton commentedon Mar 6, 2018
FWIW as a testament to the improvements brought about by LLD related to build times, the
wasm32-unknown-unknownbuilder we have in rust-lang/rust reduced from 1h53m per build to 1h10m per build when this landed, a huge reduction in our build times!mgattozzi commentedon Mar 6, 2018
Wow that's a huge reduction. Shaving 40+ minutes off is not a trivial gain at all.