LLVM 4.0 Upgrade
Since nobody has done this yet, I decided to get things started:
**Todo:**
* [x] push the relevant commits to `rust-lang/llvm` and `rust-lang/compiler-rt`
* [x] cleanup `.gitmodules`
* [x] Verify if there are any other commits from `rust-lang/llvm` which need backporting
* [x] Investigate / fix debuginfo ("`<optimized out>`") failures
* [x] Use correct emscripten version in docker image
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Closes#37609.
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**Test results:**
Everything is green 🎉
traits::select: quickly filter out predicates from other traits
this improves most pre-trans passes's performance by ~1%.
That missed the spring cleaning PR because I wanted to ship it.
r? @eddyb
cache dtorck constraints on ADTs
This avoids visiting the fields of all structs multiple times, improving item-bodies checking time by 10% (!).
Not sure whether we want this in 1.18 or 1.19. It's a big-ish patch, but the 10% win is very tempting.
r? @eddyb
rustc: generalize monomorphic_const_eval to polymorphic constants.
With the addition of `Substs` to the query key, we can now evaluate *and cache* polymorphic constants.
Fixes#23898 by replacing the crippled explicit-discriminant-only local-crate-only `lookup_variant_by_id` with `ConstVal::Variant` which can describe variants irrespective of their discriminant.
Fixes#41394 by fixing #23898 (for the original testcase) and by not looping past the first discriminant.
In some cases (e.g. <[int-var] as Add<[int-var]>>), selection can turn up
a large number of candidates. Bailing out early avoids O(n^2) performance.
This improves item-type checking time by quite a bit, resulting in ~2% of total
time-to-typeck.
this avoids parsing item attributes on each call to `item_attrs`, which takes
off 33% (!) of translation time and 50% (!) of trans-item collection time.
That method is *incredibly* hot, so this ends up saving 10% of trans
time.
BTW, we really should be doing dependency tracking there - and possibly be
taking the respective perf hit (got to find a way to make DTMs fast), but
`layout_cache` is a non-dep-tracking map.
Refactor trans some more to pave way for incremental compilation
Various refactorings paving the way for the newer approach to incremental compilation (And, in particular, to "query-ifying" trans). My partial goal is to remove `SharedCrateContext`; this PR gets about as far as I can easily get before starting to really want the red/green algorithm.
r? @eddyb
cc @michaelwoerister
Implementation of repr struct alignment RFC 1358.
The main changes around rustc::ty::Layout::struct:
* Added abi_align field which stores abi alignment before repr align is applied
* align field contains transitive repr alignment
* Added padding vec which stores padding required after fields
The main user of this information is rustc_trans::adt::struct_llfields
which determines the LLVM fields to be used by LLVM, including padding
fields.
A possible future optimisation would be to put the padding Vec in an Option, since it will be unused unless you are using repr align.
