incr.comp.: Implement query result cache and use it to cache type checking tables. #46004

michaelwoerister · Nov 15, 2017

This is a spike implementation of caching more than LLVM IR and object files when doing incremental compilation. At the moment, only the typeck_tables_of query is cached but MIR and borrow-check will follow shortly. The feature is activated by running with -Zincremental-queries in addition to -Zincremental, it is not yet active by default.

r? @nikomatsakis

michaelwoerister · Nov 15, 2017

Just found a bug in this: The new_type_index!() macro generates Encodable and Decodable implementations for DefIndex, so the specialized impls are never really called. This leads to some "vanilla" DefIndices being decoded without being rebased (e.g. in ty::UpVarId) and also without triggering the intended ICE.

Please do not merge until this is fixed.

nikomatsakis · Nov 15, 2017

nikomatsakis reviewed Nov 15, 2017

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nikomatsakis · Nov 15, 2017

src/librustc/ty/maps/on_disk_cache.rs

-            let body = Body::decode(&mut decoder).unwrap();
-            body.diagnostics.into_iter().collect()
+
+            let prev_diagnostics: FxHashMap<_, _> = {


Why do we have this new variable here? It doesn't serve an obvious purpose. =)

It does in later commits where this block is used to limit the scope of the CacheDecoder.

nikomatsakis · Nov 15, 2017

nikomatsakis reviewed Nov 15, 2017

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nikomatsakis · Nov 15, 2017

src/librustc/ty/maps/on_disk_cache.rs

+    where E: 'enc + ty_codec::TyEncoder
+{
+    fn specialized_encode(&mut self, node_id: &NodeId) -> Result<(), Self::Error> {
+        let hir_id = self.definitions.node_to_hir_id(*node_id);


Is this basically copy-n-paste from the other encoders? (Not criticizing, just trying to understand.)

Answer: this impl, anyway, is not.

Yeah, I thought about writing a macro in ty::codec that would instantiate these implementations. Upfront, I wasn't sure though how many of them would end up just being copies.

nikomatsakis · Nov 15, 2017

nikomatsakis reviewed Nov 15, 2017

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nikomatsakis · Nov 15, 2017

src/librustc/ty/maps/on_disk_cache.rs

+    }
+}
+
+impl<'a, 'tcx, 'x> SpecializedDecoder<&'tcx Substs<'tcx>> for CacheDecoder<'a, 'tcx, 'x> {


If we forget one of these impls, what happens? I think it panics? (These impls are copy-n-paste, I assume.)

Yeah, most of them should panic.

nikomatsakis · Nov 15, 2017

nikomatsakis reviewed Nov 15, 2017

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nikomatsakis · Nov 15, 2017

src/librustc/ty/maps/on_disk_cache.rs

@@ -49,6 +49,7 @@ pub struct OnDiskCache<'sess> {

    prev_cnums: Vec<(u32, String, CrateDisambiguator)>,
    cnum_map: RefCell<Option<IndexVec<CrateNum, Option<CrateNum>>>>,
+    prev_def_path_tables: Vec<DefPathTable>,


Does this signal a shift away from the "reverse lookup by hash" strategy?

No, the DefPathTables are stored for facilitating the "reverse lookup by hash". The implementation in this PR does the following:

Store a DefId unmodified in the cache.

When loading, we need to

map the serialized DefId to the session-independent DefPathHash and then

look up the new DefId by this DefPathHash.

In order to be able to do step (i), we need the old DefPathTable because it maps DefIndices to DefPathHashes.

But,... I'm not surprised that this caught your eye. There is an alternative approach that is simpler to implement and maybe more robust: Map DefIds and DefIndices to DefPathHashes at serialization time and thus get rid of the requirement to store the old DefPathTables. I'm not sure if this affects performance and space requirements in a good or in a bad way. But I'd like to implement it soonish and measure the impact.

nikomatsakis · Nov 15, 2017

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Did a first pass. Left some drive-by questions. The main "red flag" of sorts was that there seemed to be a bit more code duplication than I expected, though mostly it's just impls calling into common helpers, so perhaps it's fine. Not sure how best to avoid it, perhaps using a macro -- or there might be some specialization tricks we could use with a helper trait.

michaelwoerister · Nov 16, 2017

OK, this should be up-to-date now. I went ahead and implemented the "map to DefPathHash at serialization-time" strategy because it made a few things easier around fixing stray DefIndices.

nikomatsakis · Nov 16, 2017

nikomatsakis approved these changes Nov 16, 2017

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Lovely.

nikomatsakis · Nov 16, 2017

src/librustc/hir/def_id.rs

+
+
+#[derive(Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash)]
+pub struct LocalDefId(DefIndex);


I've wanted to add this type for so long. But can you please add a doc-comment explaining its role and why to use it, and also why it is bad to just use a raw DefIndex.

nikomatsakis · Nov 16, 2017

src/librustc/hir/def_id.rs

        self.krate == LOCAL_CRATE
    }
+
+     #[inline]


Nit: extra space.

nikomatsakis · Nov 16, 2017

src/librustc/ty/maps/on_disk_cache.rs

    prev_cnums: Vec<(u32, String, CrateDisambiguator)>,
    cnum_map: RefCell<Option<IndexVec<CrateNum, Option<CrateNum>>>>,
-    prev_def_path_tables: Vec<DefPathTable>,


nikomatsakis · Nov 16, 2017

src/librustc_metadata/decoder.rs

-        ty_codec::decode_const(self)
-    }
-}
+implement_ty_decoder!( DecodeContext<'a, 'tcx> );


nikomatsakis · Nov 16, 2017

@bors r+

bors · Nov 16, 2017

📌 Commit 4c4f7a3 has been approved by nikomatsakis

nikomatsakis · Nov 16, 2017

Er wait, I wanted something.

@bors r-

nikomatsakis · Nov 16, 2017

r=me with comment on LocalDefId explaining its role =)

michaelwoerister · Nov 16, 2017

@bors r=nikomatsakis

bors · Nov 16, 2017

📌 Commit 0a1f6dd has been approved by nikomatsakis

michaelwoerister · Nov 17, 2017

@bors p=1 (we'd like to get this tested on rust-icci asap)

bors · Nov 17, 2017

⌛️ Testing commit 0a1f6dd with merge 02eed2e...

bors · Nov 17, 2017

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 02eed2e to master...

michaelwoerister · Nov 17, 2017

Note, this is not expected to show up in any benchmarks yet since it's behind the -Zincremental-queries flag.

rust-highfive assigned nikomatsakis Nov 15, 2017

kennytm added the S-waiting-on-review label Nov 15, 2017

Fix some tidy errors in ty::codec.

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Tracking issue for "Red/Green" Dependency Tracking #42293
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Add doc comment for LocalDefId.

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Make GlobalCtxt implement Sync #45912

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incr.comp.: Make sure `cargo check` is compatible with incremental compilation. #46058

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Enable -Zincremental-queries nikomatsakis/cargo-incremental#53

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incr.comp.: Implement query result cache and use it to cache type checking tables. #46004

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nikomatsakis commented Nov 16, 2017 • edited

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nikomatsakis commented Nov 16, 2017 •

edited

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