manually implement Hash for DefId

This also reorders the fields to reduce the assembly operations for hashing
and changes two UI tests that depended on the former ordering because of
hashmap iteration order.

Author: llogiq

compiler/rustc_span/src/def_id.rs

@@ -7,6 +7,7 @@ use rustc_macros::HashStable_Generic;
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use std::borrow::Borrow;
 use std::fmt;
+use std::hash::{Hash, Hasher};
 
 rustc_index::newtype_index! {
     pub struct CrateNum {
@@ -146,9 +147,6 @@ impl StableCrateId {
     /// Computes the stable ID for a crate with the given name and
     /// `-Cmetadata` arguments.
     pub fn new(crate_name: &str, is_exe: bool, mut metadata: Vec<String>) -> StableCrateId {
-        use std::hash::Hash;
-        use std::hash::Hasher;
-
         let mut hasher = StableHasher::new();
         crate_name.hash(&mut hasher);
 
@@ -205,10 +203,38 @@ impl<D: Decoder> Decodable<D> for DefIndex {
 /// index and a def index.
 ///
 /// You can create a `DefId` from a `LocalDefId` using `local_def_id.to_def_id()`.
-#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Copy)]
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Copy)]
+// On below-64 bit systems we can simply use the derived `Hash` impl
+#[cfg_attr(not(target_pointer_width = "64"), derive(Hash))]
+// Note that the order is essential here, see below why
 pub struct DefId {
-    pub krate: CrateNum,
     pub index: DefIndex,
+    pub krate: CrateNum,
+}
+
+// On 64-bit systems, we can hash the whole `DefId` as one `u64` instead of two `u32`s. This
+// improves performance without impairing `FxHash` quality. So the below code gets compiled to a
+// noop on little endian systems because the memory layout of `DefId` is as follows:
+//
+// ```
+//     +-1--------------31-+-32-------------63-+
+//     ! index             ! krate             !
+//     +-------------------+-------------------+
+// ```
+//
+// The order here has direct impact on `FxHash` quality because we have far more `DefIndex` per
+// crate than we have `Crate`s within one compilation. Or in other words, this arrangement puts
+// more entropy in the low bits than the high bits. The reason this matters is that `FxHash`, which
+// is used throughout rustc, has problems distributing the entropy from the high bits, so reversing
+// the order would lead to a large number of collisions and thus far worse performance.
+//
+// On 64-bit big-endian systems, this compiles to a 64-bit rotation by 32 bits, which is still
+// faster than another `FxHash` round.
+#[cfg(target_pointer_width = "64")]
+impl Hash for DefId {
+    fn hash<H: Hasher>(&self, h: &mut H) {
+        (((self.krate.as_u32() as u64) << 32) | (self.index.as_u32() as u64)).hash(h)
+    }
 }
 
 impl DefId {

src/test/ui/coherence/coherence-orphan.stderr

@@ -1,14 +1,3 @@
-error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
-  --> $DIR/coherence-orphan.rs:17:1
-   |
-LL | impl !Send for Vec<isize> { }
-   | ^^^^^^^^^^^^^^^----------
-   | |              |
-   | |              `Vec` is not defined in the current crate
-   | impl doesn't use only types from inside the current crate
-   |
-   = note: define and implement a trait or new type instead
-
 error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
   --> $DIR/coherence-orphan.rs:10:1
    |
@@ -21,6 +10,17 @@ LL | impl TheTrait<usize> for isize { }
    |
    = note: define and implement a trait or new type instead
 
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/coherence-orphan.rs:17:1
+   |
+LL | impl !Send for Vec<isize> { }
+   | ^^^^^^^^^^^^^^^----------
+   | |              |
+   | |              `Vec` is not defined in the current crate
+   | impl doesn't use only types from inside the current crate
+   |
+   = note: define and implement a trait or new type instead
+
 error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0117`.

src/test/ui/methods/method-ambig-two-traits-cross-crate.stderr

@@ -4,20 +4,20 @@ error[E0034]: multiple applicable items in scope
 LL | fn main() { 1_usize.me(); }
    |                     ^^ multiple `me` found
    |
-note: candidate #1 is defined in an impl of the trait `Me2` for the type `usize`
+   = note: candidate #1 is defined in an impl of the trait `Me` for the type `usize`
+note: candidate #2 is defined in an impl of the trait `Me2` for the type `usize`
   --> $DIR/method-ambig-two-traits-cross-crate.rs:10:22
    |
 LL | impl Me2 for usize { fn me(&self) -> usize { *self } }
    |                      ^^^^^^^^^^^^^^^^^^^^^
-   = note: candidate #2 is defined in an impl of the trait `Me` for the type `usize`
 help: disambiguate the associated function for candidate #1
    |
-LL | fn main() { Me2::me(&1_usize); }
-   |             ~~~~~~~~~~~~~~~~~
-help: disambiguate the associated function for candidate #2
-   |
 LL | fn main() { Me::me(&1_usize); }
    |             ~~~~~~~~~~~~~~~~
+help: disambiguate the associated function for candidate #2
+   |
+LL | fn main() { Me2::me(&1_usize); }
+   |             ~~~~~~~~~~~~~~~~~
 
 error: aborting due to previous error