Auto merge of #119791 - Mark-Simulacrum:cut-leb128, r=cjgillot

Remove a large amount of leb128-coded integers

This removes ~41%[^1] of the leb128-encoded integers serialized during libcore compilation by changing enum tags to opportunistically use `u8` where feasible instead of the leb128 coding via `usize`.

This should have effectively zero impact on metadata file sizes, since almost all or all enum tags fit into the 7 bits available in leb128 for single-byte encodings. Perf results indicate this is basically neutral across the board except for an improvement in bootstrap time.

[^1]: More than half the remaining integers still fit into <= 128, so the leb128 coding still makes sense. 32% are zero, and 46% are <= 4.

4 files changed, 61 insertions, 42 deletions

diff --git a/compiler/rustc_macros/src/serialize.rs b/compiler/rustc_macros/src/serialize.rs
index 9ca7ce09ba6..98b53945b91 100644
--- a/compiler/rustc_macros/src/serialize.rs
+++ b/compiler/rustc_macros/src/serialize.rs
@@ -76,8 +76,17 @@ fn decodable_body(
                 ty_name,
                 variants.len()
             );
+            let tag = if variants.len() < u8::MAX as usize {
+                quote! {
+                    ::rustc_serialize::Decoder::read_u8(__decoder) as usize
+                }
+            } else {
+                quote! {
+                    ::rustc_serialize::Decoder::read_usize(__decoder)
+                }
+            };
             quote! {
-                match ::rustc_serialize::Decoder::read_usize(__decoder) {
+                match #tag {
                     #match_inner
                     n => panic!(#message, n),
                 }
@@ -206,11 +215,20 @@ fn encodable_body(
                     variant_idx += 1;
                     result
                 });
-                quote! {
-                    let disc = match *self {
-                        #encode_inner
-                    };
-                    ::rustc_serialize::Encoder::emit_usize(__encoder, disc);
+                if variant_idx < u8::MAX as usize {
+                    quote! {
+                        let disc = match *self {
+                            #encode_inner
+                        };
+                        ::rustc_serialize::Encoder::emit_u8(__encoder, disc as u8);
+                    }
+                } else {
+                    quote! {
+                        let disc = match *self {
+                            #encode_inner
+                        };
+                        ::rustc_serialize::Encoder::emit_usize(__encoder, disc);
+                    }
                 }
             };
 
diff --git a/compiler/rustc_serialize/src/serialize.rs b/compiler/rustc_serialize/src/serialize.rs
index 63bd3457eb9..287e317b10f 100644
--- a/compiler/rustc_serialize/src/serialize.rs
+++ b/compiler/rustc_serialize/src/serialize.rs
@@ -70,14 +70,6 @@ pub trait Encoder {
     }
 
     fn emit_raw_bytes(&mut self, s: &[u8]);
-
-    fn emit_enum_variant<F>(&mut self, v_id: usize, f: F)
-    where
-        F: FnOnce(&mut Self),
-    {
-        self.emit_usize(v_id);
-        f(self);
-    }
 }
 
 // Note: all the methods in this trait are infallible, which may be surprising.
@@ -132,10 +124,6 @@ pub trait Decoder {
 
     fn read_raw_bytes(&mut self, len: usize) -> &[u8];
 
-    // Although there is an `emit_enum_variant` method in `Encoder`, the code
-    // patterns in decoding are different enough to encoding that there is no
-    // need for a corresponding `read_enum_variant` method here.
-
     fn peek_byte(&self) -> u8;
     fn position(&self) -> usize;
 }
@@ -372,15 +360,18 @@ impl<'a, D: Decoder> Decodable<D> for Cow<'a, str> {
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for Option<T> {
     fn encode(&self, s: &mut S) {
         match *self {
-            None => s.emit_enum_variant(0, |_| {}),
-            Some(ref v) => s.emit_enum_variant(1, |s| v.encode(s)),
+            None => s.emit_u8(0),
+            Some(ref v) => {
+                s.emit_u8(1);
+                v.encode(s);
+            }
         }
     }
 }
 
 impl<D: Decoder, T: Decodable<D>> Decodable<D> for Option<T> {
     fn decode(d: &mut D) -> Option<T> {
-        match d.read_usize() {
+        match d.read_u8() {
             0 => None,
             1 => Some(Decodable::decode(d)),
             _ => panic!("Encountered invalid discriminant while decoding `Option`."),
@@ -391,15 +382,21 @@ impl<D: Decoder, T: Decodable<D>> Decodable<D> for Option<T> {
 impl<S: Encoder, T1: Encodable<S>, T2: Encodable<S>> Encodable<S> for Result<T1, T2> {
     fn encode(&self, s: &mut S) {
         match *self {
-            Ok(ref v) => s.emit_enum_variant(0, |s| v.encode(s)),
-            Err(ref v) => s.emit_enum_variant(1, |s| v.encode(s)),
+            Ok(ref v) => {
+                s.emit_u8(0);
+                v.encode(s);
+            }
+            Err(ref v) => {
+                s.emit_u8(1);
+                v.encode(s);
+            }
         }
     }
 }
 
 impl<D: Decoder, T1: Decodable<D>, T2: Decodable<D>> Decodable<D> for Result<T1, T2> {
     fn decode(d: &mut D) -> Result<T1, T2> {
-        match d.read_usize() {
+        match d.read_u8() {
             0 => Ok(T1::decode(d)),
             1 => Err(T2::decode(d)),
             _ => panic!("Encountered invalid discriminant while decoding `Result`."),
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 4235293823c..65702f76fda 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -203,18 +203,19 @@ impl Hash for RealFileName {
 impl<S: Encoder> Encodable<S> for RealFileName {
     fn encode(&self, encoder: &mut S) {
         match *self {
-            RealFileName::LocalPath(ref local_path) => encoder.emit_enum_variant(0, |encoder| {
+            RealFileName::LocalPath(ref local_path) => {
+                encoder.emit_u8(0);
                 local_path.encode(encoder);
-            }),
-
-            RealFileName::Remapped { ref local_path, ref virtual_name } => encoder
-                .emit_enum_variant(1, |encoder| {
-                    // For privacy and build reproducibility, we must not embed host-dependant path
-                    // in artifacts if they have been remapped by --remap-path-prefix
-                    assert!(local_path.is_none());
-                    local_path.encode(encoder);
-                    virtual_name.encode(encoder);
-                }),
+            }
+
+            RealFileName::Remapped { ref local_path, ref virtual_name } => {
+                encoder.emit_u8(1);
+                // For privacy and build reproducibility, we must not embed host-dependant path
+                // in artifacts if they have been remapped by --remap-path-prefix
+                assert!(local_path.is_none());
+                local_path.encode(encoder);
+                virtual_name.encode(encoder);
+            }
         }
     }
 }
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 8e26327196a..da8706ea715 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -3396,19 +3396,22 @@ impl Hash for TargetTriple {
 impl<S: Encoder> Encodable<S> for TargetTriple {
     fn encode(&self, s: &mut S) {
         match self {
-            TargetTriple::TargetTriple(triple) => s.emit_enum_variant(0, |s| s.emit_str(triple)),
-            TargetTriple::TargetJson { path_for_rustdoc: _, triple, contents } => s
-                .emit_enum_variant(1, |s| {
-                    s.emit_str(triple);
-                    s.emit_str(contents)
-                }),
+            TargetTriple::TargetTriple(triple) => {
+                s.emit_u8(0);
+                s.emit_str(triple);
+            }
+            TargetTriple::TargetJson { path_for_rustdoc: _, triple, contents } => {
+                s.emit_u8(1);
+                s.emit_str(triple);
+                s.emit_str(contents);
+            }
         }
     }
 }
 
 impl<D: Decoder> Decodable<D> for TargetTriple {
     fn decode(d: &mut D) -> Self {
-        match d.read_usize() {
+        match d.read_u8() {
             0 => TargetTriple::TargetTriple(d.read_str().to_owned()),
             1 => TargetTriple::TargetJson {
                 path_for_rustdoc: PathBuf::new(),