Introduce a minimum CGU size in non-incremental builds.

Because tiny CGUs make compilation less efficient *and* result in worse
generated code.

We don't do this when the number of CGUs is explicitly given, because
there are times when the requested number is very important, as
described in some comments within the commit. So the commit also
introduces a `CodegenUnits` type that distinguishes between default
values and user-specified values.

This change has a roughly neutral effect on walltimes across the
rustc-perf benchmarks; there are some speedups and some slowdowns. But
it has significant wins for most other metrics on numerous benchmarks,
including instruction counts, cycles, binary size, and max-rss. It also
reduces parallelism, which is good for reducing jobserver competition
when multiple rustc processes are running at the same time. It's smaller
benchmarks that benefit the most; larger benchmarks already have CGUs
that are all larger than the minimum size.

Here are some example before/after CGU sizes for opt builds.

- html5ever
  - CGUs: 16, mean size: 1196.1, sizes: [3908, 2992, 1706, 1652, 1572,
    1136, 1045, 948, 946, 938, 579, 471, 443, 327, 286, 189]
  - CGUs: 4, mean size: 4396.0, sizes: [6706, 3908, 3490, 3480]

- libc
  - CGUs: 12, mean size: 35.3, sizes: [163, 93, 58, 53, 37, 8, 2 (x6)]
  - CGUs: 1, mean size: 424.0, sizes: [424]

- tt-muncher
  - CGUs: 5, mean size: 1819.4, sizes: [8508, 350, 198, 34, 7]
  - CGUs: 1, mean size: 9075.0, sizes: [9075]

Note that CGUs of size 100,000+ aren't unusual in larger programs.

1 files changed, 27 insertions, 6 deletions

diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 2cc02003218..5feea83edb6 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -234,6 +234,27 @@ pub enum MetadataKind {
     Compressed,
 }
 
+#[derive(Clone, Copy)]
+pub enum CodegenUnits {
+    /// Specified by the user. In this case we try fairly hard to produce the
+    /// number of CGUs requested.
+    User(usize),
+
+    /// A default value, i.e. not specified by the user. In this case we take
+    /// more liberties about CGU formation, e.g. avoid producing very small
+    /// CGUs.
+    Default(usize),
+}
+
+impl CodegenUnits {
+    pub fn as_usize(self) -> usize {
+        match self {
+            CodegenUnits::User(n) => n,
+            CodegenUnits::Default(n) => n,
+        }
+    }
+}
+
 impl Session {
     pub fn miri_unleashed_feature(&self, span: Span, feature_gate: Option<Symbol>) {
         self.miri_unleashed_features.lock().push((span, feature_gate));
@@ -1104,7 +1125,7 @@ impl Session {
 
         // If there's only one codegen unit and LTO isn't enabled then there's
         // no need for ThinLTO so just return false.
-        if self.codegen_units() == 1 {
+        if self.codegen_units().as_usize() == 1 {
             return config::Lto::No;
         }
 
@@ -1206,19 +1227,19 @@ impl Session {
 
     /// Returns the number of codegen units that should be used for this
     /// compilation
-    pub fn codegen_units(&self) -> usize {
+    pub fn codegen_units(&self) -> CodegenUnits {
         if let Some(n) = self.opts.cli_forced_codegen_units {
-            return n;
+            return CodegenUnits::User(n);
         }
         if let Some(n) = self.target.default_codegen_units {
-            return n as usize;
+            return CodegenUnits::Default(n as usize);
         }
 
         // If incremental compilation is turned on, we default to a high number
         // codegen units in order to reduce the "collateral damage" small
         // changes cause.
         if self.opts.incremental.is_some() {
-            return 256;
+            return CodegenUnits::Default(256);
         }
 
         // Why is 16 codegen units the default all the time?
@@ -1271,7 +1292,7 @@ impl Session {
         // As a result 16 was chosen here! Mostly because it was a power of 2
         // and most benchmarks agreed it was roughly a local optimum. Not very
         // scientific.
-        16
+        CodegenUnits::Default(16)
     }
 
     pub fn teach(&self, code: &DiagnosticId) -> bool {