Use item count to estimate CGU size.

Author: nnethercote

Diff for: compiler/rustc_middle/src/mir/mono.rs

@@ -56,12 +56,13 @@ impl<'tcx> MonoItem<'tcx> {
         }
     }
 
-    pub fn size_estimate(&self, tcx: TyCtxt<'tcx>) -> usize {
+    pub fn size_estimate(&self, _tcx: TyCtxt<'tcx>) -> usize {
         match *self {
-            MonoItem::Fn(instance) => {
+            MonoItem::Fn(_instance) => {
                 // Estimate the size of a function based on how many statements
                 // it contains.
-                tcx.instance_def_size_estimate(instance.def)
+                //tcx.instance_def_size_estimate(instance.def)
+                1
             }
             // Conservatively estimate the size of a static declaration
             // or assembly to be 1.
@@ -321,8 +322,7 @@ impl<'tcx> CodegenUnit<'tcx> {
     }
 
     pub fn compute_size_estimate(&mut self, tcx: TyCtxt<'tcx>) {
-        // Estimate the size of a codegen unit as (approximately) the number of MIR
-        // statements it corresponds to.
+        // Estimate the size of a codegen unit as the number of items in it.
         self.size_estimate = self.items.keys().map(|mi| mi.size_estimate(tcx)).sum();
     }
 

Diff for: compiler/rustc_monomorphize/src/partitioning.rs

@@ -328,9 +328,9 @@ fn merge_codegen_units<'tcx>(
     // Having multiple CGUs can drastically speed up compilation. But for
     // non-incremental builds, tiny CGUs slow down compilation *and* result in
     // worse generated code. So we don't allow CGUs smaller than this (unless
-    // there is just one CGU, of course). Note that CGU sizes of 100,000+ are
+    // there is just one CGU, of course). Note that CGU sizes of 3,000+ are
     // common in larger programs, so this isn't all that large.
-    const NON_INCR_MIN_CGU_SIZE: usize = 1800;
+    const NON_INCR_MIN_CGU_SIZE: usize = 65;
 
     // Repeatedly merge the two smallest codegen units as long as:
     // - we have more CGUs than the upper limit, or