Base CGU estimates on basic block count, not statement count.

Author: nnethercote

compiler/rustc_middle/src/mir/mono.rs

@@ -61,7 +61,7 @@ impl<'tcx> MonoItem<'tcx> {
             MonoItem::Fn(instance) => {
                 // Estimate the size of a function based on how many statements
                 // it contains.
-                tcx.instance_def_size_estimate(instance.def)
+                1 + tcx.instance_def_size_estimate(instance.def)
             }
             // Conservatively estimate the size of a static declaration
             // or assembly to be 1.

compiler/rustc_monomorphize/src/partitioning.rs

@@ -322,9 +322,9 @@ 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 20,000+ 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 = 500;
 
     // Repeatedly merge the two smallest codegen units as long as:
     // - we have more CGUs than the upper limit, or

compiler/rustc_ty_utils/src/ty.rs

@@ -459,7 +459,7 @@ fn instance_def_size_estimate<'tcx>(
     match instance_def {
         InstanceDef::Item(..) | InstanceDef::DropGlue(..) => {
             let mir = tcx.instance_mir(instance_def);
-            mir.basic_blocks.iter().map(|bb| bb.statements.len() + 1).sum()
+            mir.basic_blocks.len()
         }
         // Estimate the size of other compiler-generated shims to be 1.
         _ => 1,