rustc: Reduce default CGUs to 16

Rationale explained in the included comment as well as #44941

Author: alexcrichton

src/librustc/session/config.rs

@@ -1716,7 +1716,7 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
 
     let codegen_units = codegen_units.unwrap_or_else(|| {
         match opt_level {
-            // If we're compiling at `-O0` then default to 32 codegen units.
+            // If we're compiling at `-O0` then default to 16 codegen units.
             // The number here shouldn't matter too too much as debug mode
             // builds don't rely on performance at all, meaning that lost
             // opportunities for inlining through multiple codegen units is
@@ -1734,7 +1734,21 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
             // unit takes *too* long to build we'll be guaranteed that all
             // cpus will finish pretty closely to one another and we should
             // make relatively optimal use of system resources
-            OptLevel::No => 32,
+            //
+            // Another note worth mentioning here, however, is that this number
+            // isn't *too* high. When codegen units are increased that means we
+            // currently have to codegen `#[inline]` functions into each codegen
+            // unit, which means the more codegen units we're using the more we
+            // may be generating. In other words, increasing codegen units may
+            // increase the overall work the compiler does. If we don't have
+            // enough cores to make up for this loss then increasing the number
+            // of codegen units could become an overall loss!
+            //
+            // As a result we choose a hopefully conservative value 16, which
+            // should be more than the number of cpus of most hardware compiling
+            // Rust but also not too much for 2-4 core machines to have too much
+            // loss of compile time.
+            OptLevel::No => 16,
 
             // All other optimization levels default use one codegen unit,
             // the historical default in Rust for a Long Time.