go/ssa: create fewer goroutines

This CL limits the number of goroutines created by Program.Build
to the number of P's, since building is entirely CPU bound.
Creating more than this number of goroutines just increases
churn, and potentially increases cache and allocator contention,
and increases peak memory usage and GC.

The effect on TestStdlib's "build SSA" timing metric seems
to be a reduction of around 2-3%, but it's fairly noisy.

Change-Id: Ie85e93b99c27ece245be05892818d52ae3b7fbc1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/572796
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Tim King <[email protected]>

Author: adonovan

go/ssa/builder.go

@@ -79,6 +79,7 @@ import (
 	"go/token"
 	"go/types"
 	"os"
+	"runtime"
 	"sync"
 
 	"golang.org/x/tools/internal/aliases"
@@ -2622,15 +2623,20 @@ func (prog *Program) Build() {
 			p.Build()
 		} else {
 			wg.Add(1)
+			cpuLimit <- struct{}{} // acquire a token
 			go func(p *Package) {
 				p.Build()
 				wg.Done()
+				<-cpuLimit // release a token
 			}(p)
 		}
 	}
 	wg.Wait()
 }
 
+// cpuLimit is a counting semaphore to limit CPU parallelism.
+var cpuLimit = make(chan struct{}, runtime.GOMAXPROCS(0))
+
 // Build builds SSA code for all functions and vars in package p.
 //
 // CreatePackage must have been called for all of p's direct imports