[dev.regabi] cmd/go: Use GOMAXPROCS to limit default build, compile parallelism

When people want deterministic/single-process builds, they probably
assume that GOMAXPROCS=1 will do that.  It currently does not,
neither for build parallelism nor for compiler internal parallelism.
(Current incantation for that is "go build -p=1 -gcflags=all=-c=1 ... ")

This CL makes
  "GOMAXPROCS=1 go build ..."
behave like
  "go build -p=1 -gcflags=all=-c=1 ... "

RELNOTE=yes

Change-Id: I9cfe50b7deee7334d2f1057b58385f6c98547b9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/284695
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

4 files changed, 24 insertions, 21 deletions

diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index 49d390297c..da06e831ae 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -111,7 +111,7 @@
 // 	-p n
 // 		the number of programs, such as build commands or
 // 		test binaries, that can be run in parallel.
-// 		The default is the number of CPUs available.
+// 		The default is GOMAXPROCS, normally the number of CPUs available.
 // 	-race
 // 		enable data race detection.
 // 		Supported only on linux/amd64, freebsd/amd64, darwin/amd64, windows/amd64,
diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
index c48904eacc..322247962f 100644
--- a/src/cmd/go/internal/cfg/cfg.go
+++ b/src/cmd/go/internal/cfg/cfg.go
@@ -28,18 +28,18 @@ var (
 	BuildA                 bool   // -a flag
 	BuildBuildmode         string // -buildmode flag
 	BuildContext           = defaultContext()
-	BuildMod               string             // -mod flag
-	BuildModExplicit       bool               // whether -mod was set explicitly
-	BuildModReason         string             // reason -mod was set, if set by default
-	BuildI                 bool               // -i flag
-	BuildLinkshared        bool               // -linkshared flag
-	BuildMSan              bool               // -msan flag
-	BuildN                 bool               // -n flag
-	BuildO                 string             // -o flag
-	BuildP                 = runtime.NumCPU() // -p flag
-	BuildPkgdir            string             // -pkgdir flag
-	BuildRace              bool               // -race flag
-	BuildToolexec          []string           // -toolexec flag
+	BuildMod               string                  // -mod flag
+	BuildModExplicit       bool                    // whether -mod was set explicitly
+	BuildModReason         string                  // reason -mod was set, if set by default
+	BuildI                 bool                    // -i flag
+	BuildLinkshared        bool                    // -linkshared flag
+	BuildMSan              bool                    // -msan flag
+	BuildN                 bool                    // -n flag
+	BuildO                 string                  // -o flag
+	BuildP                 = runtime.GOMAXPROCS(0) // -p flag
+	BuildPkgdir            string                  // -pkgdir flag
+	BuildRace              bool                    // -race flag
+	BuildToolexec          []string                // -toolexec flag
 	BuildToolchainName     string
 	BuildToolchainCompiler func() string
 	BuildToolchainLinker   func() string
diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index 780d639c5d..0e7af6d33f 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -71,7 +71,7 @@ and test commands:
 	-p n
 		the number of programs, such as build commands or
 		test binaries, that can be run in parallel.
-		The default is the number of CPUs available.
+		The default is GOMAXPROCS, normally the number of CPUs available.
 	-race
 		enable data race detection.
 		Supported only on linux/amd64, freebsd/amd64, darwin/amd64, windows/amd64,
diff --git a/src/cmd/go/internal/work/gc.go b/src/cmd/go/internal/work/gc.go
index 3205fcbffc..2087855b3c 100644
--- a/src/cmd/go/internal/work/gc.go
+++ b/src/cmd/go/internal/work/gc.go
@@ -239,16 +239,19 @@ CheckFlags:
 	//   - it has no successor packages to compile (usually package main)
 	//   - all paths through the build graph pass through it
 	//   - critical path scheduling says it is high priority
-	// and in such a case, set c to runtime.NumCPU.
+	// and in such a case, set c to runtime.GOMAXPROCS(0).
+	// By default this is the same as runtime.NumCPU.
 	// We do this now when p==1.
+	// To limit parallelism, set GOMAXPROCS below numCPU; this may be useful
+	// on a low-memory builder, or if a deterministic build order is required.
+	c := runtime.GOMAXPROCS(0)
 	if cfg.BuildP == 1 {
-		// No process parallelism. Max out c.
-		return runtime.NumCPU()
+		// No process parallelism, do not cap compiler parallelism.
+		return c
 	}
-	// Some process parallelism. Set c to min(4, numcpu).
-	c := 4
-	if ncpu := runtime.NumCPU(); ncpu < c {
-		c = ncpu
+	// Some process parallelism. Set c to min(4, maxprocs).
+	if c > 4 {
+		c = 4
 	}
 	return c
 }