cmd/go: fix go command fails to perform concurrent compilation

CL 344909 fixed the bug in the order of passing gcflags from cmd/go to
cmd/compile. In that process, we merged the flags passed by cmd/go and
the flags specified by "-gcflags" to one variable. That causes the
gcBackendConcurrency function fails to detect concurrency level, since
when it expects only the latter flags.

To fix this, just don't merge those two variables, so we can now
correctly detect the concurrency level and passing the right -c to
the compiler.

Fixes #48490

Change-Id: I1293a7d6b946b7fccdd5cd34a38452bf6306e115
Reviewed-on: https://go-review.googlesource.com/c/go/+/351049
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

src/cmd/go/internal/work/gc.go

@@ -75,7 +75,7 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 	}
 
 	pkgpath := pkgPath(a)
-	gcflags := []string{"-p", pkgpath}
+	defaultGcFlags := []string{"-p", pkgpath}
 	if p.Module != nil {
 		v := p.Module.GoVersion
 		if v == "" {
@@ -94,19 +94,19 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 			v = "1.16"
 		}
 		if allowedVersion(v) {
-			gcflags = append(gcflags, "-lang=go"+v)
+			defaultGcFlags = append(defaultGcFlags, "-lang=go"+v)
 		}
 	}
 	if p.Standard {
-		gcflags = append(gcflags, "-std")
+		defaultGcFlags = append(defaultGcFlags, "-std")
 	}
 	_, compilingRuntime := runtimePackages[p.ImportPath]
 	compilingRuntime = compilingRuntime && p.Standard
 	if compilingRuntime {
 		// runtime compiles with a special gc flag to check for
 		// memory allocations that are invalid in the runtime package,
 		// and to implement some special compiler pragmas.
-		gcflags = append(gcflags, "-+")
+		defaultGcFlags = append(defaultGcFlags, "-+")
 	}
 
 	// If we're giving the compiler the entire package (no C etc files), tell it that,
@@ -125,25 +125,25 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 		}
 	}
 	if extFiles == 0 {
-		gcflags = append(gcflags, "-complete")
+		defaultGcFlags = append(defaultGcFlags, "-complete")
 	}
 	if cfg.BuildContext.InstallSuffix != "" {
-		gcflags = append(gcflags, "-installsuffix", cfg.BuildContext.InstallSuffix)
+		defaultGcFlags = append(defaultGcFlags, "-installsuffix", cfg.BuildContext.InstallSuffix)
 	}
 	if a.buildID != "" {
-		gcflags = append(gcflags, "-buildid", a.buildID)
+		defaultGcFlags = append(defaultGcFlags, "-buildid", a.buildID)
 	}
 	if p.Internal.OmitDebug || cfg.Goos == "plan9" || cfg.Goarch == "wasm" {
-		gcflags = append(gcflags, "-dwarf=false")
+		defaultGcFlags = append(defaultGcFlags, "-dwarf=false")
 	}
 	if strings.HasPrefix(runtimeVersion, "go1") && !strings.Contains(os.Args[0], "go_bootstrap") {
-		gcflags = append(gcflags, "-goversion", runtimeVersion)
+		defaultGcFlags = append(defaultGcFlags, "-goversion", runtimeVersion)
 	}
 	if symabis != "" {
-		gcflags = append(gcflags, "-symabis", symabis)
+		defaultGcFlags = append(defaultGcFlags, "-symabis", symabis)
 	}
 
-	gcflags = append(gcflags, str.StringList(forcedGcflags, p.Internal.Gcflags)...)
+	gcflags := str.StringList(forcedGcflags, p.Internal.Gcflags)
 	if compilingRuntime {
 		// Remove -N, if present.
 		// It is not possible to build the runtime with no optimizations,
@@ -157,7 +157,7 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 		}
 	}
 
-	args := []interface{}{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", a.trimpath(), gcflags}
+	args := []interface{}{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", a.trimpath(), defaultGcFlags, gcflags}
 	if p.Internal.LocalPrefix != "" {
 		// Workaround #43883.
 		args = append(args, "-D", p.Internal.LocalPrefix)

src/cmd/go/testdata/script/build_concurrent_backend.txt

@@ -0,0 +1,14 @@
+# Tests golang.org/issue/48490
+# cmd/go should enable concurrent compilation by default
+
+# Skip test on darwin/arm64, see #48496.
+# TODO(cuonglm): remove this once #48496 is fixed.
+[darwin] [arm64] skip
+
+# Reset all experiments, since one of them can disable
+# concurrent compilation, e.g: fieldtrack.
+env GOEXPERIMENT=none
+
+env GOMAXPROCS=4
+go build -n -x -a fmt
+stderr ' -c=4 '