cmd/go: put user flags after code generation flag

This permits the user to override the code generation flag when they
know better. This is always a good policy for all flags automatically
inserted by the build system.

Doing this now so that I can write a test for #20290.

Update #20290

Change-Id: I5c6708a277238d571b8d037993a5a59e2a442e98
Reviewed-on: https://go-review.googlesource.com/42952
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -4009,3 +4009,31 @@ func TestNeedVersion(t *testing.T) {
 	tg.runFail("run", path)
 	tg.grepStderr("compile", "does not match go tool version")
 }
+
+// Test that user can override default code generation flags.
+func TestUserOverrideFlags(t *testing.T) {
+	if !canCgo {
+		t.Skip("skipping because cgo not enabled")
+	}
+	if runtime.GOOS != "linux" {
+		// We are testing platform-independent code, so it's
+		// OK to skip cases that work differently.
+		t.Skipf("skipping on %s because test only works if c-archive implies -shared", runtime.GOOS)
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("override.go", `package main
+
+import "C"
+
+//export GoFunc
+func GoFunc() {}
+
+func main() {}`)
+	tg.creatingTemp("override.a")
+	tg.creatingTemp("override.h")
+	tg.run("build", "-x", "-buildmode=c-archive", "-gcflags=-shared=false", tg.path("override.go"))
+	tg.grepStderr("compile .*-shared .*-shared=false", "user can not override code generation flag")
+}

src/cmd/go/internal/work/build.go

@@ -379,10 +379,10 @@ func BuildModeInit() {
 	}
 	if codegenArg != "" {
 		if gccgo {
-			buildGccgoflags = append(buildGccgoflags, codegenArg)
+			buildGccgoflags = append([]string{codegenArg}, buildGccgoflags...)
 		} else {
-			buildAsmflags = append(buildAsmflags, codegenArg)
-			buildGcflags = append(buildGcflags, codegenArg)
+			buildAsmflags = append([]string{codegenArg}, buildAsmflags...)
+			buildGcflags = append([]string{codegenArg}, buildGcflags...)
 		}
 		// Don't alter InstallSuffix when modifying default codegen args.
 		if cfg.BuildBuildmode != "default" || cfg.BuildLinkshared {