cmd/go: don't assemble all .s files in a single cmd/asm run

For the 1.8 release, go back to invoking the assembler once per .s
file, to avoid the problem in #18225. When the assembler is fixed, the
change to cmd/go/build.go can be rolled back, but the test in
cmd/go/go_test.go should remain.

Fixes #18225.
Update #15680.

Change-Id: Ibff8d0c638536efb50a2b2c280b41399332f4fe4
Reviewed-on: https://go-review.googlesource.com/34284
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/build.go

@@ -2406,8 +2406,7 @@ func (gcToolchain) gc(b *builder, p *Package, archive, obj string, asmhdr bool,
 func (gcToolchain) asm(b *builder, p *Package, obj string, sfiles []string) ([]string, error) {
 	// Add -I pkg/GOOS_GOARCH so #include "textflag.h" works in .s files.
 	inc := filepath.Join(goroot, "pkg", "include")
-	ofile := obj + "asm.o"
-	args := []interface{}{buildToolExec, tool("asm"), "-o", ofile, "-trimpath", b.work, "-I", obj, "-I", inc, "-D", "GOOS_" + goos, "-D", "GOARCH_" + goarch, buildAsmflags}
+	args := []interface{}{buildToolExec, tool("asm"), "-trimpath", b.work, "-I", obj, "-I", inc, "-D", "GOOS_" + goos, "-D", "GOARCH_" + goarch, buildAsmflags}
 	if p.ImportPath == "runtime" && goarch == "386" {
 		for _, arg := range buildAsmflags {
 			if arg == "-dynlink" {
@@ -2415,13 +2414,16 @@ func (gcToolchain) asm(b *builder, p *Package, obj string, sfiles []string) ([]s
 			}
 		}
 	}
+	var ofiles []string
 	for _, sfile := range sfiles {
-		args = append(args, mkAbs(p.Dir, sfile))
-	}
-	if err := b.run(p.Dir, p.ImportPath, nil, args...); err != nil {
-		return nil, err
+		ofile := obj + sfile[:len(sfile)-len(".s")] + ".o"
+		ofiles = append(ofiles, ofile)
+		a := append(args, "-o", ofile, mkAbs(p.Dir, sfile))
+		if err := b.run(p.Dir, p.ImportPath, nil, a...); err != nil {
+			return nil, err
+		}
 	}
-	return []string{ofile}, nil
+	return ofiles, nil
 }
 
 // toolVerify checks that the command line args writes the same output file

src/cmd/go/go_test.go

@@ -3727,3 +3727,19 @@ func TestLdBindNow(t *testing.T) {
 	tg.setenv("LD_BIND_NOW", "1")
 	tg.run("help")
 }
+
+// Issue 18225.
+// This is really a cmd/asm issue but this is a convenient place to test it.
+func TestConcurrentAsm(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	asm := `DATA ·constants<>+0x0(SB)/8,$0
+GLOBL ·constants<>(SB),8,$8
+`
+	tg.tempFile("go/src/p/a.s", asm)
+	tg.tempFile("go/src/p/b.s", asm)
+	tg.tempFile("go/src/p/p.go", `package p`)
+	tg.setenv("GOPATH", tg.path("go"))
+	tg.run("build", "p")
+}