cmd/go, cmd/compile: always optimize when building runtime

When optimizations are disabled, the compiler
cannot eliminate enough write barriers to satisfy
the runtime's nowritebarrier and nowritebarrierrec
annotations.

Enforce that requirement, and for convenience,
have cmd/go elide -N when compiling the runtime.

This came up in practice for me when running
toolstash -cmp. When toolstash -cmp detected
mismatches, it recompiled with -N, which caused
runtime compilation failures.

Change-Id: Ifcdef22c725baf2c59a09470f00124361508a8f3
Reviewed-on: https://go-review.googlesource.com/38380
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -268,6 +268,9 @@ func Main(archInit func(*Arch)) {
 	} else if flag_race || flag_msan {
 		instrumenting = true
 	}
+	if compiling_runtime && Debug['N'] != 0 {
+		log.Fatal("cannot disable optimizations while compiling runtime")
+	}
 
 	// parse -d argument
 	if debugstr != "" {

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

@@ -2176,7 +2176,8 @@ func (gcToolchain) gc(b *Builder, p *load.Package, archive, obj string, asmhdr b
 	if p.Name == "main" {
 		gcargs[1] = "main"
 	}
-	if p.Standard && (p.ImportPath == "runtime" || strings.HasPrefix(p.ImportPath, "runtime/internal")) {
+	compilingRuntime := p.Standard && (p.ImportPath == "runtime" || strings.HasPrefix(p.ImportPath, "runtime/internal"))
+	if compilingRuntime {
 		// runtime compiles with a special gc flag to emit
 		// additional reflect type data.
 		gcargs = append(gcargs, "-+")
@@ -2211,7 +2212,22 @@ func (gcToolchain) gc(b *Builder, p *load.Package, archive, obj string, asmhdr b
 		}
 	}
 
-	args := []interface{}{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", b.WorkDir, buildGcflags, gcargs, "-D", p.Internal.LocalPrefix, importArgs}
+	gcflags := buildGcflags
+	if compilingRuntime {
+		// Remove -N, if present.
+		// It is not possible to build the runtime with no optimizations,
+		// because the compiler cannot eliminate enough write barriers.
+		gcflags = make([]string, len(buildGcflags))
+		copy(gcflags, buildGcflags)
+		for i := 0; i < len(gcflags); i++ {
+			if gcflags[i] == "-N" {
+				copy(gcflags[i:], gcflags[i+1:])
+				gcflags = gcflags[:len(gcflags)-1]
+				i--
+			}
+		}
+	}
+	args := []interface{}{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", b.WorkDir, gcflags, gcargs, "-D", p.Internal.LocalPrefix, importArgs}
 	if ofile == archive {
 		args = append(args, "-pack")
 	}