cmd/go: skip writing dwarf debug info for ephemeral binaries

Update #6853

For an ephemeral binary - one created, run, and then deleted -
there is no need to write dwarf debug information, since the
binary will not be used with gdb. In this case, instruct the linker
not to spend time and disk space generating the debug information
by passing the -w flag to the linker.

Omitting dwarf information reduces the size of most binaries by 25%.
We may be more aggressive about this in the future.

LGTM=bradfitz, r
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/65890043

src/cmd/go/build.go

@@ -1714,6 +1714,10 @@ func (gcToolchain) ld(b *builder, p *Package, out string, allactions []*action,
 	if buildContext.InstallSuffix != "" {
 		ldflags = append(ldflags, "-installsuffix", buildContext.InstallSuffix)
 	}
+	if p.omitDWARF {
+		ldflags = append(ldflags, "-w")
+	}
+
 	// If the user has not specified the -extld option, then specify the
 	// appropriate linker. In case of C++ code, use the compiler named
 	// by the CXX environment variable or defaultCXX if CXX is not set.

src/cmd/go/pkg.go

@@ -89,6 +89,7 @@ type Package struct {
 	exeName      string               // desired name for temporary executable
 	coverMode    string               // preprocess Go source files with the coverage tool in this mode
 	coverVars    map[string]*CoverVar // variables created by coverage analysis
+	omitDWARF    bool                 // tell linker not to write DWARF information
 }
 
 // CoverVar holds the name of the generated coverage variables targeting the named file.

src/cmd/go/run.go

@@ -58,6 +58,7 @@ func runRun(cmd *Command, args []string) {
 	if p.Error != nil {
 		fatalf("%s", p.Error)
 	}
+	p.omitDWARF = true
 	for _, err := range p.DepsErrors {
 		errorf("%s", err)
 	}

src/cmd/go/test.go

@@ -654,6 +654,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		pkgdir:     testDir,
 		fake:       true,
 		Stale:      true,
+		omitDWARF:  !testC && !testNeedBinary,
 	}
 	if pxtest != nil {
 		pmain.imports = append(pmain.imports, pxtest)

test/run.go

@@ -200,7 +200,7 @@ func compileInDir(runcmd runCmd, dir string, names ...string) (out []byte, err e
 
 func linkFile(runcmd runCmd, goname string) (err error) {
 	pfile := strings.Replace(goname, ".go", "."+letter, -1)
-	_, err = runcmd("go", "tool", ld, "-o", "a.exe", "-L", ".", pfile)
+	_, err = runcmd("go", "tool", ld, "-w", "-o", "a.exe", "-L", ".", pfile)
 	return
 }