builder,loader: fix -ldflags -X not overriding variables with default values

When a string variable already had a source-level default value, the -ldflags
"-X" flag was silently ignored and the variable remained the default value
at runtime.

Fix by stripping the InitOrder entry for each -X variable before
LoadSSA() is called. With no entry in InitOrder, go/ssa emits no init
store, so the global stays zero-valued in the IR. makeGlobalsModule
still injects the actual -X values at final link time.

The -X values remain out of the per-package build cache with only the
variable names appearing in the cache key.

Signed-off-by: deadprogram <ron@hybridgroup.com>

Author: deadprogram

builder/build.go

@@ -255,6 +255,18 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		result.PackagePathMap[pkg.OriginalDir()] = pkg.Pkg.Path()
 	}
 
+	// Strip default initializers for -X globals from the type info before
+	// building SSA. This prevents go/ssa from emitting init stores for them,
+	// so that makeGlobalsModule can supply the correct values at final link
+	// time without any runtime init overwriting them. The -X values themselves
+	// are kept out of the per-package build cache; only the variable names
+	// appear in the cache key.
+	for _, pkg := range lprogram.Sorted() {
+		for name := range globalValues[pkg.Pkg.Path()] {
+			pkg.StripVarInitializer(name)
+		}
+	}
+
 	// Create the *ssa.Program. This does not yet build the entire SSA of the
 	// program so it's pretty fast and doesn't need to be parallelized.
 	program := lprogram.LoadSSA()

loader/loader.go

@@ -304,6 +304,29 @@ func (p *Program) Sorted() []*Package {
 	return p.sorted
 }
 
+// StripVarInitializer removes the package-level initializer for the named
+// variable from the type info. This prevents go/ssa from emitting an init
+// store for it, leaving the global zero-initialized in the IR. An external
+// value (e.g. from -ldflags -X via makeGlobalsModule) can then be linked in
+// at final link time without any runtime init overwriting it.
+//
+// Must be called after Parse() (typechecking populates InitOrder) and before
+// LoadSSA() (which consumes InitOrder to build the package init function).
+//
+// Only 1:1 var initializers (var x = expr) are matched. Multi-variable
+// initializers (var x, y = f()) are left untouched.
+func (p *Package) StripVarInitializer(name string) {
+	n := 0
+	for _, init := range p.info.InitOrder {
+		if len(init.Lhs) == 1 && init.Lhs[0].Name() == name {
+			continue // drop this initializer
+		}
+		p.info.InitOrder[n] = init
+		n++
+	}
+	p.info.InitOrder = p.info.InitOrder[:n]
+}
+
 // MainPkg returns the last package in the Sorted() slice. This is the main
 // package of the program.
 func (p *Program) MainPkg() *Package {

main_test.go

@@ -155,6 +155,19 @@ func TestBuild(t *testing.T) {
 			}
 			runTestWithConfig("ldflags.go", t, opts, nil, nil)
 		})
+
+		// Same as ldflags, but the global has a default value in source.
+		// -ldflags -X must override it, matching standard Go behaviour.
+		t.Run("ldflags-initialized", func(t *testing.T) {
+			t.Parallel()
+			opts := optionsFromTarget("", sema)
+			opts.GlobalValues = map[string]map[string]string{
+				"main": {
+					"someGlobal": "foobar",
+				},
+			}
+			runTestWithConfig("ldflags-initialized.go", t, opts, nil, nil)
+		})
 	})
 
 	if testing.Short() {

testdata/ldflags-initialized.go

@@ -0,0 +1,9 @@
+package main
+
+// This global has a default value. It should be overridable via
+// -ldflags="-X main.someGlobal=value" just like an uninitialized global.
+var someGlobal = "default"
+
+func main() {
+	println("someGlobal:", someGlobal)
+}

testdata/ldflags-initialized.txt

@@ -0,0 +1 @@
+someGlobal: foobar

testdata/ldflags.go

@@ -1,7 +1,6 @@
 package main
 
-// These globals can be changed using -ldflags="-X main.someGlobal=value".
-// At the moment, only globals without an initializer can be replaced this way.
+// This global can be changed using -ldflags="-X main.someGlobal=value".
 var someGlobal string
 
 func main() {