Repack libgcc.a to only include required objects

Previous solution by using objcopy uses a quirky behaviour of the GNU
objcopy and there is no equivalent option in llvm-objcopy.

Instead of removing symbols, extract and repack libgcc to only include
required objects.

Bug: 142585047
Test: presubmit
Change-Id: I58af74c18838f797e481da38c3265f0624fddf99

cc/builder.go

@@ -130,6 +130,17 @@ var (
 		},
 		"args", "crossCompile")
 
+	_ = pctx.SourcePathVariable("archiveRepackPath", "build/soong/scripts/archive_repack.sh")
+
+	archiveRepack = pctx.AndroidStaticRule("archiveRepack",
+		blueprint.RuleParams{
+			Depfile:     "${out}.d",
+			Deps:        blueprint.DepsGCC,
+			Command:     "CLANG_BIN=${config.ClangBin} $archiveRepackPath -i ${in} -o ${out} -d ${out}.d ${objects}",
+			CommandDeps: []string{"$archiveRepackPath"},
+		},
+		"objects")
+
 	emptyFile = pctx.AndroidStaticRule("emptyFile",
 		blueprint.RuleParams{
 			Command: "rm -f $out && touch $out",
@@ -866,6 +877,20 @@ func TransformCoverageFilesToZip(ctx android.ModuleContext,
 	return android.OptionalPath{}
 }
 
+func TransformArchiveRepack(ctx android.ModuleContext, inputFile android.Path,
+	outputFile android.WritablePath, objects []string) {
+
+	ctx.Build(pctx, android.BuildParams{
+		Rule:        archiveRepack,
+		Description: "Repack archive " + outputFile.Base(),
+		Output:      outputFile,
+		Input:       inputFile,
+		Args: map[string]string{
+			"objects": strings.Join(objects, " "),
+		},
+	})
+}
+
 func gccCmd(toolchain config.Toolchain, cmd string) string {
 	return filepath.Join(toolchain.GccRoot(), "bin", toolchain.GccTriple()+"-"+cmd)
 }

cc/toolchain_library.go

@@ -29,6 +29,9 @@ func init() {
 type toolchainLibraryProperties struct {
 	// the prebuilt toolchain library, as a path from the top of the source tree
 	Src *string `android:"arch_variant"`
+
+	// Repack the archive with only the selected objects.
+	Repack_objects_to_keep []string `android:"arch_variant"`
 }
 
 type toolchainLibraryDecorator struct {
@@ -90,6 +93,14 @@ func (library *toolchainLibraryDecorator) link(ctx ModuleContext,
 		return outputFile
 	}
 
+	if library.Properties.Repack_objects_to_keep != nil {
+		fileName := ctx.ModuleName() + staticLibraryExtension
+		outputFile := android.PathForModuleOut(ctx, fileName)
+		TransformArchiveRepack(ctx, srcPath, outputFile, library.Properties.Repack_objects_to_keep)
+
+		return outputFile
+	}
+
 	return srcPath
 }