[WebAssembly] Add --gc-sections to the link line.

This will eventually be accompanied with a change to enable -ffunction-sections
and -fdata-sections by default, which is currently delayed by some development
process issues.

llvm-svn: 256967

clang/lib/Driver/Tools.cpp

@@ -6534,6 +6534,11 @@ void wasm::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   ArgStringList CmdArgs;
   CmdArgs.push_back("-flavor");
   CmdArgs.push_back("ld");
+
+  // Enable garbage collection of unused input sections by default, since code
+  // size is of particular importance.
+  CmdArgs.push_back("--gc-sections");
+
   AddLinkerInputs(getToolChain(), Inputs, Args, CmdArgs);
   CmdArgs.push_back("-o");
   CmdArgs.push_back(Output.getFilename());

clang/test/Driver/wasm-toolchain.c

@@ -1,3 +1,3 @@
 // RUN: %clang -### -no-canonical-prefixes -target wasm32-unknown-unknown -x assembler %s 2>&1 | FileCheck -check-prefix=AS_LINK %s
 // AS_LINK: clang{{.*}}" "-cc1as" {{.*}} "-o" "[[temp:[^"]*]]"
-// AS_LINK: lld{{.*}}" "-flavor" "ld" "[[temp]]" "-o" "a.out"
+// AS_LINK: lld{{.*}}" "-flavor" "ld" "--gc-sections" "[[temp]]" "-o" "a.out"