Propagate "-O" to Clang (as "-Os").

It affects Clang IRGen, among other things.

include/swift/ClangImporter/ClangImporterOptions.h

@@ -47,6 +47,10 @@ public:
   /// header, place it in this directory.
   std::string PrecompiledHeaderOutputDir;
 
+  /// The optimizaton setting.  This doesn't typically matter for
+  /// import, but it can affect Clang's IR generation of static functions.
+  std::string Optimization;
+
   /// Disable validating the persistent PCH.
   bool PCHDisableValidation = false;
 

lib/ClangImporter/ClangImporter.cpp

@@ -686,6 +686,10 @@ addCommonInvocationArguments(std::vector<std::string> &invocationArgStrs,
     invocationArgStrs.push_back("-march=z196");
   }
 
+  if (!importerOpts.Optimization.empty()) {
+    invocationArgStrs.push_back(importerOpts.Optimization);
+  }
+
   const std::string &overrideResourceDir = importerOpts.OverrideResourceDir;
   if (overrideResourceDir.empty()) {
     llvm::SmallString<128> resourceDir(searchPathOpts.RuntimeResourcePath);
@@ -868,6 +872,7 @@ ClangImporter::create(ASTContext &ctx,
   }
 
   std::vector<const char *> invocationArgs;
+  invocationArgs.reserve(invocationArgStrs.size());
   for (auto &argStr : invocationArgStrs)
     invocationArgs.push_back(argStr.c_str());
 

lib/Frontend/CompilerInvocation.cpp

@@ -1352,7 +1352,8 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
                          IRGenOptions &IRGenOpts,
                          FrontendOptions &FEOpts,
                          DiagnosticEngine &Diags,
-                         const llvm::Triple &Triple) {
+                         const llvm::Triple &Triple,
+                         ClangImporterOptions &ClangOpts) {
   using namespace options;
 
   if (const Arg *A = Args.getLastArg(OPT_sil_inline_threshold)) {
@@ -1408,6 +1409,10 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
       IRGenOpts.Optimize = true;
       Opts.Optimization = SILOptions::SILOptMode::Optimize;
     }
+
+    if (IRGenOpts.Optimize) {
+      ClangOpts.Optimization = "-Os";
+    }
   }
 
   if (Args.getLastArg(OPT_AssumeSingleThreaded)) {
@@ -1799,7 +1804,7 @@ bool CompilerInvocation::parseArgs(ArrayRef<const char *> Args,
   }
 
   if (ParseSILArgs(SILOpts, ParsedArgs, IRGenOpts, FrontendOpts, Diags,
-                   LangOpts.Target)) {
+                   LangOpts.Target, ClangImporterOpts)) {
     return true;
   }
 

test/IRGen/Inputs/c_functions.h

@@ -20,3 +20,7 @@ static inline void log_a_thing(const a_thing thing) {
  useInt(thing.val[0]);
  useInt(thing.val[7]);
 }
+
+static inline unsigned int return7(void) {
+  return 7;
+}

test/IRGen/clang_inline_opt.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-frontend -import-objc-header %S/Inputs/c_functions.h -primary-file %s -O -emit-ir -disable-llvm-optzns | %FileCheck %s
+
+func return10() -> UInt32 {
+	return return7() + 3
+}
+
+// Sanity check that we tell Clang to generate optimizable code when
+// we're optimizing.
+
+// CHECK: define internal i32 @return7() [[CLANG_ATTRS:#[0-9]+]] {
+
+// CHECK: attributes [[CLANG_ATTRS]] = {
+// CHECK-NOT: noinline
+// CHECK-SAME: }