Pass -Oz/-Os along to the backend

[paquette]

Summary

This is dependent on https://reviews.llvm.org/D45916 (Enable MachineOutliner by default under -Oz for AArch64).

This patch teaches clang to pass along size optimization information along to the backend to facilitate use of the MachineOutliner.

[paquette]

Add context to diff.

[paquette]

We decided it'd be better to just use function attributes for the outliner, so I'm abandoning this.

lib/CodeGen/BackendUtil.cpp

   }
 }
 
+static CodeGenSizeOpt::Level getCGSizeOptLevel(const CodeGenOptions &CodeGenOpts) {
+  switch (CodeGenOpts.OptimizeSize) {
+    default:
+      llvm_unreachable("Invalid size optimization level!");
+    case 0:
+      return CodeGenSizeOpt::None;
+    case 1:
+      return CodeGenSizeOpt::OptSize; // Os
+    case 2:
+      return CodeGenSizeOpt::MinSize; // Oz
+  }
+}
+
 static Optional<llvm::CodeModel::Model>
 getCodeModel(const CodeGenOptions &CodeGenOpts) {
   unsigned CodeModel = llvm::StringSwitch<unsigned>(CodeGenOpts.CodeModel)

       llvm::join(TargetOpts.Features.begin(), TargetOpts.Features.end(), ",");
   llvm::Reloc::Model RM = CodeGenOpts.RelocationModel;
   CodeGenOpt::Level OptLevel = getCGOptLevel(CodeGenOpts);
-
+  CodeGenSizeOpt::Level SizeOptLevel = getCGSizeOptLevel(CodeGenOpts);
   llvm::TargetOptions Options;
   initTargetOptions(Options, CodeGenOpts, TargetOpts, LangOpts, HSOpts);
   TM.reset(TheTarget->createTargetMachine(Triple, TargetOpts.CPU, FeaturesStr,
-                                          Options, RM, CM, OptLevel));
+                                          Options, RM, CM, OptLevel,
+                                          SizeOptLevel));
 }
 
 bool EmitAssemblyHelper::AddEmitPasses(legacy::PassManager &CodeGenPasses,

test/CodeGen/arm64-outline.c

+// RUN: %clang_cc1 -emit-llvm -triple arm64--- -Oz -S -mllvm -debug-pass=Structure %s -o /dev/null 2> FileCheck %s
+// CHECK: Machine Outliner
+// RUN: %clang_cc1 -emit-llvm -Oz -S -mllvm -debug-pass=Structure %s -o /dev/null 2> FileCheck %s
+// CHECK-NOT: Machine Outliner
+// Ensure that -Oz is properly passed down to the backend by checking if the 
+// Machine Outliner is only added by default in ARM64.
+
+void foo(void) {
+}