[PATCH] [ARM] Save option "arm-long-calls" to the IR as a function attribute

[Akira Hatanaka]

Hi echristo, dexonsmith,

This patch is part of the work to make LTO and function multi-versioning work correctly.

Currently, -mlong-calls, which is converted to cc1 option -arm-long-calls, is ignored when building with LTO because the option isn't passed to the linker or libLTO. This patch saves the option in the IR as a function attribute to fix this problem.

The corresponding llvm patch is here:
http://reviews.llvm.org/D9364

There are a few things to discuss:

1. Should "arm-long-calls" be a binary attribute or a tri-state like "unsafe-fp-math" that takes a value "true" or "false"? I made it a binary attribute because it simplifies the backend and frontend without breaking backward compatibility, but might be use cases that I'm unaware of in which this approach wouldn't work.

2. Since we are saving the option in the IR, should we stop passing it as a cc1 backend option and stop passing it to llvm::cl::ParseCommandLineOptions? It's not needed if this attribute is a tri-state, but is needed if it's a binary to preserve backward compatibility. By "backward compatibility", I mean the following commands should produce the same result before and after this patch is committed:

1. clang -target armv7-apple-ios -static -mlong-calls old.bc -o old
2. clang -target armv7-apple-ios -static old.bc -o old

Here, old.bc is generated by an older version of clang that doesn't save "arm-long-calls" in the IR.

http://reviews.llvm.org/D9414

Files:
  include/clang/Frontend/CodeGenOptions.h
  lib/CodeGen/CGCall.cpp
  lib/Frontend/CompilerInvocation.cpp
  test/CodeGen/fn-attr.c

Index: include/clang/Frontend/CodeGenOptions.h
===================================================================
--- include/clang/Frontend/CodeGenOptions.h
+++ include/clang/Frontend/CodeGenOptions.h
@@ -151,6 +151,9 @@
   /// A list of command-line options to forward to the LLVM backend.
   std::vector<std::string> BackendOptions;
 
+  /// A list of function attributes to save to the IR.
+  std::vector<std::pair<std::string, std::string>> FunctionAttributes;
+
   /// A list of dependent libraries.
   std::vector<std::string> DependentLibraries;
 
Index: lib/CodeGen/CGCall.cpp
===================================================================
--- lib/CodeGen/CGCall.cpp
+++ lib/CodeGen/CGCall.cpp
@@ -1455,6 +1455,9 @@
       FuncAttrs.addAttribute(llvm::Attribute::NoBuiltin);
   } else {
     // Attributes that should go on the function, but not the call site.
+    for (auto &KV : CodeGenOpts.FunctionAttributes)
+      FuncAttrs.addAttribute(KV.first, KV.second);
+
     if (!CodeGenOpts.DisableFPElim) {
       FuncAttrs.addAttribute("no-frame-pointer-elim", "false");
     } else if (CodeGenOpts.OmitLeafFramePointer) {
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -340,6 +340,14 @@
   }
 }
 
+static void getFunctionAttributes(CodeGenOptions &Opts) {
+  StringRef Opt = "-arm-long-calls", Key = Opt.drop_front(1);
+
+  if (std::find(Opts.BackendOptions.begin(), Opts.BackendOptions.end(),
+                Opt) != Opts.BackendOptions.end())
+    Opts.FunctionAttributes.push_back(std::make_pair(Key, ""));
+}
+
 static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
                              DiagnosticsEngine &Diags,
                              const TargetOptions &TargetOpts) {
@@ -643,6 +651,8 @@
                       Args.getAllArgValues(OPT_fsanitize_recover_EQ), Diags,
                       Opts.SanitizeRecover);
 
+  getFunctionAttributes(Opts);
+
   return Success;
 }
 
Index: test/CodeGen/fn-attr.c
===================================================================
--- /dev/null
+++ test/CodeGen/fn-attr.c
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -triple thumbv7-apple-ios5 -backend-option -arm-long-calls -emit-llvm -o - %s | FileCheck -check-prefix=LONGCALL %s
+// RUN: %clang_cc1 -triple thumbv7-apple-ios5 -emit-llvm -o - %s | FileCheck -check-prefix=NOLONGCALL %s
+
+// LONGCALL: attributes #0 = { {{.*}} "arm-long-calls
+// NOLONGCALL-NOT: attributes #0 = { {{.*}} "arm-long-calls
+
+int foo1(int a) { return a; }

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