Pass split-machine-functions to code generator when flto is used

-fsplit-machine-functions is an optimization in codegen phase. when -flto is use, clang generate IR bitcode in .o files, and linker will call into these codegen optimization passes. Current clang driver doesn't pass this option to linker when both -fsplit-machine-functions and -flto are used, so the optimization is silently ignored.  My fix generates linker option -plugin-opt=-split-machine-functions for this case. It allows the linker to pass "split-machine-functions" to code generator to turn on that optimization.  It works for both gold and lld.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D121969

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -574,6 +574,13 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
     CmdArgs.push_back("-plugin-opt=-data-sections");
   }
 
+  // Pass an option to enable split machine functions.
+  if (auto *A = Args.getLastArg(options::OPT_fsplit_machine_functions,
+                                options::OPT_fno_split_machine_functions)) {
+    if (A->getOption().matches(options::OPT_fsplit_machine_functions))
+      CmdArgs.push_back("-plugin-opt=-split-machine-functions");
+  }
+
   if (Arg *A = getLastProfileSampleUseArg(Args)) {
     StringRef FName = A->getValue();
     if (!llvm::sys::fs::exists(FName))

clang/test/Driver/fsplit-machine-functions2.c

@@ -0,0 +1,12 @@
+// Test -fsplit-machine-functions option pass-through with lto
+// RUN: %clang -### -target x86_64-unknown-linux -flto -fsplit-machine-functions %s 2>&1 | FileCheck %s -check-prefix=CHECK-PASS
+
+// Test no pass-through to ld without lto
+// RUN: %clang -### -target x86_64-unknown-linux -fsplit-machine-functions %s 2>&1 | FileCheck %s -check-prefix=CHECK-NOPASS
+
+// Test the mix of -fsplit-machine-functions and -fno-split-machine-functions
+// RUN: %clang -### -target x86_64-unknown-linux -flto -fsplit-machine-functions -fno-split-machine-functions %s 2>&1 | FileCheck %s -check-prefix=CHECK-NOPASS
+// RUN: %clang -### -target x86_64-unknown-linux -flto -fno-split-machine-functions -fsplit-machine-functions %s 2>&1 | FileCheck %s -check-prefix=CHECK-PASS
+
+// CHECK-PASS:          "-plugin-opt=-split-machine-functions"
+// CHECK-NOPASS-NOT:    "-plugin-opt=-split-machine-functions"