[clang] Add a command line flag for the Machine Function Splitter.

This patch adds a command line flag for the machine function splitter
(added in rG94faadaca4e1).

-fsplit-machine-functions
Split machine functions using profile information (x86 ELF). On
other targets an error is emitted. If profile information is not
provided a warning is emitted notifying the user that profile
information is required.

Differential Revision: https://reviews.llvm.org/D87047

clang/include/clang/Basic/CodeGenOptions.def

@@ -162,6 +162,7 @@ CODEGENOPT(NoImplicitFloat   , 1, 0) ///< Set when -mno-implicit-float is enable
 CODEGENOPT(NullPointerIsValid , 1, 0) ///< Assume Null pointer deference is defined.
 CODEGENOPT(CorrectlyRoundedDivSqrt, 1, 0) ///< -cl-fp32-correctly-rounded-divide-sqrt
 CODEGENOPT(UniqueInternalLinkageNames, 1, 0) ///< Internal Linkage symbols get unique names.
+CODEGENOPT(SplitMachineFunctions, 1, 0) ///< Split machine functions using profile information.
 
 /// When false, this attempts to generate code as if the result of an
 /// overflowing conversion matches the overflowing behavior of a target's native

clang/include/clang/Driver/Options.td

@@ -1996,6 +1996,9 @@ defm unique_internal_linkage_names : OptInFFlag<"unique-internal-linkage-names",
 defm unique_section_names : OptOutFFlag<"unique-section-names",
   "", "Don't use unique names for text and data sections">;
 
+defm split_machine_functions: OptInFFlag<"split-machine-functions",
+  "Enable", "Disable", " late function splitting using profile information (x86 ELF)">;
+
 defm strict_return : OptOutFFlag<"strict-return", "",
   "Don't treat control flow paths that fall off the end of a non-void function as unreachable">;
 

clang/lib/CodeGen/BackendUtil.cpp

@@ -514,6 +514,7 @@ static void initTargetOptions(DiagnosticsEngine &Diags,
       Options.BBSectionsFuncListBuf = std::move(*MBOrErr);
   }
 
+  Options.EnableMachineFunctionSplitter = CodeGenOpts.SplitMachineFunctions;
   Options.FunctionSections = CodeGenOpts.FunctionSections;
   Options.DataSections = CodeGenOpts.DataSections;
   Options.UniqueSectionNames = CodeGenOpts.UniqueSectionNames;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4911,6 +4911,26 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                    options::OPT_fno_unique_basic_block_section_names, false))
     CmdArgs.push_back("-funique-basic-block-section-names");
 
+  if (Arg *A = Args.getLastArg(options::OPT_fsplit_machine_functions,
+                               options::OPT_fno_split_machine_functions)) {
+    // This codegen pass is only available on x86-elf targets.
+    if (Triple.isX86() && Triple.isOSBinFormatELF()) {
+      if (A->getOption().matches(options::OPT_fsplit_machine_functions)) {
+        // If the flag is enabled but no profile information is available then
+        // emit a warning.
+        if (getLastProfileUseArg(Args) || getLastProfileSampleUseArg(Args)) {
+          A->render(Args, CmdArgs);
+        } else {
+          D.Diag(diag::warn_drv_diagnostics_hotness_requires_pgo)
+              << A->getAsString(Args);
+        }
+      }
+    } else {
+      D.Diag(diag::err_drv_unsupported_opt_for_target)
+          << A->getAsString(Args) << TripleStr;
+    }
+  }
+
   Args.AddLastArg(CmdArgs, options::OPT_finstrument_functions,
                   options::OPT_finstrument_functions_after_inlining,
                   options::OPT_finstrument_function_entry_bare);

clang/lib/Frontend/CompilerInvocation.cpp

@@ -998,6 +998,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.UniqueInternalLinkageNames =
       Args.hasArg(OPT_funique_internal_linkage_names);
 
+  Opts.SplitMachineFunctions = Args.hasArg(OPT_fsplit_machine_functions);
+
   Opts.MergeFunctions = Args.hasArg(OPT_fmerge_functions);
 
   Opts.NoUseJumpTables = Args.hasArg(OPT_fno_jump_tables);

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

@@ -0,0 +1,9 @@
+// RUN: %clang -### -target x86_64 -fprofile-use=default.profdata -fsplit-machine-functions %s -c 2>&1 | FileCheck -check-prefix=CHECK-OPT %s
+// RUN: %clang -### -target x86_64 -fprofile-use=default.profdata -fsplit-machine-functions -fno-split-machine-functions %s -c 2>&1 | FileCheck -check-prefix=CHECK-NOOPT %s
+// RUN: %clang -### -target x86_64 -fsplit-machine-functions %s 2>&1 | FileCheck -check-prefix=CHECK-WARN %s
+// RUN: not %clang -c -target arm-unknown-linux -fsplit-machine-functions %s 2>&1 | FileCheck -check-prefix=CHECK-TRIPLE %s
+
+// CHECK-OPT:       "-fsplit-machine-functions"
+// CHECK-NOOPT-NOT: "-fsplit-machine-functions"
+// CHECK-WARN:      warning: argument '-fsplit-machine-functions' requires profile-guided optimization information
+// CHECK-TRIPLE:    error: unsupported option '-fsplit-machine-functions' for target