[LTO] Add option to enable LTOUnit splitting, and disable unless needed

Summary:
Adds a new -f[no]split-lto-unit flag that is disabled by default to
control module splitting during ThinLTO. It is automatically enabled
for -fsanitize=cfi and -fwhole-program-vtables.

The new EnableSplitLTOUnit codegen flag is passed down to llvm
via a new module flag of the same name.

Depends on D53890.

Reviewers: pcc

Subscribers: ormris, mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D53891

llvm-svn: 350949

4 files changed, 34 insertions, 10 deletions

diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 1311af91686..3e0651978b4 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -814,6 +814,8 @@ void EmitAssemblyHelper::EmitAssembly(BackendAction Action,
         if (!ThinLinkOS)
           return;
       }
+      TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
+                               CodeGenOpts.EnableSplitLTOUnit);
       PerModulePasses.add(createWriteThinLTOBitcodePass(
           *OS, ThinLinkOS ? &ThinLinkOS->os() : nullptr));
     } else {
@@ -824,12 +826,15 @@ void EmitAssemblyHelper::EmitAssembly(BackendAction Action,
            !CodeGenOpts.DisableLLVMPasses &&
            llvm::Triple(TheModule->getTargetTriple()).getVendor() !=
                llvm::Triple::Apple);
-      if (EmitLTOSummary && !TheModule->getModuleFlag("ThinLTO"))
-        TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
+      if (EmitLTOSummary) {
+        if (!TheModule->getModuleFlag("ThinLTO"))
+          TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
+        TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
+                                 CodeGenOpts.EnableSplitLTOUnit);
+      }
 
-      PerModulePasses.add(
-          createBitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists,
-                                  EmitLTOSummary));
+      PerModulePasses.add(createBitcodeWriterPass(
+          *OS, CodeGenOpts.EmitLLVMUseLists, EmitLTOSummary));
     }
     break;
 
@@ -1054,6 +1059,8 @@ void EmitAssemblyHelper::EmitAssemblyWithNewPassManager(
         if (!ThinLinkOS)
           return;
       }
+      TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
+                               CodeGenOpts.EnableSplitLTOUnit);
       MPM.addPass(ThinLTOBitcodeWriterPass(*OS, ThinLinkOS ? &ThinLinkOS->os()
                                                            : nullptr));
     } else {
@@ -1064,11 +1071,14 @@ void EmitAssemblyHelper::EmitAssemblyWithNewPassManager(
            !CodeGenOpts.DisableLLVMPasses &&
            llvm::Triple(TheModule->getTargetTriple()).getVendor() !=
                llvm::Triple::Apple);
-      if (EmitLTOSummary && !TheModule->getModuleFlag("ThinLTO"))
-        TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
-
-      MPM.addPass(BitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists,
-                                    EmitLTOSummary));
+      if (EmitLTOSummary) {
+        if (!TheModule->getModuleFlag("ThinLTO"))
+          TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
+        TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
+                                 CodeGenOpts.EnableSplitLTOUnit);
+      }
+      MPM.addPass(
+          BitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists, EmitLTOSummary));
     }
     break;
 
diff --git a/clang/lib/Driver/SanitizerArgs.cpp b/clang/lib/Driver/SanitizerArgs.cpp
index 6667cbb347c..1a46073aaa3 100644
--- a/clang/lib/Driver/SanitizerArgs.cpp
+++ b/clang/lib/Driver/SanitizerArgs.cpp
@@ -207,6 +207,8 @@ bool SanitizerArgs::needsUnwindTables() const {
   return Sanitizers.Mask & NeedsUnwindTables;
 }
 
+bool SanitizerArgs::needsLTO() const { return Sanitizers.Mask & NeedsLTO; }
+
 SanitizerArgs::SanitizerArgs(const ToolChain &TC,
                              const llvm::opt::ArgList &Args) {
   SanitizerMask AllRemove = 0;  // During the loop below, the accumulated set of
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index eb88a71e492..4b999925273 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -5226,6 +5226,17 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back("-fwhole-program-vtables");
   }
 
+  bool RequiresSplitLTOUnit = WholeProgramVTables || Sanitize.needsLTO();
+  bool SplitLTOUnit =
+      Args.hasFlag(options::OPT_fsplit_lto_unit,
+                   options::OPT_fno_split_lto_unit, RequiresSplitLTOUnit);
+  if (RequiresSplitLTOUnit && !SplitLTOUnit)
+    D.Diag(diag::err_drv_argument_not_allowed_with)
+        << "-fno-split-lto-unit"
+        << (WholeProgramVTables ? "-fwhole-program-vtables" : "-fsanitize=cfi");
+  if (SplitLTOUnit)
+    CmdArgs.push_back("-fsplit-lto-unit");
+
   if (Arg *A = Args.getLastArg(options::OPT_fexperimental_isel,
                                options::OPT_fno_experimental_isel)) {
     CmdArgs.push_back("-mllvm");
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 1e857355b33..3e6528c2598 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -907,6 +907,7 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
       Diags.Report(diag::err_drv_invalid_value) << A->getAsString(Args) << S;
   }
   Opts.LTOUnit = Args.hasFlag(OPT_flto_unit, OPT_fno_lto_unit, false);
+  Opts.EnableSplitLTOUnit = Args.hasArg(OPT_fsplit_lto_unit);
   if (Arg *A = Args.getLastArg(OPT_fthinlto_index_EQ)) {
     if (IK.getLanguage() != InputKind::LLVM_IR)
       Diags.Report(diag::err_drv_argument_only_allowed_with)