[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

1 files changed, 11 insertions, 0 deletions

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");