[ThinLTO] Enable index-only WPD from clang

Summary:
To trigger the index-only Whole Program Devirt support added to LLVM, we
need to be able to specify -fno-split-lto-unit in conjunction with
-fwhole-program-vtables. Keep the default for -fwhole-program-vtables as
-fsplit-lto-unit, but don't error on that option combination.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, steven_wu, dexonsmith, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68029

llvm-svn: 373370

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5456,14 +5456,13 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back("-fwhole-program-vtables");
   }
 
-  bool RequiresSplitLTOUnit = WholeProgramVTables || Sanitize.needsLTO();
+  bool DefaultsSplitLTOUnit = 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");
+                   options::OPT_fno_split_lto_unit, DefaultsSplitLTOUnit);
+  if (Sanitize.needsLTO() && !SplitLTOUnit)
+    D.Diag(diag::err_drv_argument_not_allowed_with) << "-fno-split-lto-unit"
+                                                    << "-fsanitize=cfi";
   if (SplitLTOUnit)
     CmdArgs.push_back("-fsplit-lto-unit");
 

clang/test/Driver/split-lto-unit.c

@@ -6,5 +6,5 @@
 
 // UNIT: "-fsplit-lto-unit"
 // NOUNIT-NOT: "-fsplit-lto-unit"
-// ERROR1: error: invalid argument '-fno-split-lto-unit' not allowed with '-fwhole-program-vtables'
+// ERROR1-NOT: error: invalid argument
 // ERROR2: error: invalid argument '-fno-split-lto-unit' not allowed with '-fsanitize=cfi'