Add -fforce-emit-vtables

Summary:
 In many cases we can't devirtualize
 because definition of vtable is not present. Most of the
 time it is caused by inline virtual function not beeing
 emitted. Forcing emitting of vtable adds a reference of these
 inline virtual functions.
 Note that GCC was always doing it.

Reviewers: rjmccall, rsmith, amharc, kuhar

Subscribers: llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D47108

Co-authored-by: Krzysztof Pszeniczny <krzysztof.pszeniczny@gmail.com>
llvm-svn: 334600

4 files changed, 25 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/ItaniumCXXABI.cpp b/clang/lib/CodeGen/ItaniumCXXABI.cpp
index d84a10c6ba2..6ac7d29752d 100644
--- a/clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ b/clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -1707,11 +1707,19 @@ bool ItaniumCXXABI::canSpeculativelyEmitVTable(const CXXRecordDecl *RD) const {
   if (CGM.getLangOpts().AppleKext)
     return false;
 
-  // If we don't have any not emitted inline virtual function, and if vtable is
-  // not hidden, then we are safe to emit available_externally copy of vtable.
+  // If the vtable is hidden then it is not safe to emit an available_externally
+  // copy of vtable.
+  if (isVTableHidden(RD))
+    return false;
+
+  if (CGM.getCodeGenOpts().ForceEmitVTables)
+    return true;
+
+  // If we don't have any not emitted inline virtual function then we are safe
+  // to emit an available_externally copy of vtable.
   // FIXME we can still emit a copy of the vtable if we
   // can emit definition of the inline functions.
-  return !hasAnyUnusedVirtualInlineFunction(RD) && !isVTableHidden(RD);
+  return !hasAnyUnusedVirtualInlineFunction(RD);
 }
 static llvm::Value *performTypeAdjustment(CodeGenFunction &CGF,
                                           Address InitialPtr,
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 3eee463bb13..d392f68897c 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -3472,6 +3472,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                    options::OPT_fno_strict_vtable_pointers,
                    false))
     CmdArgs.push_back("-fstrict-vtable-pointers");
+  if (Args.hasFlag(options::OPT_fforce_emit_vtables,
+                   options::OPT_fno_force_emit_vtables,
+                   false))
+    CmdArgs.push_back("-fforce-emit-vtables");
   if (!Args.hasFlag(options::OPT_foptimize_sibling_calls,
                     options::OPT_fno_optimize_sibling_calls))
     CmdArgs.push_back("-mdisable-tail-calls");
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index a959cdb2347..2aa74c38e2b 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -719,6 +719,7 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.StrictEnums = Args.hasArg(OPT_fstrict_enums);
   Opts.StrictReturn = !Args.hasArg(OPT_fno_strict_return);
   Opts.StrictVTablePointers = Args.hasArg(OPT_fstrict_vtable_pointers);
+  Opts.ForceEmitVTables = Args.hasArg(OPT_fforce_emit_vtables);
   Opts.UnsafeFPMath = Args.hasArg(OPT_menable_unsafe_fp_math) ||
                       Args.hasArg(OPT_cl_unsafe_math_optimizations) ||
                       Args.hasArg(OPT_cl_fast_relaxed_math);
@@ -2735,6 +2736,9 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
       Args.getAllArgValues(OPT_fxray_never_instrument);
   Opts.XRayAttrListFiles = Args.getAllArgValues(OPT_fxray_attr_list);
 
+  // -fforce-emit-vtables
+  Opts.ForceEmitVTables = Args.hasArg(OPT_fforce_emit_vtables);
+
   // -fallow-editor-placeholders
   Opts.AllowEditorPlaceholders = Args.hasArg(OPT_fallow_editor_placeholders);
 
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 55517fc3f66..f5e48210924 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -6125,6 +6125,12 @@ void Sema::CheckCompletedCXXClass(CXXRecordDecl *Record) {
     Record->setParamDestroyedInCallee(true);
   else if (Record->hasNonTrivialDestructor())
     Record->setParamDestroyedInCallee(CanPass);
+
+  if (getLangOpts().ForceEmitVTables) {
+    // If we want to emit all the vtables, we need to mark it as used.  This
+    // is especially required for cases like vtable assumption loads.
+    MarkVTableUsed(Record->getInnerLocStart(), Record);
+  }
 }
 
 /// Look up the special member function that would be called by a special