Currently clang does not emit unused static constants. GCC emits these

constants by default when there is no optimization.

GCC's option -fno-keep-static-consts can be used to not emit
unused static constants.

In Clang, since default behavior does not keep unused static constants, 
-fkeep-static-consts can be used to emit these if required. This could be 
useful for producing identification strings like SVN identifiers 
inside the object file even though the string isn't used by the program.

Differential Revision: https://reviews.llvm.org/D40925


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@340439 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Driver/Options.td

@@ -893,7 +893,8 @@ def fforce_enable_int128 : Flag<["-"], "fforce-enable-int128">,
 def fno_force_enable_int128 : Flag<["-"], "fno-force-enable-int128">,
   Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Disable support for int128_t type">;
-
+def fkeep_static_consts : Flag<["-"], "fkeep-static-consts">, Group<f_Group>, Flags<[CC1Option]>,
+  HelpText<"Keep static const variables even if unused">;
 def ffixed_point : Flag<["-"], "ffixed-point">, Group<f_Group>,
                    Flags<[CC1Option]>, HelpText<"Enable fixed point types">;
 def fno_fixed_point : Flag<["-"], "fno-fixed-point">, Group<f_Group>,

include/clang/Frontend/CodeGenOptions.def

@@ -341,6 +341,9 @@ CODEGENOPT(Addrsig, 1, 0)
 
 ENUM_CODEGENOPT(SignReturnAddress, SignReturnAddressScope, 2, None)
 
+/// Whether to emit unused static constants.
+CODEGENOPT(KeepStaticConsts, 1, 0)
+
 #undef CODEGENOPT
 #undef ENUM_CODEGENOPT
 #undef VALUE_CODEGENOPT

lib/CodeGen/CodeGenModule.cpp

@@ -1350,6 +1350,12 @@ void CodeGenModule::SetCommonAttributes(GlobalDecl GD, llvm::GlobalValue *GV) {
 
   if (D && D->hasAttr<UsedAttr>())
     addUsedGlobal(GV);
+
+  if (CodeGenOpts.KeepStaticConsts && D && isa<VarDecl>(D)) {
+    const auto *VD = cast<VarDecl>(D);
+    if (VD->getType().isConstQualified() && VD->getStorageClass() == SC_Static)
+      addUsedGlobal(GV);
+  }
 }
 
 bool CodeGenModule::GetCPUAndFeaturesAttributes(const Decl *D,
@@ -1985,6 +1991,13 @@ bool CodeGenModule::MustBeEmitted(const ValueDecl *Global) {
   if (LangOpts.EmitAllDecls)
     return true;
 
+  if (CodeGenOpts.KeepStaticConsts) {
+    const auto *VD = dyn_cast<VarDecl>(Global);
+    if (VD && VD->getType().isConstQualified() &&
+        VD->getStorageClass() == SC_Static)
+      return true;
+  }
+
   return getContext().DeclMustBeEmitted(Global);
 }
 

lib/Driver/ToolChains/Clang.cpp

@@ -4008,6 +4008,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.AddLastArg(CmdArgs, options::OPT_fno_operator_names);
   Args.AddLastArg(CmdArgs, options::OPT_femulated_tls,
                   options::OPT_fno_emulated_tls);
+  Args.AddLastArg(CmdArgs, options::OPT_fkeep_static_consts);
 
   // AltiVec-like language extensions aren't relevant for assembling.
   if (!isa<PreprocessJobAction>(JA) || Output.getType() != types::TY_PP_Asm)

lib/Frontend/CompilerInvocation.cpp

@@ -1145,6 +1145,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
           << A->getAsString(Args) << A->getValue();
   }
 
+  Opts.KeepStaticConsts = Args.hasArg(OPT_fkeep_static_consts);
+
   return Success;
 }
 

test/CodeGen/keep-static-consts.cpp

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -fkeep-static-consts -emit-llvm %s -o - -triple=x86_64-unknown-linux-gnu | FileCheck %s
+
+// CHECK: @_ZL7srcvers = internal constant [4 x i8] c"xyz\00", align 1
+// CHECK: @llvm.used = appending global [1 x i8*] [i8* getelementptr inbounds ([4 x i8], [4 x i8]* @_ZL7srcvers, i32 0, i32 0)], section "llvm.metadata"
+static const char srcvers[] = "xyz";
+extern const int b = 1;