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

diff --git a/include/clang/Driver/Options.td b/include/clang/Driver/Options.td
index 6e45f2aef6390520bf733fcc18e62af2be39e676..5d27a9a7dc90972bf53f4efe6a84f50d9f5a86f3 100644 (file)
--- a/include/clang/Driver/Options.td
+++ b/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>,

diff --git a/include/clang/Frontend/CodeGenOptions.def b/include/clang/Frontend/CodeGenOptions.def
index 06aeea0ab89cf731f163614e89cd215504633fd7..4968e7b2bf3b72017a9c60da3c427bd4a487855f 100644 (file)
--- a/include/clang/Frontend/CodeGenOptions.def
+++ b/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

diff --git a/lib/CodeGen/CodeGenModule.cpp b/lib/CodeGen/CodeGenModule.cpp
index aaaf6c6d754cabc26b6c073d535c9726f652016d..29692eedb53a185101b103ef369518bc6ca012c0 100644 (file)
--- a/lib/CodeGen/CodeGenModule.cpp
+++ b/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);
 }
 

diff --git a/lib/Driver/ToolChains/Clang.cpp b/lib/Driver/ToolChains/Clang.cpp
index 135d40fc7008f68d23ea769bc865605f1530aebc..45c63d543016b71bdaf08f0f8221942669c8784d 100644 (file)
--- a/lib/Driver/ToolChains/Clang.cpp
+++ b/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)

diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 8e836b163efe770d3967e3d6314c76351b682553..78ed8f78207fba2522b4dd8ae53c20cf4c19831a 100644 (file)
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/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;
 }
 

diff --git a/test/CodeGen/keep-static-consts.cpp b/test/CodeGen/keep-static-consts.cpp
new file mode 100644 (file)
index 0000000..3cdac7a
--- /dev/null
+++ b/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;