From: Elizabeth Andrews <elizabeth.andrews@intel.com>
Date: Wed, 22 Aug 2018 19:05:19 +0000 (+0000)
Subject: Currently clang does not emit unused static constants. GCC emits these
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=6a00520d81d71f6c1df16288cf7f65b8ab9ac124;p=clang

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 6e45f2aef6..5d27a9a7dc 100644
--- 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 06aeea0ab8..4968e7b2bf 100644
--- 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 aaaf6c6d75..29692eedb5 100644
--- 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 135d40fc70..45c63d5430 100644
--- 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 8e836b163e..78ed8f7820 100644
--- 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
index 0000000000..3cdac7ad1a
--- /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;