[PATCH] D40925: Add option -fkeep-static-consts

[Elizabeth Andrews via Phabricator via cfe-commits]

eandrews updated this revision to Diff 161307.
eandrews edited the summary of this revision.
eandrews added a comment.

This patch fell through the cracks earlier. I apologize. Based on Reid's and Erich's feedback, I am now adding the variable to @llvm.used. Additionally I modified the if statements to ensure only static variables are considered.

I also compared the IR when using the option -fkeep-static-consts while compiling the program, vs adding __attribute__((used)) to individual variables inside the program. The generated IR had no difference. I assume this means all required information to prevent the variable from being removed in later stages are now being emitted?


https://reviews.llvm.org/D40925

Files:
  include/clang/Basic/LangOptions.def
  include/clang/Driver/Options.td
  lib/AST/ASTContext.cpp
  lib/CodeGen/CodeGenModule.cpp
  lib/Driver/ToolChains/Clang.cpp
  lib/Frontend/CompilerInvocation.cpp
  test/CodeGen/keep-static-consts.cpp


Index: test/CodeGen/keep-static-consts.cpp
===================================================================
--- /dev/null
+++ 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;
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -2485,6 +2485,7 @@
   Opts.HalfArgsAndReturns = Args.hasArg(OPT_fallow_half_arguments_and_returns)
                             | Opts.NativeHalfArgsAndReturns;
   Opts.GNUAsm = !Args.hasArg(OPT_fno_gnu_inline_asm);
+  Opts.KeepStaticConsts = Args.hasArg(OPT_fkeep_static_consts);
 
   // __declspec is enabled by default for the PS4 by the driver, and also
   // enabled for Microsoft Extensions or Borland Extensions, here.
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -3996,6 +3996,7 @@
   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)
Index: lib/CodeGen/CodeGenModule.cpp
===================================================================
--- lib/CodeGen/CodeGenModule.cpp
+++ lib/CodeGen/CodeGenModule.cpp
@@ -1350,6 +1350,12 @@
 
   if (D && D->hasAttr<UsedAttr>())
     addUsedGlobal(GV);
+
+  if (LangOpts.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,
Index: lib/AST/ASTContext.cpp
===================================================================
--- lib/AST/ASTContext.cpp
+++ lib/AST/ASTContext.cpp
@@ -9741,6 +9741,14 @@
   if (D->hasAttr<AliasAttr>() || D->hasAttr<UsedAttr>())
     return true;
 
+  // Emit static constants even if they are not used if KeepStaticConsts is set.
+  if (LangOpts.KeepStaticConsts) {
+    const auto *VD = dyn_cast<VarDecl>(D);
+    if (VD && VD->getType().isConstQualified() &&
+        VD->getStorageClass() == SC_Static)
+      return true;
+  }
+
   if (const auto *FD = dyn_cast<FunctionDecl>(D)) {
     // Forward declarations aren't required.
     if (!FD->doesThisDeclarationHaveABody())
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -893,7 +893,8 @@
 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>,
Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -308,6 +308,8 @@
 LANGOPT(PaddingOnUnsignedFixedPoint, 1, 0,
         "unsigned fixed point types having one extra padding bit")
 
+BENIGN_LANGOPT(KeepStaticConsts      , 1, 0, "keep static const variables even if unused")
+
 #undef LANGOPT
 #undef COMPATIBLE_LANGOPT
 #undef BENIGN_LANGOPT


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D40925.161307.patch
Type: text/x-patch
Size: 4226 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20180817/b5d0345c/attachment.bin>