Re-land r337333, "Teach Clang to emit address-significance tables.",

which was reverted in r337336.

The problem that required a revert was fixed in r337338.

Also added a missing "REQUIRES: x86-registered-target" to one of
the tests.

Original commit message:
> Teach Clang to emit address-significance tables.
>
> By default, we emit an address-significance table on all ELF
> targets when the integrated assembler is enabled. The emission of an
> address-significance table can be controlled with the -faddrsig and
> -fno-addrsig flags.
>
> Differential Revision: https://reviews.llvm.org/D48155

llvm-svn: 337339

clang/docs/ReleaseNotes.rst

@@ -98,6 +98,14 @@ Non-comprehensive list of changes in this release
   finding out the warning hierarchy, and which of them are enabled by default
   or for a particular compiler invocation.
 
+- By default, Clang emits an address-significance table into
+  every ELF object file when using the integrated assembler.
+  Address-significance tables allow linkers to implement `safe ICF
+  <https://research.google.com/pubs/archive/36912.pdf>`_ without the false
+  positives that can result from other implementation techniques such as
+  relocation scanning. The ``-faddrsig`` and ``-fno-addrsig`` flags can be
+  used to control whether to emit the address-significance table.
+
 - ...
 
 New Compiler Flags

clang/docs/UsersManual.rst

@@ -1382,6 +1382,15 @@ are listed below.
         // value of -fmax-type-align.
       }
 
+.. option:: -faddrsig, -fno-addrsig
+
+   Controls whether Clang emits an address-significance table into the object
+   file. Address-significance tables allow linkers to implement `safe ICF
+   <https://research.google.com/pubs/archive/36912.pdf>`_ without the false
+   positives that can result from other implementation techniques such as
+   relocation scanning. Address-significance tables are enabled by default
+   on ELF targets when using the integrated assembler. This flag currently
+   only has an effect on ELF targets.
 
 Profile Guided Optimization
 ---------------------------

clang/include/clang/Driver/Options.td

@@ -758,6 +758,10 @@ def fno_profile_instr_use : Flag<["-"], "fno-profile-instr-use">,
 def fno_profile_use : Flag<["-"], "fno-profile-use">,
     Alias<fno_profile_instr_use>;
 
+def faddrsig : Flag<["-"], "faddrsig">, Group<f_Group>, Flags<[CoreOption, CC1Option]>,
+  HelpText<"Emit an address-significance table">;
+def fno_addrsig : Flag<["-"], "fno-addrsig">, Group<f_Group>, Flags<[CoreOption]>,
+  HelpText<"Don't emit an address-significance table">;
 def fblocks : Flag<["-"], "fblocks">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Enable the 'blocks' language feature">;
 def fbootclasspath_EQ : Joined<["-"], "fbootclasspath=">, Group<f_Group>;

clang/include/clang/Frontend/CodeGenOptions.def

@@ -335,6 +335,9 @@ CODEGENOPT(EmbedSource, 1, 0)
 /// Whether to emit all vtables
 CODEGENOPT(ForceEmitVTables, 1, 0)
 
+/// Whether to emit an address-significance table into the object file.
+CODEGENOPT(Addrsig, 1, 0)
+
 
 #undef CODEGENOPT
 #undef ENUM_CODEGENOPT

clang/lib/CodeGen/BackendUtil.cpp

@@ -454,6 +454,7 @@ static void initTargetOptions(llvm::TargetOptions &Options,
   Options.ExplicitEmulatedTLS = CodeGenOpts.ExplicitEmulatedTLS;
   Options.DebuggerTuning = CodeGenOpts.getDebuggerTuning();
   Options.EmitStackSizeSection = CodeGenOpts.StackSizeSection;
+  Options.EmitAddrsig = CodeGenOpts.Addrsig;
 
   if (CodeGenOpts.EnableSplitDwarf)
     Options.MCOptions.SplitDwarfFile = CodeGenOpts.SplitDwarfFile;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4777,6 +4777,11 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     }
   }
 
+  if (Args.hasFlag(options::OPT_faddrsig, options::OPT_fno_addrsig,
+                   getToolChain().getTriple().isOSBinFormatELF() &&
+                       getToolChain().useIntegratedAs()))
+    CmdArgs.push_back("-faddrsig");
+
   // Finally add the compile command to the compilation.
   if (Args.hasArg(options::OPT__SLASH_fallback) &&
       Output.getType() == types::TY_Object &&

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1119,6 +1119,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
 
   Opts.EmitVersionIdentMetadata = Args.hasFlag(OPT_Qy, OPT_Qn, true);
 
+  Opts.Addrsig = Args.hasArg(OPT_faddrsig);
+
   return Success;
 }
 

clang/test/CodeGen/addrsig.c

@@ -0,0 +1,20 @@
+// REQUIRES: x86-registered-target
+
+// RUN: %clang_cc1 -triple=x86_64-unknown-linux -S %s -faddrsig -O -o - | FileCheck --check-prefix=ADDRSIG %s
+// RUN: %clang_cc1 -triple=x86_64-unknown-linux -S %s -O -o - | FileCheck --check-prefix=NO-ADDRSIG %s
+
+// ADDRSIG: .addrsig
+// ADDRSIG: .addrsig_sym g1
+// ADDRSIG-NOT: .addrsig_sym g2
+
+// NO-ADDRSIG-NOT: .addrsig
+
+extern const int g1[], g2[];
+
+const int *f1() {
+  return g1;
+}
+
+int f2() {
+  return g2[0];
+}

clang/test/Driver/addrsig.c

@@ -0,0 +1,8 @@
+// RUN: %clang -### -target x86_64-unknown-linux -c %s 2>&1 | FileCheck -check-prefix=ADDRSIG %s
+// RUN: %clang -### -target x86_64-unknown-linux -fno-integrated-as -c %s 2>&1 | FileCheck -check-prefix=NO-ADDRSIG %s
+// RUN: %clang -### -target x86_64-unknown-linux -fno-integrated-as -faddrsig -c %s 2>&1 | FileCheck -check-prefix=ADDRSIG %s
+// RUN: %clang -### -target x86_64-unknown-linux -fno-addrsig -c %s 2>&1 | FileCheck -check-prefix=NO-ADDRSIG %s
+// RUN: %clang -### -target x86_64-apple-darwin -c %s 2>&1 | FileCheck -check-prefix=NO-ADDRSIG %s
+
+// ADDRSIG: -faddrsig
+// NO-ADDRSIG-NOT: -faddrsig