New clang option -mpie-copy-relocationss to indicate support for linker copy relocations when linking as PIE

[tmsriram]

Summary

With linker copy relocations, PIE can generate better code for external global variable accesses. This patch adds a new option to clang to specify this. Please see http://reviews.llvm.org/D24849 for the patch to LLVM to optimize global accesses when this is available.

[• rafael]

Is there a gcc option or they just assume they are targeting the
linker that was around when gcc was built?

+ if (Args.hasFlag(options::OPT_mpiecopyrelocs, options::OPT_mno_piecopyrelocs,
+ false)) {
+ CmdArgs.push_back("-piecopyrelocs");
+ }

you don't need the {}

+def piecopyrelocs : Flag<["-"], "piecopyrelocs">,
+ HelpText<"Linker copy relocations support when linking as PIE">;

I think you are missing a verb: Linker copy relocations *are* supported.

But how about just "Position independent executables can have copy relocations"?

Cheers,
Rafael

[tmsriram]

• Option name changed to -mpie-copy-relocations
• Option sets MCPIECopyRelocations in MCOptions

[rnk]

In D19996#422906, @rafael wrote:

Is there a gcc option or they just assume they are targeting the
linker that was around when gcc was built?

IIRC there's no GCC option. They base their decision on the configure-time check like you suggested.

include/clang/Driver/Options.td

1696 ↗

(On Diff #74757)

I agree with Rafael, this needs a verb, like "Use copy relocations in PIE builds" or something.

1698 ↗

(On Diff #74757)

The "no" option variants don't need to be CC1 options.

1699 ↗

(On Diff #74757)

I don't think we need help text for the "no" variant. I think we add help text to "no" flags when the flag is on by default, for example -fno-exceptions.

[tmsriram]

• Changed help text to "Avail copy relocations support for PIE builds"
• Removed mno-pie... as a CC1 option
• Removed help text from mno-pie..

[rnk]

lgtm

include/clang/Driver/Options.td

1696 ↗

(On Diff #74757)

I guess "avail" is a verb, but I've never seen anyone use it. I'd really rather use "use". :)

[mgrang]

lib/Driver/Tools.cpp

4502 ↗

(On Diff #75179)

Please fix indentation.

lib/Frontend/CompilerInvocation.cpp

589 ↗

(On Diff #75179)

You should also check for the negative flag here:

Opts.PIECopyRelocations = Args.hasFlag(options::OPT_mpie_copy_relocations, options::OPT_mno_pie_copy_relocations, false);

[tmsriram]

lib/Frontend/CompilerInvocation.cpp

589 ↗

(On Diff #75179)

mpie_copy_relocations is the only CC1 flag here and that is pushed into CC1 command after checking for the negative.

[tmsriram]

• Fix Help text.
• Fix indentation.

cfe/trunk/include/clang/Driver/Options.td

 def msmall_data_threshold_EQ : Joined <["-"], "msmall-data-threshold=">, Group<m_Group>;
 def msoft_float : Flag<["-"], "msoft-float">, Group<m_Group>, Flags<[CC1Option]>,
   HelpText<"Use software floating point">;
 def mno_implicit_float : Flag<["-"], "mno-implicit-float">, Group<m_Group>,
   HelpText<"Don't generate implicit floating point instructions">;
 def mimplicit_float : Flag<["-"], "mimplicit-float">, Group<m_Group>;
 def mrecip : Flag<["-"], "mrecip">, Group<m_Group>;
 def mrecip_EQ : CommaJoined<["-"], "mrecip=">, Group<m_Group>, Flags<[CC1Option]>;
+def mpie_copy_relocations : Flag<["-"], "mpie-copy-relocations">, Group<m_Group>,
+  Flags<[CC1Option]>,
+  HelpText<"Use copy relocations support for PIE builds">;
+def mno_pie_copy_relocations : Flag<["-"], "mno-pie-copy-relocations">, Group<m_Group>;
 def mx87 : Flag<["-"], "mx87">, Group<m_x86_Features_Group>;
 def m80387 : Flag<["-"], "m80387">, Alias<mx87>;
 def msse2 : Flag<["-"], "msse2">, Group<m_x86_Features_Group>;
 def msse3 : Flag<["-"], "msse3">, Group<m_x86_Features_Group>;
 def msse4a : Flag<["-"], "msse4a">, Group<m_x86_Features_Group>;
 def msse4_1 : Flag<["-"], "msse4.1">, Group<m_x86_Features_Group>;
 def msse4_2 : Flag<["-"], "msse4.2">, Group<m_x86_Features_Group>;
 def msse4 : Flag<["-"], "msse4">, Alias<msse4_2>;

cfe/trunk/include/clang/Frontend/CodeGenOptions.def

 
 /// Number of path components to strip when emitting checks. (0 == full
 /// filename)
 VALUE_CODEGENOPT(EmitCheckPathComponentsToStrip, 32, 0)
 
 /// Whether to report the hotness of the code region for optimization remarks.
 CODEGENOPT(DiagnosticsWithHotness, 1, 0)
 
+/// Whether copy relocations support is available when building as PIE.
+CODEGENOPT(PIECopyRelocations, 1, 0)
+
 #undef CODEGENOPT
 #undef ENUM_CODEGENOPT
 #undef VALUE_CODEGENOPT
 

cfe/trunk/lib/CodeGen/BackendUtil.cpp

void EmitAssemblyHelper::CreateTargetMachine(bool MustCreateTM) {
   Options.DebuggerTuning = CodeGenOpts.getDebuggerTuning();
 
   Options.MCOptions.MCRelaxAll = CodeGenOpts.RelaxAll;
   Options.MCOptions.MCSaveTempLabels = CodeGenOpts.SaveTempLabels;
   Options.MCOptions.MCUseDwarfDirectory = !CodeGenOpts.NoDwarfDirectoryAsm;
   Options.MCOptions.MCNoExecStack = CodeGenOpts.NoExecStack;
   Options.MCOptions.MCIncrementalLinkerCompatible =
       CodeGenOpts.IncrementalLinkerCompatible;
+  Options.MCOptions.MCPIECopyRelocations =
+      CodeGenOpts.PIECopyRelocations;
   Options.MCOptions.MCFatalWarnings = CodeGenOpts.FatalWarnings;
   Options.MCOptions.AsmVerbose = CodeGenOpts.AsmVerbose;
   Options.MCOptions.PreserveAsmComments = CodeGenOpts.PreserveAsmComments;
   Options.MCOptions.ABIName = TargetOpts.ABI;
 
   TM.reset(TheTarget->createTargetMachine(Triple, TargetOpts.CPU, FeaturesStr,
                                           Options, RM, CM, OptLevel));
 }

cfe/trunk/lib/Driver/Tools.cpp

#endif
   if (KernelOrKext && getToolChain().getTriple().isOSDarwin())
     CmdArgs.push_back("-fforbid-guard-variables");
 
   if (Args.hasFlag(options::OPT_mms_bitfields, options::OPT_mno_ms_bitfields,
                    false)) {
     CmdArgs.push_back("-mms-bitfields");
   }
 
+  if (Args.hasFlag(options::OPT_mpie_copy_relocations,
+                   options::OPT_mno_pie_copy_relocations,
+                   false)) {
+    CmdArgs.push_back("-mpie-copy-relocations");
+  }
+
   // This is a coarse approximation of what llvm-gcc actually does, both
   // -fasynchronous-unwind-tables and -fnon-call-exceptions interact in more
   // complicated ways.
   bool AsynchronousUnwindTables =
       Args.hasFlag(options::OPT_fasynchronous_unwind_tables,
                    options::OPT_fno_asynchronous_unwind_tables,
                    (getToolChain().IsUnwindTablesDefault() ||
                     getToolChain().getSanitizerArgs().needsUnwindTables()) &&

cfe/trunk/lib/Frontend/CompilerInvocation.cpp

static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.BackendOptions = Args.getAllArgValues(OPT_backend_option);
   Opts.NumRegisterParameters = getLastArgIntValue(Args, OPT_mregparm, 0, Diags);
   Opts.NoExecStack = Args.hasArg(OPT_mno_exec_stack);
   Opts.FatalWarnings = Args.hasArg(OPT_massembler_fatal_warnings);
   Opts.EnableSegmentedStacks = Args.hasArg(OPT_split_stacks);
   Opts.RelaxAll = Args.hasArg(OPT_mrelax_all);
   Opts.IncrementalLinkerCompatible =
       Args.hasArg(OPT_mincremental_linker_compatible);
+  Opts.PIECopyRelocations =
+      Args.hasArg(OPT_mpie_copy_relocations);
   Opts.OmitLeafFramePointer = Args.hasArg(OPT_momit_leaf_frame_pointer);
   Opts.SaveTempLabels = Args.hasArg(OPT_msave_temp_labels);
   Opts.NoDwarfDirectoryAsm = Args.hasArg(OPT_fno_dwarf_directory_asm);
   Opts.SoftFloat = Args.hasArg(OPT_msoft_float);
   Opts.StrictEnums = Args.hasArg(OPT_fstrict_enums);
   Opts.StrictVTablePointers = Args.hasArg(OPT_fstrict_vtable_pointers);
   Opts.UnsafeFPMath = Args.hasArg(OPT_menable_unsafe_fp_math) ||
                       Args.hasArg(OPT_cl_unsafe_math_optimizations) ||