[PATCH] D39079: New clang option -fno-plt to avoid PLT for external calls

[Sriraman Tallam via Phabricator via cfe-commits]

tmsriram created this revision.

New clang option -fno-plt which avoids the PLT and lazy binding while making external calls.

GCC supports -fno-plt, https://gcc.gnu.org/ml/gcc-patches/2015-05/msg00001.html. This patch adds this to clang which marks all externally defined functions with the "nolazybind" attribute.  This LLVM patch skips the PLT for calls to these functions, https://reviews.llvm.org/D39065.


https://reviews.llvm.org/D39079

Files:
  include/clang/Driver/Options.td
  include/clang/Frontend/CodeGenOptions.def
  lib/CodeGen/CGCall.cpp
  lib/Driver/ToolChains/Clang.cpp
  lib/Frontend/CompilerInvocation.cpp


Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -650,6 +650,7 @@
       Args.hasArg(OPT_mincremental_linker_compatible);
   Opts.PIECopyRelocations =
       Args.hasArg(OPT_mpie_copy_relocations);
+  Opts.NoPLT = Args.hasArg(OPT_fno_plt);
   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);
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -3386,6 +3386,10 @@
     CmdArgs.push_back("-mpie-copy-relocations");
   }
 
+  if (Args.hasFlag(options::OPT_fno_plt, options::OPT_fplt, false)) {
+    CmdArgs.push_back("-fno-plt");
+  }
+
   // -fhosted is default.
   // TODO: Audit uses of KernelOrKext and see where it'd be more appropriate to
   // use Freestanding.
Index: lib/CodeGen/CGCall.cpp
===================================================================
--- lib/CodeGen/CGCall.cpp
+++ lib/CodeGen/CGCall.cpp
@@ -1852,6 +1852,16 @@
       !(TargetDecl && TargetDecl->hasAttr<NoSplitStackAttr>()))
     FuncAttrs.addAttribute("split-stack");
 
+  // Add NonLazyBind attribute to function declarations when -fno-plt
+  // is used.
+  if (TargetDecl && CodeGenOpts.NoPLT) {
+    if (auto *Fn = dyn_cast<FunctionDecl>(TargetDecl)) {
+      if (!Fn->isDefined() && !AttrOnCallSite) {
+        FuncAttrs.addAttribute(llvm::Attribute::NonLazyBind);
+      }
+    }
+  }
+
   if (!AttrOnCallSite) {
     bool DisableTailCalls =
         CodeGenOpts.DisableTailCalls ||
Index: include/clang/Frontend/CodeGenOptions.def
===================================================================
--- include/clang/Frontend/CodeGenOptions.def
+++ include/clang/Frontend/CodeGenOptions.def
@@ -294,6 +294,8 @@
 /// Whether to emit .debug_gnu_pubnames section instead of .debug_pubnames.
 CODEGENOPT(GnuPubnames, 1, 0)
 
+CODEGENOPT(NoPLT, 1, 0)
+
 #undef CODEGENOPT
 #undef ENUM_CODEGENOPT
 #undef VALUE_CODEGENOPT
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -1363,6 +1363,10 @@
 def fno_pic : Flag<["-"], "fno-pic">, Group<f_Group>;
 def fpie : Flag<["-"], "fpie">, Group<f_Group>;
 def fno_pie : Flag<["-"], "fno-pie">, Group<f_Group>;
+def fplt : Flag<["-"], "fplt">, Group<f_Group>, Flags<[CC1Option]>,
+  HelpText<"Use the PLT to make function calls">;
+def fno_plt : Flag<["-"], "fno-plt">, Group<f_Group>, Flags<[CC1Option]>,
+  HelpText<"Do not use the PLT to make function calls">;
 def fropi : Flag<["-"], "fropi">, Group<f_Group>;
 def fno_ropi : Flag<["-"], "fno-ropi">, Group<f_Group>;
 def frwpi : Flag<["-"], "frwpi">, Group<f_Group>;


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