[Driver] Add -Wa, options --crel and --allow-experimental-crel

The two options are discussed in a few comments around
#91280 (comment)

* -Wa,--crel: error "-Wa,--allow-experimental-crel must be specified to use -Wa,--crel..."
* -Wa,--allow-experimental-crel: no-op
* -Wa,--crel,--allow-experimental-crel: enable CREL in the integrated assembler (#91280)

MIPS's little-endian n64 ABI messed up the `r_info` field in
relocations. While this could be fixed with CREL, my intention is to
avoid complication in assembler/linker. The implementation simply
doesn't allow CREL for MIPS.

Link: https://discourse.llvm.org/t/rfc-crel-a-compact-relocation-format-for-elf/77600

Pull Request: #97378

Author: MaskRay

clang/include/clang/Basic/CodeGenOptions.def

@@ -36,6 +36,7 @@ VALUE_CODEGENOPT(Name, Bits, Default)
 #endif
 
 CODEGENOPT(DisableIntegratedAS, 1, 0) ///< -no-integrated-as
+CODEGENOPT(Crel, 1, 0) ///< -Wa,--crel
 CODEGENOPT(RelaxELFRelocations, 1, 1) ///< -Wa,-mrelax-relocations={yes,no}
 CODEGENOPT(AsmVerbose        , 1, 0) ///< -dA, -fverbose-asm.
 CODEGENOPT(PreserveAsmComments, 1, 1) ///< -dA, -fno-preserve-as-comments.

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -801,6 +801,10 @@ def warn_drv_missing_multilib : Warning<
 def note_drv_available_multilibs : Note<
   "available multilibs are:%0">;
 
+def err_drv_experimental_crel : Error<
+  "-Wa,--allow-experimental-crel must be specified to use -Wa,--crel. "
+  "CREL is experimental and uses a non-standard section type code">;
+
 def warn_android_unversioned_fallback : Warning<
   "using unversioned Android target directory %0 for target %1; unversioned "
   "directories will not be used in Clang 19 -- provide a versioned directory "

clang/include/clang/Driver/Options.td

@@ -7027,6 +7027,9 @@ def massembler_no_warn : Flag<["-"], "massembler-no-warn">,
 def massembler_fatal_warnings : Flag<["-"], "massembler-fatal-warnings">,
   HelpText<"Make assembler warnings fatal">,
   MarshallingInfoFlag<CodeGenOpts<"FatalWarnings">>;
+def crel : Flag<["--"], "crel">,
+  HelpText<"Enable CREL relocation format (ELF only)">,
+  MarshallingInfoFlag<CodeGenOpts<"Crel">>;
 def mrelax_relocations_no : Flag<["-"], "mrelax-relocations=no">,
     HelpText<"Disable x86 relax relocations">,
     MarshallingInfoNegativeFlag<CodeGenOpts<"RelaxELFRelocations">>;

clang/lib/CodeGen/BackendUtil.cpp

@@ -470,6 +470,7 @@ static bool initTargetOptions(DiagnosticsEngine &Diags,
   Options.MCOptions.AsmVerbose = CodeGenOpts.AsmVerbose;
   Options.MCOptions.Dwarf64 = CodeGenOpts.Dwarf64;
   Options.MCOptions.PreserveAsmComments = CodeGenOpts.PreserveAsmComments;
+  Options.MCOptions.Crel = CodeGenOpts.Crel;
   Options.MCOptions.X86RelaxRelocations = CodeGenOpts.RelaxELFRelocations;
   Options.MCOptions.CompressDebugSections =
       CodeGenOpts.getCompressDebugSections();

clang/lib/Driver/ToolChains/Clang.cpp

@@ -2500,6 +2500,8 @@ static void CollectArgsForIntegratedAssembler(Compilation &C,
   // arg after parsing the '-I' arg.
   bool TakeNextArg = false;
 
+  const llvm::Triple &Triple = C.getDefaultToolChain().getTriple();
+  bool Crel = false, ExperimentalCrel = false;
   bool UseRelaxRelocations = C.getDefaultToolChain().useRelaxRelocations();
   bool UseNoExecStack = false;
   const char *MipsTargetFeature = nullptr;
@@ -2623,6 +2625,12 @@ static void CollectArgsForIntegratedAssembler(Compilation &C,
                  Value == "-nocompress-debug-sections" ||
                  Value == "--nocompress-debug-sections") {
         CmdArgs.push_back(Value.data());
+      } else if (Value == "--crel") {
+        Crel = true;
+      } else if (Value == "--no-crel") {
+        Crel = false;
+      } else if (Value == "--allow-experimental-crel") {
+        ExperimentalCrel = true;
       } else if (Value == "-mrelax-relocations=yes" ||
                  Value == "--mrelax-relocations=yes") {
         UseRelaxRelocations = true;
@@ -2688,6 +2696,16 @@ static void CollectArgsForIntegratedAssembler(Compilation &C,
   }
   if (ImplicitIt.size())
     AddARMImplicitITArgs(Args, CmdArgs, ImplicitIt);
+  if (Crel) {
+    if (!ExperimentalCrel)
+      D.Diag(diag::err_drv_experimental_crel);
+    if (Triple.isOSBinFormatELF() && !Triple.isMIPS()) {
+      CmdArgs.push_back("--crel");
+    } else {
+      D.Diag(diag::err_drv_unsupported_opt_for_target)
+          << "-Wa,--crel" << D.getTargetTriple();
+    }
+  }
   if (!UseRelaxRelocations)
     CmdArgs.push_back("-mrelax-relocations=no");
   if (UseNoExecStack)

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -1133,6 +1133,27 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
 
   addMachineOutlinerArgs(D, Args, CmdArgs, ToolChain.getEffectiveTriple(),
                          /*IsLTO=*/true, PluginOptPrefix);
+
+  for (const Arg *A : Args.filtered(options::OPT_Wa_COMMA)) {
+    bool Crel = false;
+    for (StringRef V : A->getValues()) {
+      if (V == "--crel")
+        Crel = true;
+      else if (V == "--no-crel")
+        Crel = false;
+      else
+        continue;
+      A->claim();
+    }
+    if (Crel) {
+      if (Triple.isOSBinFormatELF() && !Triple.isMIPS()) {
+        CmdArgs.push_back(Args.MakeArgString(Twine(PluginOptPrefix) + "-crel"));
+      } else {
+        D.Diag(diag::err_drv_unsupported_opt_for_target)
+            << "-Wa,--crel" << D.getTargetTriple();
+      }
+    }
+  }
 }
 
 /// Adds the '-lcgpu' and '-lmgpu' libraries to the compilation to include the

clang/test/Driver/crel.c

@@ -0,0 +1,31 @@
+// RUN: not %clang -### -c --target=x86_64 -Wa,--crel %s 2>&1 | FileCheck %s --check-prefix=NOEXP
+
+// NOEXP: error: -Wa,--allow-experimental-crel must be specified to use -Wa,--crel. CREL is experimental and uses a non-standard section type code
+
+// RUN: %clang -### -c --target=x86_64 -Wa,--crel,--allow-experimental-crel %s -Werror 2>&1 | FileCheck %s
+// RUN: %clang -### -c --target=x86_64 -Wa,--crel,--no-crel,--allow-experimental-crel %s -Werror 2>&1 | FileCheck %s --check-prefix=NO
+// RUN: %clang -### -c --target=x86_64 -Wa,--allow-experimental-crel %s -Werror 2>&1 | FileCheck %s --check-prefix=NO
+// RUN: not %clang -### -c --target=arm64-apple-darwin -Wa,--crel,--allow-experimental-crel %s 2>&1 | FileCheck %s --check-prefix=ERR
+// RUN: not %clang -### -c --target=mips64 -Wa,--crel,--allow-experimental-crel %s 2>&1 | FileCheck %s --check-prefix=ERR
+
+// RUN: %clang -### -c --target=aarch64 -Werror -Wa,--crel,--allow-experimental-crel -x assembler %s -Werror 2>&1 | FileCheck %s --check-prefix=ASM
+// RUN: not %clang -### -c --target=mips64 -Wa,--crel,--allow-experimental-crel -x assembler %s 2>&1 | FileCheck %s --check-prefix=ERR
+
+// CHECK:  "-cc1" {{.*}}"--crel"
+// NO:     "-cc1"
+// NO-NOT: "--crel"
+// ASM:    "-cc1as" {{.*}}"--crel"
+// ERR: error: unsupported option '-Wa,--crel' for target '{{.*}}'
+
+/// The --allow-experimental-crel error check is exempted for -fno-integrated-as.
+// RUN: %clang -### -c --target=aarch64 -fno-integrated-as -Wa,--crel %s -Werror 2>&1 | FileCheck %s --check-prefix=GAS
+
+// GAS: "--crel"
+
+/// The --allow-experimental-crel error check doesn't apply to LTO.
+// RUN: %clang -### --target=x86_64-linux -Werror -flto -Wa,--crel %s 2>&1 | FileCheck %s --check-prefix=LTO
+
+// LTO: "-plugin-opt=-crel"
+
+// RUN: touch %t.o
+// RUN: not %clang -### --target=mips64-linux-gnu -flto -Wa,--crel %t.o 2>&1 | FileCheck %s --check-prefix=ERR

clang/test/Misc/cc1as-crel.s

@@ -0,0 +1,6 @@
+// REQUIRES: x86-registered-target
+// RUN: %clang -cc1as -triple x86_64 %s -filetype obj --crel -o %t.o
+// RUN: llvm-readelf -S %t.o | FileCheck %s
+
+// CHECK: .crel.text CREL
+call foo

clang/tools/driver/cc1as_main.cpp

@@ -164,6 +164,9 @@ struct AssemblerInvocation {
   LLVM_PREFERRED_TYPE(bool)
   unsigned EmitCompactUnwindNonCanonical : 1;
 
+  LLVM_PREFERRED_TYPE(bool)
+  unsigned Crel : 1;
+
   /// The name of the relocation model to use.
   std::string RelocationModel;
 
@@ -204,6 +207,7 @@ struct AssemblerInvocation {
     EmbedBitcode = 0;
     EmitDwarfUnwind = EmitDwarfUnwindType::Default;
     EmitCompactUnwindNonCanonical = false;
+    Crel = false;
   }
 
   static bool CreateFromArgs(AssemblerInvocation &Res,
@@ -373,6 +377,7 @@ bool AssemblerInvocation::CreateFromArgs(AssemblerInvocation &Opts,
 
   Opts.EmitCompactUnwindNonCanonical =
       Args.hasArg(OPT_femit_compact_unwind_non_canonical);
+  Opts.Crel = Args.hasArg(OPT_crel);
 
   Opts.AsSecureLogFile = Args.getLastArgValue(OPT_as_secure_log_file);
 
@@ -430,6 +435,7 @@ static bool ExecuteAssemblerImpl(AssemblerInvocation &Opts,
   MCOptions.EmitDwarfUnwind = Opts.EmitDwarfUnwind;
   MCOptions.EmitCompactUnwindNonCanonical = Opts.EmitCompactUnwindNonCanonical;
   MCOptions.MCSaveTempLabels = Opts.SaveTemporaryLabels;
+  MCOptions.Crel = Opts.Crel;
   MCOptions.X86RelaxRelocations = Opts.RelaxELFRelocations;
   MCOptions.CompressDebugSections = Opts.CompressDebugSections;
   MCOptions.AsSecureLogFile = Opts.AsSecureLogFile;