[Driver] Use .init_array for all gcc installations and simplify Generic_ELF -fno-use-init-array rules

[MaskRay]

Summary

D39317 made clang use .init_array when no gcc installations is found.
This change changes all gcc installations to use .init_array .

GCC 4.7 by default stopped providing .ctors/.dtors compatible crt files,
and stopped emitting .ctors for attribute((constructor)).
.init_array should always work.

FreeBSD rules are moved to FreeBSD.cpp to make Generic_ELF rules clean.

[rnk]

I think we should do this, but let's add a release note for it. There's a flag, so users always have a workaround if clang starts doing the wrong thing for them.

[MaskRay]

Add release note

[rnk]

lgtm

clang/docs/ReleaseNotes.rst

 
 - For X86 target, -march=skylake-avx512, -march=icelake-client,
   -march=icelake-server, -march=cascadelake, -march=cooperlake will default to
   not using 512-bit zmm registers in vectorized code unless 512-bit intrinsics
   are used in the source code. 512-bit operations are known to cause the CPUs
   to run at a lower frequency which can impact performance. This behavior can be
   changed by passing -mprefer-vector-width=512 on the command line.
 
+* clang now defaults to ``.init_array`` on Linux. It used to use ``.ctors`` if
+  the found gcc installation is older than 4.7.0. Add ``-fno-use-init-array`` to
+  get the old behavior (``.ctors``).
+
 New Compiler Flags
 ------------------
 
 - The -fgnuc-version= flag now controls the value of ``__GNUC__`` and related
   macros. This flag does not enable or disable any GCC extensions implemented in
   Clang. Setting the version to zero causes Clang to leave ``__GNUC__`` and
   other GNU-namespaced macros, such as ``__GXX_WEAK__``, undefined.
 

clang/lib/Driver/ToolChains/FreeBSD.h

llvm::ExceptionHandling GetExceptionModel(
       const llvm::opt::ArgList &Args) const override;
   bool IsUnwindTablesDefault(const llvm::opt::ArgList &Args) const override;
   bool isPIEDefault() const override;
   SanitizerMask getSupportedSanitizers() const override;
   unsigned GetDefaultDwarfVersion() const override;
   // Until dtrace (via CTF) and LLDB can deal with distributed debug info,
   // FreeBSD defaults to standalone/full debug info.
   bool GetDefaultStandaloneDebug() const override { return true; }
+  void
+  addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
+                        llvm::opt::ArgStringList &CC1Args,
+                        Action::OffloadKind DeviceOffloadKind) const override;
 
 protected:
   Tool *buildAssembler() const override;
   Tool *buildLinker() const override;
 };
 
 } // end namespace toolchains
 } // end namespace driver
 } // end namespace clang
 
 #endif // LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_FREEBSD_H

clang/lib/Driver/ToolChains/FreeBSD.cpp

if (IsX86 || IsX86_64) {
     Res |= SanitizerKind::SafeStack;
     Res |= SanitizerKind::Fuzzer;
     Res |= SanitizerKind::FuzzerNoLink;
   }
   if (IsX86_64)
     Res |= SanitizerKind::Memory;
   return Res;
 }
+
+void FreeBSD::addClangTargetOptions(const ArgList &DriverArgs,
+                                    ArgStringList &CC1Args,
+                                    Action::OffloadKind) const {
+  if (!DriverArgs.hasFlag(options::OPT_fuse_init_array,
+                          options::OPT_fno_use_init_array,
+                          getTriple().getOSMajorVersion() >= 12))
+    CC1Args.push_back("-fno-use-init-array");
+}

clang/lib/Driver/ToolChains/Gnu.cpp

Generic_GCC::TranslateArgs(const llvm::opt::DerivedArgList &Args, StringRef,
   return nullptr;
 }
 
 void Generic_ELF::anchor() {}
 
 void Generic_ELF::addClangTargetOptions(const ArgList &DriverArgs,
                                         ArgStringList &CC1Args,
                                         Action::OffloadKind) const {
-  const Generic_GCC::GCCVersion &V = GCCInstallation.getVersion();
-  bool UseInitArrayDefault =
-      getTriple().getArch() == llvm::Triple::aarch64 ||
-      getTriple().getArch() == llvm::Triple::aarch64_be ||
-      (getTriple().isOSFreeBSD() &&
-       getTriple().getOSMajorVersion() >= 12) ||
-      (getTriple().getOS() == llvm::Triple::Linux &&
-       ((!GCCInstallation.isValid() || !V.isOlderThan(4, 7, 0)) ||
-        getTriple().isAndroid())) ||
-      getTriple().getOS() == llvm::Triple::NaCl ||
-      (getTriple().getVendor() == llvm::Triple::MipsTechnologies &&
-       !getTriple().hasEnvironment()) ||
-      getTriple().getOS() == llvm::Triple::Solaris ||
-      getTriple().getArch() == llvm::Triple::riscv32 ||
-      getTriple().getArch() == llvm::Triple::riscv64;
-
   if (!DriverArgs.hasFlag(options::OPT_fuse_init_array,
-                          options::OPT_fno_use_init_array, UseInitArrayDefault))
+                          options::OPT_fno_use_init_array, true))
     CC1Args.push_back("-fno-use-init-array");
 }

clang/test/Driver/constructors.c

 // RUN:     --sysroot=%S/Inputs/fake_install_tree \
 // RUN:     --gcc-toolchain="" \
 // RUN:   | FileCheck --check-prefix=CHECK-INIT-ARRAY %s
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1       \
 // RUN:     -target i386-unknown-linux \
 // RUN:     --sysroot=%S/Inputs/basic_linux_tree \
 // RUN:     --gcc-toolchain="" \
-// RUN:   | FileCheck --check-prefix=CHECK-NO-INIT-ARRAY %s
+// RUN:   | FileCheck --check-prefix=CHECK-INIT-ARRAY %s
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1       \
 // RUN:     -fuse-init-array \
 // RUN:     -target i386-unknown-linux \
 // RUN:     --sysroot=%S/Inputs/basic_linux_tree \
 // RUN:     --gcc-toolchain="" \
 // RUN:   | FileCheck --check-prefix=CHECK-INIT-ARRAY %s
 //