Use -fuse-init-array if no gcc installation is found.

[thakis]

Summary

clang currently uses .init_array instead of .ctors on Linux if it detects gcc 4.7+. Make it so that it also uses .init_array if no gcc installation is found at all – if there's no old gcc, there's nothing we need to be compatible with.

icecc for example runs clang in a very small chroot, so before this change clang would use .ctors if run under icecc. And lld currently silently mislinks inputs with .ctors sections, so before this clang + icecc + lld would produce broken binaries.

[smeenai]

And lld currently silently mislinks inputs with .ctors sections

Is there a bug for this? I see https://bugs.llvm.org/show_bug.cgi?id=31224, which links to https://reviews.llvm.org/D35509, but neither seems active.

[rsmith]

I don't think working around an lld bug is a particularly good justification for this, but I do think it makes sense to switch this from being a "compatibility with GCC 4.7+" thing to being a "compatibility with GCC <= 4.6" thing. Please update the release notes to mention this change.

[thakis]

r316713, thanks!

lib/Driver/ToolChains/Gnu.cpp

 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().getOS() == llvm::Triple::Linux &&
-       (!V.isOlderThan(4, 7, 0) || getTriple().isAndroid())) ||
+       ((!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;
 
   if (DriverArgs.hasFlag(options::OPT_fuse_init_array,
                          options::OPT_fno_use_init_array, UseInitArrayDefault))
     CC1Args.push_back("-fuse-init-array");
 }

test/Driver/constructors.c

 // Test whether or not the driver instructs the backend to use .init_array
 // sections for global constructors.
 //
 // CHECK-INIT-ARRAY: -fuse-init-array
 // CHECK-NO-INIT-ARRAY-NOT: -fuse-init-array
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1       \
 // RUN:     -target i386-unknown-linux \
+// RUN:     --sysroot=%S/Inputs/resource_dir \
+// 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/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:     -fno-use-init-array \
 // RUN:     -target i386-unknown-linux \
 // RUN:     --sysroot=%S/Inputs/fake_install_tree \