[PowerPC] Emit warning for ieeelongdouble on older GNU toolchain

GCC 12 should have proper support for IEEE-754 compliant 128-bit
floating point in libstdc++. So warning is needed when linking against
older libstdc++ versions or LLVM libc++.

Glibc starts supporting float128 in both header and libraries since
2.32.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D112906

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -380,6 +380,9 @@ def warn_drv_deprecated_arg : Warning<
   "argument '%0' is deprecated, use '%1' instead">, InGroup<Deprecated>;
 def warn_drv_assuming_mfloat_abi_is : Warning<
   "unknown platform, assuming -mfloat-abi=%0">;
+def warn_drv_unsupported_float_abi_by_lib : Warning<
+  "float ABI '%0' is not supported by current library">,
+  InGroup<DiagGroup<"unsupported-abi">>;
 def warn_ignoring_ftabstop_value : Warning<
   "ignoring invalid -ftabstop value '%0', using default value %1">;
 def warn_drv_overriding_flag_option : Warning<

clang/lib/Driver/ToolChains/PPCLinux.cpp

@@ -8,11 +8,50 @@
 
 #include "PPCLinux.h"
 #include "clang/Driver/Driver.h"
+#include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/Options.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 
 using namespace clang::driver::toolchains;
 using namespace llvm::opt;
+using namespace llvm::sys;
+
+// Glibc older than 2.32 doesn't fully support IEEE float128. Here we check
+// glibc version by looking at dynamic linker name.
+static bool GlibcSupportsFloat128(const std::string &Linker) {
+  llvm::SmallVector<char, 16> Path;
+
+  // Resolve potential symlinks to linker.
+  if (fs::real_path(Linker, Path))
+    return false;
+  llvm::StringRef LinkerName =
+      path::filename(llvm::StringRef(Path.data(), Path.size()));
+
+  // Since glibc 2.34, the installed .so file is not symlink anymore. But we can
+  // still safely assume it's newer than 2.32.
+  if (LinkerName.startswith("ld64.so"))
+    return true;
+
+  if (!LinkerName.startswith("ld-2."))
+    return false;
+  unsigned Minor = (LinkerName[5] - '0') * 10 + (LinkerName[6] - '0');
+  if (Minor < 32)
+    return false;
+
+  return true;
+}
+
+PPCLinuxToolChain::PPCLinuxToolChain(const Driver &D,
+                                     const llvm::Triple &Triple,
+                                     const llvm::opt::ArgList &Args)
+    : Linux(D, Triple, Args) {
+  if (Arg *A = Args.getLastArg(options::OPT_mabi_EQ)) {
+    StringRef ABIName = A->getValue();
+    if (ABIName == "ieeelongdouble" && !SupportIEEEFloat128(D, Triple, Args))
+      D.Diag(diag::warn_drv_unsupported_float_abi_by_lib) << ABIName;
+  }
+}
 
 void PPCLinuxToolChain::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
                                                   ArgStringList &CC1Args) const {
@@ -26,3 +65,20 @@ void PPCLinuxToolChain::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
 
   Linux::AddClangSystemIncludeArgs(DriverArgs, CC1Args);
 }
+
+bool PPCLinuxToolChain::SupportIEEEFloat128(
+    const Driver &D, const llvm::Triple &Triple,
+    const llvm::opt::ArgList &Args) const {
+  if (!Triple.isLittleEndian() || !Triple.isPPC64())
+    return false;
+
+  if (Args.hasArg(options::OPT_nostdlib, options::OPT_nostdlibxx))
+    return true;
+
+  bool HasUnsupportedCXXLib =
+      ToolChain::GetCXXStdlibType(Args) == CST_Libcxx &&
+      GCCInstallation.getVersion().isOlderThan(12, 1, 0);
+
+  return GlibcSupportsFloat128(Linux::getDynamicLinker(Args)) &&
+         !(D.CCCIsCXX() && HasUnsupportedCXXLib);
+}

clang/lib/Driver/ToolChains/PPCLinux.h

@@ -18,12 +18,15 @@ namespace toolchains {
 class LLVM_LIBRARY_VISIBILITY PPCLinuxToolChain : public Linux {
 public:
   PPCLinuxToolChain(const Driver &D, const llvm::Triple &Triple,
-                    const llvm::opt::ArgList &Args)
+                    const llvm::opt::ArgList &Args);
-      : Linux(D, Triple, Args) {}
 
   void
   AddClangSystemIncludeArgs(const llvm::opt::ArgList &DriverArgs,
                             llvm::opt::ArgStringList &CC1Args) const override;
+
+private:
+  bool SupportIEEEFloat128(const Driver &D, const llvm::Triple &Triple,
+                           const llvm::opt::ArgList &Args) const;
 };
 
 } // end namespace toolchains

clang/test/Driver/ppc-float-abi-warning.cpp

@@ -0,0 +1,13 @@
+// REQUIRES: powerpc-registered-target
+// RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s \
+// RUN:  --gcc-toolchain=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-11.2.0 \
+// RUN:  -mabi=ieeelongdouble -stdlib=libstdc++ 2>&1 | FileCheck %s
+// RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s \
+// RUN:  -mabi=ieeelongdouble -stdlib=libc++ 2>&1 | FileCheck %s
+// RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s\
+// RUN:  -mabi=ieeelongdouble -stdlib=libc++ -Wno-unsupported-abi 2>&1 | \
+// RUN:  FileCheck %s --check-prefix=NOWARN
+
+// CHECK: warning: float ABI 'ieeelongdouble' is not supported by current library
+// NOWARN-NOT: warning: float ABI 'ieeelongdouble' is not supported by current library
+long double foo(long double x) { return x; }