[PowerPC] Support ELFv1/ELFv2 ABI selection via features

While LLVM now supports both ELFv1 and ELFv2 ABIs, their use is currently
hard-coded via the target triple: powerpc64-linux is always ELFv1, while
powerpc64le-linux is always ELFv2.

These are of course the most common scenarios, but in principle it is
possible to support the ELFv2 ABI on big-endian or the ELFv1 ABI on
little-endian systems (and GCC does support that), and there are some
special use cases for that (e.g. certain Linux kernel versions could
only be built using ELFv1 on LE).

This patch implements the LLVM side of supporting this.  As precedent
on other platforms suggests, ABI options are passed to the back-end as
features.  Thus, this patch implements two features "elfv1" and "elfv2"
that select the desired ABI if present.  (If not, the LLVM uses the
same default rules as now.)

llvm-svn: 214072

llvm/lib/Target/PowerPC/PPC.td

@@ -107,6 +107,16 @@ def DeprecatedDST    : SubtargetFeature<"", "DeprecatedDST", "true",
 // POPCNTB      p5 through p7      popcntb and related instructions
 // VSX          p7                 vector-scalar instruction set
 
+//===----------------------------------------------------------------------===//
+// ABI Selection                                                              //
+//===----------------------------------------------------------------------===//
+
+def FeatureELFv1 : SubtargetFeature<"elfv1", "TargetABI", "PPC_ABI_ELFv1",
+                                    "Use the ELFv1 ABI">;
+
+def FeatureELFv2 : SubtargetFeature<"elfv2", "TargetABI", "PPC_ABI_ELFv2",
+                                    "Use the ELFv2 ABI">;
+
 //===----------------------------------------------------------------------===//
 // Classes used for relation maps.
 //===----------------------------------------------------------------------===//

llvm/lib/Target/PowerPC/PPCSubtarget.cpp

@@ -78,7 +78,7 @@ PPCSubtarget::PPCSubtarget(const std::string &TT, const std::string &CPU,
                            const std::string &FS, PPCTargetMachine &TM,
                            bool is64Bit, CodeGenOpt::Level OptLevel)
     : PPCGenSubtargetInfo(TT, CPU, FS), IsPPC64(is64Bit), TargetTriple(TT),
-      OptLevel(OptLevel),
+      OptLevel(OptLevel), TargetABI(PPC_ABI_UNKNOWN),
       FrameLowering(initializeSubtargetDependencies(CPU, FS)),
       DL(getDataLayoutString(*this)), InstrInfo(*this), JITInfo(*this),
       TLInfo(TM), TSInfo(&DL) {}
@@ -203,6 +203,16 @@ void PPCSubtarget::resetSubtargetFeatures(StringRef CPU, StringRef FS) {
   // issues in those instructions can be addressed.
   if (IsLittleEndian)
     HasVSX = false;
+
+  // Determine default ABI.
+  if (TargetABI == PPC_ABI_UNKNOWN) {
+    if (!isDarwin() && IsPPC64) {
+      if (IsLittleEndian)
+        TargetABI = PPC_ABI_ELFv2;
+      else
+        TargetABI = PPC_ABI_ELFv1;
+    }
+  }
 }
 
 /// hasLazyResolverStub - Return true if accesses to the specified global have

llvm/lib/Target/PowerPC/PPCSubtarget.h

@@ -109,6 +109,12 @@ protected:
   /// OptLevel - What default optimization level we're emitting code for.
   CodeGenOpt::Level OptLevel;
 
+  enum {
+    PPC_ABI_UNKNOWN,
+    PPC_ABI_ELFv1,
+    PPC_ABI_ELFv2
+  } TargetABI;
+
   PPCFrameLowering FrameLowering;
   const DataLayout DL;
   PPCInstrInfo InstrInfo;
@@ -227,9 +233,7 @@ public:
 
   bool isDarwinABI() const { return isDarwin(); }
   bool isSVR4ABI() const { return !isDarwin(); }
-  /// FIXME: Should use a command-line option.
-  bool isELFv2ABI() const { return isPPC64() && isSVR4ABI() &&
-                                   isLittleEndian(); }
+  bool isELFv2ABI() const { return TargetABI == PPC_ABI_ELFv2; }
 
   bool enableEarlyIfConversion() const override { return hasISEL(); }