[COFF] Use 32-bit jump table entries in .rdata for Win64

Summary:
We were already using 32-bit jump table entries, but this was a
consequence of the default PIC model on Win64, and not an intentional
design decision. This patch ensures that we always use 32-bit label
difference jump table entries on Win64 regardless of the PIC model. This
is a good idea because it saves executable size and object file size.

Moving the jump tables to .rdata cleans up the disassembled object code
and reduces the available ROP targets, but it requires adding one more
RIP-relative lea to the code.  COFF doesn't have relocations to express
the difference between two arbitrary symbols, so we can't use the jump
table label in the label difference like we do elsewhere.

Fixes PR31488

Reviewers: majnemer, compnerd

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28141

llvm-svn: 290694

3 files changed, 33 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index eb2a28f574a..d19ca47ec1a 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -1055,6 +1055,13 @@ MCSection *TargetLoweringObjectFileCOFF::getSectionForJumpTable(
                                      COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE, UniqueID);
 }
 
+bool TargetLoweringObjectFileCOFF::shouldPutJumpTableInFunctionSection(
+    bool UsesLabelDifference, const Function &F) const {
+  // We can always create relative relocations, so use another section
+  // that can be marked non-executable.
+  return false;
+}
+
 void TargetLoweringObjectFileCOFF::emitModuleFlags(
     MCStreamer &Streamer, ArrayRef<Module::ModuleFlagEntry> ModuleFlags,
     const TargetMachine &TM) const {
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index b293dfa98f8..d0c77030acd 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -1926,10 +1926,20 @@ unsigned X86TargetLowering::getJumpTableEncoding() const {
   if (isPositionIndependent() && Subtarget.isPICStyleGOT())
     return MachineJumpTableInfo::EK_Custom32;
 
+  // On Win64, we want to use both label differences and a separate section.
+  if (Subtarget.isTargetWin64())
+    return MachineJumpTableInfo::EK_LabelDifference32;
+
   // Otherwise, use the normal jump table encoding heuristics.
   return TargetLowering::getJumpTableEncoding();
 }
 
+bool X86TargetLowering::isJumpTableRelative() const {
+  if (Subtarget.isTargetWin64())
+    return true;
+  return TargetLowering::isJumpTableRelative();
+}
+
 bool X86TargetLowering::useSoftFloat() const {
   return Subtarget.useSoftFloat();
 }
@@ -1948,11 +1958,19 @@ X86TargetLowering::LowerCustomJumpTableEntry(const MachineJumpTableInfo *MJTI,
 /// Returns relocation base for the given PIC jumptable.
 SDValue X86TargetLowering::getPICJumpTableRelocBase(SDValue Table,
                                                     SelectionDAG &DAG) const {
+  // COFF doesn't have relocations to take the difference between two arbitrary
+  // symbols. The assembler, however, can resolve a fixup between the function
+  // entry and a basic block label, so use the function entry as the base.
+  if (Subtarget.isTargetWin64())
+    return DAG.getGlobalAddress(DAG.getMachineFunction().getFunction(), SDLoc(),
+                                getPointerTy(DAG.getDataLayout()));
+
   if (!Subtarget.is64Bit())
     // This doesn't have SDLoc associated with it, but is not really the
     // same as a Register.
     return DAG.getNode(X86ISD::GlobalBaseReg, SDLoc(),
                        getPointerTy(DAG.getDataLayout()));
+
   return Table;
 }
 
@@ -1961,6 +1979,13 @@ SDValue X86TargetLowering::getPICJumpTableRelocBase(SDValue Table,
 const MCExpr *X86TargetLowering::
 getPICJumpTableRelocBaseExpr(const MachineFunction *MF, unsigned JTI,
                              MCContext &Ctx) const {
+  // COFF doesn't have relocations to take the difference between two arbitrary
+  // symbols. The assembler, however, can resolve a fixup between the function
+  // entry and a basic block label, so use the function entry as the base.
+  if (Subtarget.isTargetWin64())
+    return MCSymbolRefExpr::create(
+        getTargetMachine().getSymbol(MF->getFunction()), Ctx);
+
   // X86-64 uses RIP relative addressing based on the jump table label.
   if (Subtarget.isPICStyleRIPRel())
     return TargetLowering::getPICJumpTableRelocBaseExpr(MF, JTI, Ctx);
diff --git a/llvm/lib/Target/X86/X86ISelLowering.h b/llvm/lib/Target/X86/X86ISelLowering.h
index 37f9353042b..643727d7aed 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.h
+++ b/llvm/lib/Target/X86/X86ISelLowering.h
@@ -684,6 +684,7 @@ namespace llvm {
                                const X86Subtarget &STI);
 
     unsigned getJumpTableEncoding() const override;
+    bool isJumpTableRelative() const override;
     bool useSoftFloat() const override;
 
     MVT getScalarShiftAmountTy(const DataLayout &, EVT) const override {