[x86] Use 16-bit addressing where possible in 16-bit mode

Where "where possible" means that it's an immediate value and it's below
0x10000. In fact GAS will either truncate or error with larger values,
and will insist on using the addr32 prefix to get 32-bit addressing. So
perhaps we should do that, in a later patch.

llvm-svn: 198758

1 files changed, 18 insertions, 14 deletions

diff --git a/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp b/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
index d3879e6c4cf..ae4fc2b3b71 100644
--- a/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
+++ b/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
@@ -57,6 +57,24 @@ public:
     return (STI.getFeatureBits() & X86::Mode16Bit) != 0;
   }
 
+  /// Is16BitMemOperand - Return true if the specified instruction has
+  /// a 16-bit memory operand. Op specifies the operand # of the memoperand.
+  bool Is16BitMemOperand(const MCInst &MI, unsigned Op) const {
+    const MCOperand &BaseReg  = MI.getOperand(Op+X86::AddrBaseReg);
+    const MCOperand &IndexReg = MI.getOperand(Op+X86::AddrIndexReg);
+    const MCOperand &Disp     = MI.getOperand(Op+X86::AddrDisp);
+
+    if (is16BitMode() && BaseReg.getReg() == 0 &&
+        Disp.isImm() && Disp.getImm() < 0x10000)
+      return true;
+    if ((BaseReg.getReg() != 0 &&
+         X86MCRegisterClasses[X86::GR16RegClassID].contains(BaseReg.getReg())) ||
+        (IndexReg.getReg() != 0 &&
+         X86MCRegisterClasses[X86::GR16RegClassID].contains(IndexReg.getReg())))
+      return true;
+    return false;
+  }
+
   unsigned GetX86RegNum(const MCOperand &MO) const {
     return Ctx.getRegisterInfo()->getEncodingValue(MO.getReg()) & 0x7;
   }
@@ -250,20 +268,6 @@ static bool Is64BitMemOperand(const MCInst &MI, unsigned Op) {
 }
 #endif
 
-/// Is16BitMemOperand - Return true if the specified instruction has
-/// a 16-bit memory operand. Op specifies the operand # of the memoperand.
-static bool Is16BitMemOperand(const MCInst &MI, unsigned Op) {
-  const MCOperand &BaseReg  = MI.getOperand(Op+X86::AddrBaseReg);
-  const MCOperand &IndexReg = MI.getOperand(Op+X86::AddrIndexReg);
-
-  if ((BaseReg.getReg() != 0 &&
-       X86MCRegisterClasses[X86::GR16RegClassID].contains(BaseReg.getReg())) ||
-      (IndexReg.getReg() != 0 &&
-       X86MCRegisterClasses[X86::GR16RegClassID].contains(IndexReg.getReg())))
-    return true;
-  return false;
-}
-
 /// StartsWithGlobalOffsetTable - Check if this expression starts with
 ///  _GLOBAL_OFFSET_TABLE_ and if it is of the form
 ///  _GLOBAL_OFFSET_TABLE_-symbol. This is needed to support PIC on ELF