[X86] Merge disp8 and cdisp8 handling into a single helper function to reduce some code.

[craig.topper]

Summary

We currently handle EVEX and non-EVEX separately in two places. By sinking the EVEX
check into the existing helper for CDisp8 we can simplify these two places.

[RKSimon]

LGTM

llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

 static void emitConstant(uint64_t Val, unsigned Size, raw_ostream &OS) {
   // Output the constant in little endian byte order.
   for (unsigned i = 0; i != Size; ++i) {
     emitByte(Val & 255, OS);
     Val >>= 8;
   }
 }
 
-/// \returns true if this signed displacement fits in a 8-bit sign-extended
-/// field.
-static bool isDisp8(int Value) { return Value == (int8_t)Value; }
-
-/// \returns true if this signed displacement fits in a 8-bit compressed
-/// dispacement field.
-static bool isCDisp8(uint64_t TSFlags, int Value, int &CValue) {
-  assert(((TSFlags & X86II::EncodingMask) == X86II::EVEX) &&
-         "Compressed 8-bit displacement is only valid for EVEX inst.");
+/// Determine if this immediate can fit in a disp8 or a compressed disp8 for
+/// EVEX instructions. \p will be set to the value to pass to the ImmOffset
+/// parameter of emitImmediate.
+static bool isDispOrCDisp8(uint64_t TSFlags, int Value, int &ImmOffset) {
+  bool HasEVEX = (TSFlags & X86II::EncodingMask) == X86II::EVEX;
 
-  unsigned CD8_Scale =
+  int CD8_Scale =
       (TSFlags & X86II::CD8_Scale_Mask) >> X86II::CD8_Scale_Shift;
-  if (CD8_Scale == 0) {
-    CValue = Value;
-    return isDisp8(Value);
-  }
+  if (!HasEVEX || CD8_Scale == 0)
+    return isInt<8>(Value);
+
+  assert(isPowerOf2_32(CD8_Scale) && "Unexpected CD8 scale!");
+  if (Value & (CD8_Scale - 1)) // Unaligned offset
+    return false;
 
-  unsigned Mask = CD8_Scale - 1;
-  assert((CD8_Scale & Mask) == 0 && "Invalid memory object size.");
-  if (Value & Mask) // Unaligned offset
+  int CDisp8 = Value / CD8_Scale;
+  if (!isInt<8>(CDisp8))
     return false;
-  Value /= (int)CD8_Scale;
-  bool Ret = (Value == (int8_t)Value);
 
-  if (Ret)
-    CValue = Value;
-  return Ret;
+  // ImmOffset will be added to Value in emitImmediate leaving just CDisp8.
+  ImmOffset = CDisp8 - Value;
+  return true;
 }
 
 /// \returns the appropriate fixup kind to use for an immediate in an
 /// instruction with the specified TSFlags.
 static MCFixupKind getImmFixupKind(uint64_t TSFlags) {
   unsigned Size = X86II::getSizeOfImm(TSFlags);
   bool isPCRel = X86II::isImmPCRel(TSFlags);
 
void X86MCCodeEmitter::emitMemModRMByte(const MCInst &MI, unsigned Op,
                                         SmallVectorImpl<MCFixup> &Fixups,
                                         const MCSubtargetInfo &STI,
                                         bool ForceSIB) const {
   const MCOperand &Disp = MI.getOperand(Op + X86::AddrDisp);
   const MCOperand &Base = MI.getOperand(Op + X86::AddrBaseReg);
   const MCOperand &Scale = MI.getOperand(Op + X86::AddrScaleAmt);
   const MCOperand &IndexReg = MI.getOperand(Op + X86::AddrIndexReg);
   unsigned BaseReg = Base.getReg();
-  bool HasEVEX = (TSFlags & X86II::EncodingMask) == X86II::EVEX;
 
   // Handle %rip relative addressing.
   if (BaseReg == X86::RIP ||
       BaseReg == X86::EIP) { // [disp32+rIP] in X86-64 mode
     assert(STI.hasFeature(X86::Mode64Bit) &&
            "Rip-relative addressing requires 64-bit mode");
     assert(IndexReg.getReg() == 0 && !ForceSIB &&
            "Invalid rip-relative address");
if (BaseReg) {
 
         // Allow base/index to appear in either order (although GAS doesn't).
         if (IndexReg16 & 2)
           RMfield = (RMfield & 1) | ((7 - IndexReg16) << 1);
         else
           RMfield = (IndexReg16 & 1) | ((7 - RMfield) << 1);
       }
 
-      if (Disp.isImm() && isDisp8(Disp.getImm())) {
+      if (Disp.isImm() && isInt<8>(Disp.getImm())) {
         if (Disp.getImm() == 0 && RMfield != 6) {
           // There is no displacement; just the register.
           emitByte(modRMByte(0, RegOpcodeField, RMfield), OS);
           return;
         }
         // Use the [REG]+disp8 form, including for [BP] which cannot be encoded.
         emitByte(modRMByte(1, RegOpcodeField, RMfield), OS);
         emitImmediate(Disp, MI.getLoc(), 1, FK_Data_1, StartByte, OS, Fixups);
if (BaseRegNo != N86::EBP) {
           emitByte(modRMByte(0, RegOpcodeField, BaseRegNo), OS);
           return;
         }
       }
     }
 
     // Otherwise, if the displacement fits in a byte, encode as [REG+disp8].
     if (Disp.isImm()) {
-      if (!HasEVEX && isDisp8(Disp.getImm())) {
-        emitByte(modRMByte(1, RegOpcodeField, BaseRegNo), OS);
-        emitImmediate(Disp, MI.getLoc(), 1, FK_Data_1, StartByte, OS, Fixups);
-        return;
-      }
-      // Try EVEX compressed 8-bit displacement first; if failed, fall back to
-      // 32-bit displacement.
-      int CDisp8 = 0;
-      if (HasEVEX && isCDisp8(TSFlags, Disp.getImm(), CDisp8)) {
+      int ImmOffset = 0;
+      if (isDispOrCDisp8(TSFlags, Disp.getImm(), ImmOffset)) {
         emitByte(modRMByte(1, RegOpcodeField, BaseRegNo), OS);
         emitImmediate(Disp, MI.getLoc(), 1, FK_Data_1, StartByte, OS, Fixups,
-                      CDisp8 - Disp.getImm());
+                      ImmOffset);
         return;
       }
     }
 
     // Otherwise, emit the most general non-SIB encoding: [REG+disp32]
     emitByte(modRMByte(2, RegOpcodeField, BaseRegNo), OS);
     unsigned Opcode = MI.getOpcode();
     unsigned FixupKind = Opcode == X86::MOV32rm ? X86::reloc_signed_4byte_relax
                                                 : X86::reloc_signed_4byte;
     emitImmediate(Disp, MI.getLoc(), 4, MCFixupKind(FixupKind), StartByte, OS,
                   Fixups);
     return;
   }
 
   // We need a SIB byte, so start by outputting the ModR/M byte first
   assert(IndexReg.getReg() != X86::ESP && IndexReg.getReg() != X86::RSP &&
          "Cannot use ESP as index reg!");
 
   bool ForceDisp32 = false;
   bool ForceDisp8 = false;
-  int CDisp8 = 0;
   int ImmOffset = 0;
   if (BaseReg == 0) {
     // If there is no base register, we emit the special case SIB byte with
     // MOD=0, BASE=5, to JUST get the index, scale, and displacement.
     emitByte(modRMByte(0, RegOpcodeField, 4), OS);
     ForceDisp32 = true;
   } else if (!Disp.isImm()) {
     // Emit the normal disp32 encoding.
     emitByte(modRMByte(2, RegOpcodeField, 4), OS);
     ForceDisp32 = true;
   } else if (Disp.getImm() == 0 &&
              // Base reg can't be anything that ends up with '5' as the base
              // reg, it is the magic [*] nomenclature that indicates no base.
              BaseRegNo != N86::EBP) {
     // Emit no displacement ModR/M byte
     emitByte(modRMByte(0, RegOpcodeField, 4), OS);
-  } else if (!HasEVEX && isDisp8(Disp.getImm())) {
-    // Emit the disp8 encoding.
-    emitByte(modRMByte(1, RegOpcodeField, 4), OS);
-    ForceDisp8 = true; // Make sure to force 8 bit disp if Base=EBP
-  } else if (HasEVEX && isCDisp8(TSFlags, Disp.getImm(), CDisp8)) {
+  } else if (isDispOrCDisp8(TSFlags, Disp.getImm(), ImmOffset)) {
     // Emit the disp8 encoding.
     emitByte(modRMByte(1, RegOpcodeField, 4), OS);
     ForceDisp8 = true; // Make sure to force 8 bit disp if Base=EBP
-    ImmOffset = CDisp8 - Disp.getImm();
   } else {
     // Emit the normal disp32 encoding.
     emitByte(modRMByte(2, RegOpcodeField, 4), OS);
   }
 
   // Calculate what the SS field value should be...
   static const unsigned SSTable[] = {~0U, 0, 1, ~0U, 2, ~0U, ~0U, ~0U, 3};
   unsigned SS = SSTable[Scale.getImm()];