X86 segment reg override (#512)

* arch: Add segment_override attribute to Instruction.

* arch: Update x86 decoder wrt to default segment register.

include/remill/Arch/Instruction.h

@@ -215,6 +215,12 @@ class Instruction {
   // Is this instruction decoded within the context of a delay slot?
   bool in_delay_slot;
 
+  // For x86 it is possible to specify a prefix that overrides the default
+  // segment register. This attribute by itself is currently not used directly
+  // by the lifter - it is expeted `Operand`s will include segment reg where appropriate
+  // but it can be used in different applications.
+  const Register *segment_override = nullptr;
+
   enum Category {
     kCategoryInvalid,
     kCategoryNormal,

lib/Arch/Instruction.cpp

@@ -692,6 +692,10 @@ std::string Instruction::Serialize(void) const {
     ss << " " << function;
   }
 
+  if (segment_override) {
+    ss << "(SEGMENT_OVERRIDE " << segment_override->name << ")";
+  }
+
   for (const auto &op : operands) {
     ss << " " << op.Serialize();
   }

lib/Arch/X86/Arch.cpp

@@ -424,7 +424,6 @@ static void DecodeMemory(Instruction &inst, const xed_decoded_inst_t *xedd,
   auto iform = xed_decoded_inst_get_iform_enum(xedd);
   auto iclass = xed_decoded_inst_get_iclass(xedd);
   auto op_name = xed_operand_name(xedo);
-  auto segment = xed_decoded_inst_get_seg_reg(xedd, mem_index);
   auto base = xed_decoded_inst_get_base_reg(xedd, mem_index);
   auto index = xed_decoded_inst_get_index_reg(xedd, mem_index);
   auto disp = xed_decoded_inst_get_memory_displacement(xedd, mem_index);
@@ -438,24 +437,34 @@ static void DecodeMemory(Instruction &inst, const xed_decoded_inst_t *xedd,
     size = 16;
   }
 
-  // Deduce the implicit segment register if it is absent.
-  if (XED_REG_INVALID == segment) {
-    segment = XED_REG_DS;
+  auto raw_segment_reg = xed_decoded_inst_get_seg_reg(xedd, mem_index);
+  auto deduce_segment = [&](auto segment_reg) {
+    // Deduce the implicit segment register if it is absent.
+    if (XED_REG_INVALID != segment_reg) {
+      return segment_reg;
+    }
     if (XED_REG_RSP == base_wide || XED_REG_RBP == base_wide) {
-      segment = XED_REG_SS;
+      return XED_REG_SS;
+    }
+    return XED_REG_DS;
+  };
+  auto ignore_segment = [&](auto segment_reg) {
+    // On AMD64, only the `FS` and `GS` segments are non-zero.
+    if (Is64Bit(inst.arch_name) && XED_REG_FS != segment_reg &&
+        XED_REG_GS != segment_reg) {
+      return XED_REG_INVALID;
     }
-  }
 
-  // On AMD64, only the `FS` and `GS` segments are non-zero.
-  if (Is64Bit(inst.arch_name) && XED_REG_FS != segment &&
-      XED_REG_GS != segment) {
-    segment = XED_REG_INVALID;
+    // AGEN operands, e.g. for the `LEA` instuction, can be marked with an
+    // explicit segment, but it is ignored.
+    if (XED_OPERAND_AGEN == op_name) {
+      return XED_REG_INVALID;
+    }
 
-  // AGEN operands, e.g. for the `LEA` instuction, can be marked with an
-  // explicit segment, but it is ignored.
-  } else if (XED_OPERAND_AGEN == op_name) {
-    segment = XED_REG_INVALID;
-  }
+    // No need to ignore it
+    return segment_reg;
+  };
+  auto segment_reg = ignore_segment(deduce_segment(raw_segment_reg));
 
   // Special case: `POP [xSP + ...] uses the value of `xSP` after incrementing
   // it by the stack width.
@@ -470,7 +479,7 @@ static void DecodeMemory(Instruction &inst, const xed_decoded_inst_t *xedd,
   op.addr.address_size =
       xed_decoded_inst_get_memop_address_width(xedd, mem_index);
 
-  op.addr.segment_base_reg = SegBaseRegOp(segment, op.addr.address_size);
+  op.addr.segment_base_reg = SegBaseRegOp(segment_reg, op.addr.address_size);
   op.addr.base_reg = RegOp(base);
   op.addr.index_reg = RegOp(index);
   op.addr.scale = XED_REG_INVALID != index ? static_cast<int64_t>(scale) : 0;
@@ -987,6 +996,13 @@ bool X86Arch::DecodeInstruction(uint64_t address, std::string_view inst_bytes,
   // instuction implementation.
   auto xedi = xed_decoded_inst_inst(xedd);
   auto num_operands = xed_decoded_inst_noperands(xedd);
+
+  auto xedv = xed_decoded_inst_operands_const(xedd);
+  if (xed_operand_values_has_segment_prefix(xedv)) {
+    auto reg_name = xed_reg_enum_t2str(xed_operand_values_segment_prefix(xedv));
+    inst.segment_override = RegisterByName(reg_name);
+  }
+
   for (auto i = 0U; i < num_operands; ++i) {
     auto xedo = xed_inst_operand(xedi, i);
     if (XED_OPVIS_SUPPRESSED != xed_operand_operand_visibility(xedo)) {