[WebAssembly] Initial Disassembler.

This implements a new table-gen emitter to create tables for
a wasm disassembler, and a dissassembler to use them.

Comes with 2 tests, that tests a few instructions manually. Is also able to
disassemble large .wasm files with objdump reasonably.

Not working so well, to be addressed in followups:
- objdump appears to be passing an incorrect starting point.
- since the disassembler works an instruction at a time, and it is
  disassembling stack instruction, it has no idea of pseudo register assignments.
  These registers are required for the instruction printing code that follows.
  For now, all such registers appear in the output as $0.

Patch by Wouter van Oortmerssen

Differential Revision: https://reviews.llvm.org/D45848

llvm-svn: 332052

1 files changed, 121 insertions, 5 deletions

diff --git a/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp b/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp
index f75832fdd9e..2f0960271e3 100644
--- a/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp
+++ b/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp
@@ -19,16 +19,23 @@
 #include "WebAssembly.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCDisassembler/MCDisassembler.h"
+#include "llvm/MC/MCFixedLenDisassembler.h"
 #include "llvm/MC/MCInst.h"
 #include "llvm/MC/MCInstrInfo.h"
 #include "llvm/MC/MCSubtargetInfo.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/Endian.h"
+#include "llvm/Support/LEB128.h"
 #include "llvm/Support/TargetRegistry.h"
+
 using namespace llvm;
 
 #define DEBUG_TYPE "wasm-disassembler"
 
+using DecodeStatus = MCDisassembler::DecodeStatus;
+
+#include "WebAssemblyGenDisassemblerTables.inc"
+
 namespace {
 class WebAssemblyDisassembler final : public MCDisassembler {
   std::unique_ptr<const MCInstrInfo> MCII;
@@ -60,11 +67,120 @@ extern "C" void LLVMInitializeWebAssemblyDisassembler() {
                                          createWebAssemblyDisassembler);
 }
 
-MCDisassembler::DecodeStatus WebAssemblyDisassembler::getInstruction(
-    MCInst &MI, uint64_t &Size, ArrayRef<uint8_t> Bytes, uint64_t /*Address*/,
-    raw_ostream &OS, raw_ostream &CS) const {
+static int nextByte(ArrayRef<uint8_t> Bytes, uint64_t &Size) {
+  if (Size >= Bytes.size())
+    return -1;
+  auto V = Bytes[Size];
+  Size++;
+  return V;
+}
 
-  // TODO: Implement disassembly.
+static bool parseLEBImmediate(MCInst &MI, uint64_t &Size,
+                              ArrayRef<uint8_t> Bytes, bool Signed) {
+  unsigned N = 0;
+  const char *Error = nullptr;
+  auto Val = Signed ? decodeSLEB128(Bytes.data() + Size, &N,
+                                    Bytes.data() + Bytes.size(), &Error)
+                    : static_cast<int64_t>(
+                          decodeULEB128(Bytes.data() + Size, &N,
+                                        Bytes.data() + Bytes.size(), &Error));
+  if (Error)
+    return false;
+  Size += N;
+  MI.addOperand(MCOperand::createImm(Val));
+  return true;
+}
+
+template <typename T>
+bool parseFPImmediate(MCInst &MI, uint64_t &Size, ArrayRef<uint8_t> Bytes) {
+  if (Size + sizeof(T) > Bytes.size())
+    return false;
+  T Val;
+  memcpy(&Val, Bytes.data() + Size, sizeof(T));
+  support::endian::byte_swap<T, support::endianness::little>(Val);
+  Size += sizeof(T);
+  MI.addOperand(MCOperand::createFPImm(static_cast<double>(Val)));
+  return true;
+}
 
-  return MCDisassembler::Fail;
+MCDisassembler::DecodeStatus WebAssemblyDisassembler::getInstruction(
+    MCInst &MI, uint64_t &Size, ArrayRef<uint8_t> Bytes, uint64_t /*Address*/,
+    raw_ostream & /*OS*/, raw_ostream &CS) const {
+  CommentStream = &CS;
+  Size = 0;
+  auto Opc = nextByte(Bytes, Size);
+  if (Opc < 0)
+    return MCDisassembler::Fail;
+  const auto *WasmInst = &InstructionTable0[Opc];
+  // If this is a prefix byte, indirect to another table.
+  if (WasmInst->ET == ET_Prefix) {
+    WasmInst = nullptr;
+    // Linear search, so far only 2 entries.
+    for (auto PT = PrefixTable; PT->Table; PT++) {
+      if (PT->Prefix == Opc) {
+        WasmInst = PT->Table;
+        break;
+      }
+    }
+    if (!WasmInst)
+      return MCDisassembler::Fail;
+    Opc = nextByte(Bytes, Size);
+    if (Opc < 0)
+      return MCDisassembler::Fail;
+    WasmInst += Opc;
+  }
+  if (WasmInst->ET == ET_Unused)
+    return MCDisassembler::Fail;
+  // At this point we must have a valid instruction to decode.
+  assert(WasmInst->ET == ET_Instruction);
+  MI.setOpcode(WasmInst->Opcode);
+  // Parse any operands.
+  for (uint8_t OPI = 0; OPI < WasmInst->NumOperands; OPI++) {
+    switch (WasmInst->Operands[OPI]) {
+    // ULEB operands:
+    case WebAssembly::OPERAND_BASIC_BLOCK:
+    case WebAssembly::OPERAND_LOCAL:
+    case WebAssembly::OPERAND_GLOBAL:
+    case WebAssembly::OPERAND_FUNCTION32:
+    case WebAssembly::OPERAND_OFFSET32:
+    case WebAssembly::OPERAND_P2ALIGN:
+    case WebAssembly::OPERAND_TYPEINDEX:
+    case MCOI::OPERAND_IMMEDIATE: {
+      if (!parseLEBImmediate(MI, Size, Bytes, false))
+        return MCDisassembler::Fail;
+      break;
+    }
+    // SLEB operands:
+    case WebAssembly::OPERAND_I32IMM:
+    case WebAssembly::OPERAND_I64IMM:
+    case WebAssembly::OPERAND_SIGNATURE: {
+      if (!parseLEBImmediate(MI, Size, Bytes, true))
+        return MCDisassembler::Fail;
+      break;
+    }
+    // FP operands.
+    case WebAssembly::OPERAND_F32IMM: {
+      if (!parseFPImmediate<float>(MI, Size, Bytes))
+        return MCDisassembler::Fail;
+      break;
+    }
+    case WebAssembly::OPERAND_F64IMM: {
+      if (!parseFPImmediate<double>(MI, Size, Bytes))
+        return MCDisassembler::Fail;
+      break;
+    }
+    case MCOI::OPERAND_REGISTER: {
+      // These are NOT actually in the instruction stream, but MC is going to
+      // expect operands to be present for them!
+      // FIXME: can MC re-generate register assignments or do we have to
+      // do this? Since this function decodes a single instruction, we don't
+      // have the proper context for tracking an operand stack here.
+      MI.addOperand(MCOperand::createReg(0));
+      break;
+    }
+    default:
+      llvm_unreachable("Unknown operand type in WebAssemblyDisassembler");
+    }
+  }
+  return MCDisassembler::Success;
 }