[WebAssembly] Implement ref.null

This patch adds a new "heap type" operand kind to the WebAssembly MC
layer, used by ref.null. Currently the possible values are "extern" and
"func"; when typed function references come, though, this operand may be
a type index.

Note that the "heap type" production is still known as "refedtype" in
the draft proposal; changing its name in the spec is
ongoing (https://github.com/WebAssembly/reference-types/issues/123).

The register form of ref.null is still untested.

Differential Revision: https://reviews.llvm.org/D90608

1 files changed, 22 insertions, 0 deletions

diff --git a/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp b/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp
index 8c93b59..1b7cc09 100644
--- a/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp
+++ b/llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp
@@ -241,6 +241,28 @@ MCDisassembler::DecodeStatus WebAssemblyDisassembler::getInstruction(
       }
       break;
     }
+    // heap_type operands, for e.g. ref.null:
+    case WebAssembly::OPERAND_HEAPTYPE: {
+      int64_t Val;
+      uint64_t PrevSize = Size;
+      if (!nextLEB(Val, Bytes, Size, true))
+        return MCDisassembler::Fail;
+      if (Val < 0 && Size == PrevSize + 1) {
+        // The HeapType encoding is like BlockType, in that encodings that
+        // decode as negative values indicate ValTypes.  In practice we expect
+        // either wasm::ValType::EXTERNREF or wasm::ValType::FUNCREF here.
+        //
+        // The positive SLEB values are reserved for future expansion and are
+        // expected to be type indices in the typed function references
+        // proposal, and should disassemble as MCSymbolRefExpr as in BlockType
+        // above.
+        MI.addOperand(MCOperand::createImm(Val & 0x7f));
+      } else {
+        MI.addOperand(
+            MCOperand::createImm(int64_t(WebAssembly::HeapType::Invalid)));
+      }
+      break;
+    }
     // FP operands.
     case WebAssembly::OPERAND_F32IMM: {
       if (!parseImmediate<float>(MI, Size, Bytes))