[wasm] fix Simd ExtractLane to take immediate instead of param

src/wasm/ast-decoder.cc

Index: src/wasm/ast-decoder.cc
diff --git a/src/wasm/ast-decoder.cc b/src/wasm/ast-decoder.cc
index 798f9d393a5bf10dba8e1c5b5ebb944e1d0f171e..20ec180b8ae7507f0e9c8ee99ab139b4685b616d 100644
--- a/src/wasm/ast-decoder.cc
+++ b/src/wasm/ast-decoder.cc
@@ -388,6 +388,10 @@ class WasmDecoder : public Decoder {
         FOREACH_ASMJS_COMPAT_OPCODE(DECLARE_OPCODE_CASE)
         FOREACH_SIMD_OPCODE(DECLARE_OPCODE_CASE)
 #undef DECLARE_OPCODE_CASE
+#define DECLARE_OPCODE_CASE(name, opcode, sig) case kExpr##name:
+        FOREACH_SIMD_MISC_OPCODE(DECLARE_OPCODE_CASE)

[gdeepti, 2016/09/01 00:52:52]

Nit: Maybe use a different name here?  FOREACH_SIMD_IMPLICIT_LANE_OPCODE maybe?
Actually that doesn't sound like the right name either, perhaps others can chime
in with a better name - it would be nice for the name to be more descriptive of
why these need to be defined separately instead of a Misc category. 

[aseemgarg, 2016/09/02 22:40:12]

Done.

+#undef DECLARE_OPCODE_CASE
+        return 1;
       default:
         UNREACHABLE();
         return 0;
@@ -456,7 +460,10 @@ class WasmDecoder : public Decoder {
         ReturnArityOperand operand(this, pc);
         return 1 + operand.length;
       }
-
+#define DECLARE_OPCODE_CASE(name, opcode, sig) case kExpr##name:
+        FOREACH_SIMD_OPCODE(DECLARE_OPCODE_CASE) { return 2; }

[gdeepti, 2016/09/01 00:52:52]

This doesn't seem right, for generic SIMD  opcodes shouldn't the length be
computed based on signature?  For example, this would fail for the Select
opcode. 

Also, define named constants for return values. 

[aseemgarg, 2016/09/02 22:40:12]

This just checks the size of opcode. That is 2 (simd_prefix + opcode) for 0
operand opcodes and 3 (extra immediate) for 1 operand opcodes.
Also, seems like this function doesn't really use named constants..

+        FOREACH_SIMD_MISC_OPCODE(DECLARE_OPCODE_CASE) { return 3; }

[bradnelson, 2016/09/01 00:51:34]

Not keen on the MISC name, _0OPERAND + _1OPERAND ?
The shuffles and swizzles will be in this category too.

[aseemgarg, 2016/09/01 00:57:15]

Changed name. Lot of opcodes will come in this category including replace lanes
etc. We can move them as we implement them.

+#undef DECLARE_OPCODE_CASE
       default:
         return 1;
     }
@@ -1271,7 +1278,7 @@ class WasmFullDecoder : public WasmDecoder {
             len++;
             byte simd_index = *(pc_ + 1);
             opcode = static_cast<WasmOpcode>(opcode << 8 | simd_index);
-            DecodeSimdOpcode(opcode);
+            len += DecodeSimdOpcode(opcode);
             break;
           }
           default:
@@ -1396,15 +1403,30 @@ class WasmFullDecoder : public WasmDecoder {
     return 1 + operand.length;
   }
 
-  void DecodeSimdOpcode(WasmOpcode opcode) {
-    FunctionSig* sig = WasmOpcodes::Signature(opcode);
-    compiler::NodeVector inputs(sig->parameter_count(), zone_);
-    for (size_t i = sig->parameter_count(); i > 0; i--) {
-      Value val = Pop(static_cast<int>(i - 1), sig->GetParam(i - 1));
-      inputs[i - 1] = val.node;
+  unsigned DecodeSimdOpcode(WasmOpcode opcode) {
+    unsigned len = 0;
+    switch (opcode) {
+      case kExprI32x4ExtractLane: {
+        uint8_t lane = this->checked_read_u8(pc_, 2, "lane number");
+        DCHECK(lane >= 0 && lane <= 3);
+        TFNode* input = Pop(0, LocalType::kSimd128).node;
+        TFNode* node = BUILD(SimdExtractLane, opcode, lane, input);
+        Push(LocalType::kWord32, node);
+        len++;
+        break;
+      }
+      default: {
+        FunctionSig* sig = WasmOpcodes::Signature(opcode);
+        compiler::NodeVector inputs(sig->parameter_count(), zone_);
+        for (size_t i = sig->parameter_count(); i > 0; i--) {
+          Value val = Pop(static_cast<int>(i - 1), sig->GetParam(i - 1));
+          inputs[i - 1] = val.node;
+        }
+        TFNode* node = BUILD(SimdOp, opcode, inputs);
+        Push(GetReturnType(sig), node);
+      }
     }
-    TFNode* node = BUILD(SimdOp, opcode, inputs);
-    Push(GetReturnType(sig), node);
+    return len;
   }
 
   void DispatchToTargets(Control* next_block, const Value& val) {