Implement UnalignedLoad and UnalignedStore in WASM using LoadByte/Shift/Or and StoreByte/Shift/And.

src/compiler/wasm-compiler.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/wasm-compiler.h"
 
 #include "src/isolate-inl.h"
 
 #include "src/base/platform/elapsed-timer.h"
 #include "src/base/platform/platform.h"
@@ skipping 2573 matching lines @@
     size_t limit = size - offset - memsize;
     CHECK(limit <= kMaxUInt32);
     cond = graph()->NewNode(
         jsgraph()->machine()->Uint32LessThanOrEqual(), index,
         jsgraph()->Int32Constant(static_cast<uint32_t>(limit)));
   }
 
   trap_->AddTrapIfFalse(wasm::kTrapMemOutOfBounds, cond, position);
 }
 
+MachineType WasmGraphBuilder::GetTypeForUnalignedAccess(uint32_t alignment,
+                                                        bool signExtend) {
+  switch (alignment) {
+    case 0:
+      return signExtend ? MachineType::Int8() : MachineType::Uint8();
+    case 1:
+      return signExtend ? MachineType::Int16() : MachineType::Uint16();
+    case 2:
+      return signExtend ? MachineType::Int32() : MachineType::Uint32();
+    default:
+      UNREACHABLE();
+  }
+}
+
+Node* WasmGraphBuilder::BuildUnalignedLoad(wasm::LocalType type,
+                                           MachineType memtype, Node* index,
+                                           uint32_t offset,
+                                           uint32_t alignment) {
+  Node* result;
+  Node* load;
+  bool extendTo64Bit = false;
+
+  wasm::WasmOpcode shiftOpcode;
+  wasm::WasmOpcode orOpcode;
+  Node* shiftConst;
+
+  auto calculateOffset = [this](Node* baseOffset, int numberOfBytes, int stride,
+                                int current) -> Node* {

[titzer, 2016/05/17 11:57:39]

Is there a way to refactor this to avoid a lambda? I would it more clear to have
a helper function in WasmCompiler rather than a lambda that closes over "this".

+    int offset;
+
+#if defined(V8_TARGET_LITTLE_ENDIAN)
+    offset = numberOfBytes - stride - current;
+#elif defined(V8_TARGET_BIG_ENDIAN)
+    offset = current;
+#else
+#error Unsupported endianness
+#endif
+
+    if (offset == 0) {
+      return baseOffset;
+    } else {
+      return Binop(wasm::kExprI32Add, baseOffset,
+                   jsgraph()->Int32Constant(offset));
+    }
+  };
+
+  bool signExtend = memtype.IsSigned();
+
+  bool isFloat = IsFloatingPoint(memtype.representation());
+  int stride =
+      1 << ElementSizeLog2Of(
+          GetTypeForUnalignedAccess(alignment, false).representation());
+  int numberOfBytes = 1 << ElementSizeLog2Of(memtype.representation());
+  int numberOfLoads = numberOfBytes / stride;
+
+  DCHECK(numberOfBytes % stride == 0);
+  DCHECK(numberOfLoads >= 2);
+
+  switch (type) {
+    case wasm::kAstI64:
+    case wasm::kAstF64:
+      shiftOpcode = wasm::kExprI64Shl;
+      orOpcode = wasm::kExprI64Ior;
+      result = jsgraph()->Int64Constant(0);
+      shiftConst = jsgraph()->Int64Constant(8 * stride);
+      extendTo64Bit = true;
+      break;
+    case wasm::kAstI32:
+    case wasm::kAstF32:
+      shiftOpcode = wasm::kExprI32Shl;
+      orOpcode = wasm::kExprI32Ior;
+      result = jsgraph()->Int32Constant(0);
+      shiftConst = jsgraph()->Int32Constant(8 * stride);
+      break;
+    default:
+      UNREACHABLE();
+  }
+
+  Node* baseOffset = MemBuffer(offset);
+
+  for (int i = 0; i < numberOfBytes; i += stride) {
+    result = Binop(shiftOpcode, result, shiftConst);
+    load =
+        graph()->NewNode(jsgraph()->machine()->Load(
+                             GetTypeForUnalignedAccess(alignment, signExtend)),
+                         calculateOffset(baseOffset, numberOfBytes, stride, i),
+                         index, *effect_, *control_);
+    *effect_ = load;
+    if (extendTo64Bit) {
+      if (signExtend) {
+        load =
+            graph()->NewNode(jsgraph()->machine()->ChangeInt32ToInt64(), load);
+      } else {
+        load = graph()->NewNode(jsgraph()->machine()->ChangeUint32ToUint64(),
+                                load);
+      }
+    }
+    signExtend = false;
+    result = Binop(orOpcode, result, load);
+  }
+
+  // Convert to float
+  if (isFloat) {
+    switch (type) {
+      case wasm::kAstF32:
+        result = Unop(wasm::kExprF32ReinterpretI32, result);
+        break;
+      case wasm::kAstF64:
+        result = Unop(wasm::kExprF64ReinterpretI64, result);
+        break;
+      default:
+        UNREACHABLE();
+    }
+  }
+
+  return result;
+}
+
 Node* WasmGraphBuilder::LoadMem(wasm::LocalType type, MachineType memtype,
                                 Node* index, uint32_t offset,
+                                uint32_t alignment,
                                 wasm::WasmCodePosition position) {
   Node* load;
 
   if (module_ && module_->asm_js()) {
     // asm.js semantics use CheckedLoad (i.e. OOB reads return 0ish).
     DCHECK_EQ(0, offset);
     const Operator* op = jsgraph()->machine()->CheckedLoad(memtype);
     load = graph()->NewNode(op, MemBuffer(0), index, MemSize(0), *effect_,
                             *control_);
+    *effect_ = load;
   } else {
     // WASM semantics throw on OOB. Introduce explicit bounds check.
     BoundsCheckMem(memtype, index, offset, position);
-    load = graph()->NewNode(jsgraph()->machine()->Load(memtype),
-                            MemBuffer(offset), index, *effect_, *control_);
+    bool aligned = alignment >= ElementSizeLog2Of(memtype.representation());
+
+    if (aligned ||
+        jsgraph()->machine()->UnalignedLoadSupported(memtype, alignment)) {
+      load = graph()->NewNode(jsgraph()->machine()->Load(memtype),
+                              MemBuffer(offset), index, *effect_, *control_);
+      *effect_ = load;
+    } else {
+      load = BuildUnalignedLoad(type, memtype, index, offset, alignment);
+    }
   }
 
-  *effect_ = load;
-
   if (type == wasm::kAstI64 &&
       ElementSizeLog2Of(memtype.representation()) < 3) {
     // TODO(titzer): TF zeroes the upper bits of 64-bit loads for subword sizes.
     if (memtype.IsSigned()) {
       // sign extend
       load = graph()->NewNode(jsgraph()->machine()->ChangeInt32ToInt64(), load);
     } else {
       // zero extend
       load =
           graph()->NewNode(jsgraph()->machine()->ChangeUint32ToUint64(), load);
     }
   }
 
   return load;
 }
 
+Node* WasmGraphBuilder::BuildUnalignedStore(MachineType memtype, Node* index,
+                                            uint32_t offset, uint32_t alignment,
+                                            Node* val) {
+  Node* store;
+  Node* newValue;
+
+  wasm::WasmOpcode shiftOpcode;
+
+  Node* shiftConst;
+  bool extendTo64Bit = false;
+
+  auto calculateOffset = [this](Node* baseOffset, int numberOfBytes, int stride,
+                                int current) -> Node* {
+    int offset;
+#if defined(V8_TARGET_LITTLE_ENDIAN)
+    offset = current;
+#elif defined(V8_TARGET_BIG_ENDIAN)
+    offset = numberOfBytes - stride - current;
+#else
+#error Unsupported endianness
+#endif
+    if (offset == 0) {
+      return baseOffset;
+    } else {
+      return Binop(wasm::kExprI32Add, baseOffset,
+                   jsgraph()->Int32Constant(offset));
+    }
+  };
+
+  bool isFloat = IsFloatingPoint(memtype.representation());
+  int stride = 1 << ElementSizeLog2Of(
+                   GetTypeForUnalignedAccess(alignment).representation());
+  int numberOfBytes = 1 << ElementSizeLog2Of(memtype.representation());
+  int numberOfLoads = numberOfBytes / stride;
+
+  DCHECK(numberOfBytes % stride == 0);
+  DCHECK(numberOfLoads >= 2);
+
+  StoreRepresentation rep(GetTypeForUnalignedAccess(alignment).representation(),
+                          kNoWriteBarrier);
+
+  if (ElementSizeLog2Of(memtype.representation()) <= 2) {
+    shiftOpcode = wasm::kExprI32ShrU;
+    shiftConst = jsgraph()->Int32Constant(8 * stride);
+  } else {
+    shiftOpcode = wasm::kExprI64ShrU;
+    shiftConst = jsgraph()->Int64Constant(8 * stride);
+    extendTo64Bit = true;
+  }
+
+  newValue = val;
+  if (isFloat) {
+    switch (memtype.representation()) {
+      case MachineRepresentation::kFloat64:
+        newValue = Unop(wasm::kExprI64ReinterpretF64, val);
+        break;
+      case MachineRepresentation::kFloat32:
+        newValue = Unop(wasm::kExprI32ReinterpretF32, val);
+        break;
+      default:
+        UNREACHABLE();
+    }
+  }
+
+  Node* baseOffset = MemBuffer(offset);
+
+  for (int i = 0; i < numberOfBytes - stride; i += stride) {
+    store = graph()->NewNode(
+        jsgraph()->machine()->Store(rep),
+        calculateOffset(baseOffset, numberOfBytes, stride, i), index,
+        extendTo64Bit ? Unop(wasm::kExprI32ConvertI64, newValue) : newValue,
+        *effect_, *control_);
+    newValue = Binop(shiftOpcode, newValue, shiftConst);
+    *effect_ = store;
+  }
+  store = graph()->NewNode(
+      jsgraph()->machine()->Store(rep),
+      calculateOffset(baseOffset, numberOfBytes, stride,
+                      numberOfBytes - stride),
+      index,
+      extendTo64Bit ? Unop(wasm::kExprI32ConvertI64, newValue) : newValue,
+      *effect_, *control_);
+  *effect_ = store;
+  return val;
+}
+
 Node* WasmGraphBuilder::StoreMem(MachineType memtype, Node* index,
-                                 uint32_t offset, Node* val,
+                                 uint32_t offset, uint32_t alignment, Node* val,
                                  wasm::WasmCodePosition position) {
   Node* store;
   if (module_ && module_->asm_js()) {
     // asm.js semantics use CheckedStore (i.e. ignore OOB writes).
     DCHECK_EQ(0, offset);
     const Operator* op =
         jsgraph()->machine()->CheckedStore(memtype.representation());
     store = graph()->NewNode(op, MemBuffer(0), index, MemSize(0), val, *effect_,
                              *control_);
+    *effect_ = store;
   } else {
     // WASM semantics throw on OOB. Introduce explicit bounds check.
     BoundsCheckMem(memtype, index, offset, position);
     StoreRepresentation rep(memtype.representation(), kNoWriteBarrier);
-    store =
-        graph()->NewNode(jsgraph()->machine()->Store(rep), MemBuffer(offset),
-                         index, val, *effect_, *control_);
+    bool aligned = alignment >= ElementSizeLog2Of(memtype.representation());
+
+    if (aligned ||
+        jsgraph()->machine()->UnalignedStoreSupported(memtype, alignment)) {
+      StoreRepresentation rep(memtype.representation(), kNoWriteBarrier);
+      store =
+          graph()->NewNode(jsgraph()->machine()->Store(rep), MemBuffer(offset),
+                           index, val, *effect_, *control_);
+      *effect_ = store;
+    } else {
+      store = BuildUnalignedStore(memtype, index, offset, alignment, val);
+    }
   }
-  *effect_ = store;
+
   return store;
 }
 
 
 void WasmGraphBuilder::PrintDebugName(Node* node) {
   PrintF("#%d:%s", node->id(), node->op()->mnemonic());
 }
 
 
 Node* WasmGraphBuilder::String(const char* string) {
@@ skipping 210 matching lines @@
     double* decode_ms) {
   base::ElapsedTimer decode_timer;
   if (FLAG_trace_wasm_decode_time) {
     decode_timer.Start();
   }
   // Create a TF graph during decoding.
   Graph* graph = new (zone) Graph(zone);
   CommonOperatorBuilder* common = new (zone) CommonOperatorBuilder(zone);
   MachineOperatorBuilder* machine = new (zone) MachineOperatorBuilder(
       zone, MachineType::PointerRepresentation(),
-      InstructionSelector::SupportedMachineOperatorFlags());
+      InstructionSelector::SupportedMachineOperatorFlags(),
+      InstructionSelector::AlignmentRequirements());
   JSGraph* jsgraph =
       new (zone) JSGraph(isolate, graph, common, nullptr, nullptr, machine);
   SourcePositionTable* source_position_table =
       new (zone) SourcePositionTable(graph);
   WasmGraphBuilder builder(zone, jsgraph, function->sig, source_position_table);
   wasm::FunctionBody body = {
       module_env, function->sig, module_env->module->module_start,
       module_env->module->module_start + function->code_start_offset,
       module_env->module->module_start + function->code_end_offset};
   wasm::TreeResult result =
@@ skipping 186 matching lines @@
                                  const wasm::WasmFunction* function) {
   WasmCompilationUnit* unit =
       CreateWasmCompilationUnit(thrower, isolate, module_env, function, 0);
   ExecuteCompilation(unit);
   return FinishCompilation(unit);
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8