[wasm] Introduce EmitVarInt and EmitWithVarInt

Use them to encode int32 constants properly.
This reduces the generated wasm size in the unity benchmark from
21.6 MB to 16.8 MB (-22.2%).
This hopefully also translates to increased performance especially on
mobile because of lower memory usage.

R=bradnelson@chromium.org, titzer@chromium.org

Review-Url: https://codereview.chromium.org/2692943002
Cr-Commit-Position: refs/heads/master@{#43200}

src/asmjs/asm-wasm-builder.cc

@@ -683,35 +683,26 @@ class AsmWasmBuilderImpl final : public AstVisitor<AsmWasmBuilderImpl> {
 
     if (type->IsA(AsmType::Signed())) {
       int32_t i = 0;
-      if (!value->ToInt32(&i)) {
-        UNREACHABLE();
-      }
-      byte code[] = {WASM_I32V(i)};
-      current_function_builder_->EmitCode(code, sizeof(code));
+      CHECK(value->ToInt32(&i));
+      current_function_builder_->EmitI32Const(i);
     } else if (type->IsA(AsmType::Unsigned()) || type->IsA(AsmType::FixNum())) {
       uint32_t u = 0;
-      if (!value->ToUint32(&u)) {
-        UNREACHABLE();
-      }
-      int32_t i = static_cast<int32_t>(u);
-      byte code[] = {WASM_I32V(i)};
-      current_function_builder_->EmitCode(code, sizeof(code));
+      CHECK(value->ToUint32(&u));
+      current_function_builder_->EmitI32Const(bit_cast<int32_t>(u));
     } else if (type->IsA(AsmType::Int())) {
       // The parser can collapse !0, !1 etc to true / false.
       // Allow these as int literals.
       if (expr->raw_value()->IsTrue()) {
-        byte code[] = {WASM_I32V(1)};
+        byte code[] = {WASM_ONE};
         current_function_builder_->EmitCode(code, sizeof(code));
       } else if (expr->raw_value()->IsFalse()) {
-        byte code[] = {WASM_I32V(0)};
+        byte code[] = {WASM_ZERO};
         current_function_builder_->EmitCode(code, sizeof(code));
       } else if (expr->raw_value()->IsNumber()) {
         // This can happen when -x becomes x * -1 (due to the parser).
         int32_t i = 0;
-        if (!value->ToInt32(&i) || i != -1) {
-          UNREACHABLE();
-        }
-        byte code[] = {WASM_I32V(i)};
+        CHECK(value->ToInt32(&i) && i == -1);
+        byte code[] = {WASM_I32V_1(-1)};
         current_function_builder_->EmitCode(code, sizeof(code));
       } else {
         UNREACHABLE();

src/wasm/wasm-module-builder.cc

@@ -62,13 +62,20 @@ WasmFunctionBuilder::WasmFunctionBuilder(WasmModuleBuilder* builder)
       direct_calls_(builder->zone()),
       asm_offsets_(builder->zone(), 8) {}
 
+void WasmFunctionBuilder::EmitVarInt(int32_t val) {
+  byte buffer[5];
+  byte* ptr = buffer;
+  LEBHelper::write_i32v(&ptr, val);
+  DCHECK_GE(5, ptr - buffer);
+  body_.insert(body_.end(), buffer, ptr);
+}
+
 void WasmFunctionBuilder::EmitVarUint(uint32_t val) {
-  byte buffer[8];
+  byte buffer[5];
   byte* ptr = buffer;
   LEBHelper::write_u32v(&ptr, val);
-  for (byte* p = buffer; p < ptr; p++) {
-    body_.push_back(*p);
-  }
+  DCHECK_GE(5, ptr - buffer);
+  body_.insert(body_.end(), buffer, ptr);
 }
 
 void WasmFunctionBuilder::SetSignature(FunctionSig* sig) {
@@ -116,6 +123,11 @@ void WasmFunctionBuilder::EmitWithU8U8(WasmOpcode opcode, const byte imm1,
   body_.push_back(imm2);
 }
 
+void WasmFunctionBuilder::EmitWithVarInt(WasmOpcode opcode, int32_t immediate) {
+  body_.push_back(static_cast<byte>(opcode));
+  EmitVarInt(immediate);
+}
+
 void WasmFunctionBuilder::EmitWithVarUint(WasmOpcode opcode,
                                           uint32_t immediate) {
   body_.push_back(static_cast<byte>(opcode));
@@ -123,13 +135,7 @@ void WasmFunctionBuilder::EmitWithVarUint(WasmOpcode opcode,
 }
 
 void WasmFunctionBuilder::EmitI32Const(int32_t value) {
-  if (-64 <= value && value <= 63) {
-    EmitWithU8(kExprI32Const, static_cast<byte>(value & 0x7F));
-  } else {
-    // TODO(titzer): variable-length signed and unsigned i32 constants.
-    byte code[] = {WASM_I32V_5(value)};
-    EmitCode(code, sizeof(code));
-  }
+  EmitWithVarInt(kExprI32Const, value);
 }
 
 void WasmFunctionBuilder::EmitDirectCallIndex(uint32_t index) {

src/wasm/wasm-module-builder.h

@@ -121,6 +121,7 @@ class V8_EXPORT_PRIVATE WasmFunctionBuilder : public ZoneObject {
   // Building methods.
   void SetSignature(FunctionSig* sig);
   uint32_t AddLocal(ValueType type);
+  void EmitVarInt(int32_t val);
   void EmitVarUint(uint32_t val);
   void EmitCode(const byte* code, uint32_t code_size);
   void Emit(WasmOpcode opcode);
@@ -130,6 +131,7 @@ class V8_EXPORT_PRIVATE WasmFunctionBuilder : public ZoneObject {
   void EmitI32Const(int32_t val);
   void EmitWithU8(WasmOpcode opcode, const byte immediate);
   void EmitWithU8U8(WasmOpcode opcode, const byte imm1, const byte imm2);
+  void EmitWithVarInt(WasmOpcode opcode, int32_t immediate);
   void EmitWithVarUint(WasmOpcode opcode, uint32_t immediate);
   void EmitDirectCallIndex(uint32_t index);
   void ExportAs(Vector<const char> name);