[V8][asm.js] Adds support to global const variables.

This CL adds support for:
https://discourse.wicg.io/t/allow-const-global-variables/684

It allows global const variables to be used as if they were numeric
literals. For example:

  const f0 = fround(0);
  ...
  function foo() {
    var v = f0;  // no type annotation.
    ...
    return f0;  // no return type annotation.
  }

BUG= https://bugs.chromium.org/p/v8/issues/detail?id=5540

Review-Url: https://chromiumcodereview.appspot.com/2435823002
Cr-Commit-Position: refs/heads/master@{#40477}

src/asmjs/asm-typer.cc

@@ -762,9 +762,15 @@ AsmType* AsmTyper::ValidateGlobalDeclaration(Assignment* assign) {
   bool global_variable = false;
   if (value->IsLiteral() || value->IsCall()) {
     AsmType* type = nullptr;
-    RECURSE(type = VariableTypeAnnotations(value, true));
+    VariableInfo::Mutability mutability;
+    if (target_variable->mode() == CONST) {
+      mutability = VariableInfo::kConstGlobal;
+    } else {
+      mutability = VariableInfo::kMutableGlobal;
+    }
+    RECURSE(type = VariableTypeAnnotations(value, mutability));
     target_info = new (zone_) VariableInfo(type);
-    target_info->set_mutability(VariableInfo::kMutableGlobal);
+    target_info->set_mutability(mutability);
     global_variable = true;
   } else if (value->IsProperty()) {
     target_info = ImportLookup(value->AsProperty());
@@ -828,6 +834,23 @@ AsmType* AsmTyper::ValidateGlobalDeclaration(Assignment* assign) {
     RECURSE(type = NewHeapView(value->AsCallNew()));
     target_info = new (zone_) VariableInfo(type);
     target_info->set_mutability(VariableInfo::kImmutableGlobal);
+  } else if (auto* proxy = value->AsVariableProxy()) {
+    auto* var_info = Lookup(proxy->var());
+
+    if (var_info == nullptr) {
+      FAIL(value, "Undeclared identifier in global initializer");
+    }
+
+    if (var_info->mutability() != VariableInfo::kConstGlobal) {
+      FAIL(value, "Identifier used to initialize a global must be a const");
+    }
+
+    target_info = new (zone_) VariableInfo(var_info->type());
+    if (target_variable->mode() == CONST) {
+      target_info->set_mutability(VariableInfo::kConstGlobal);
+    } else {
+      target_info->set_mutability(VariableInfo::kMutableGlobal);
+    }
   }
 
   if (target_info == nullptr) {
@@ -2684,13 +2707,27 @@ AsmType* AsmTyper::ReturnTypeAnnotations(ReturnStatement* statement) {
     FAIL(statement, "Invalid literal in return statement.");
   }
 
+  if (auto* proxy = ret_expr->AsVariableProxy()) {
+    auto* var_info = Lookup(proxy->var());
+
+    if (var_info == nullptr) {
+      FAIL(statement, "Undeclared identifier in return statement.");
+    }
+
+    if (var_info->mutability() != VariableInfo::kConstGlobal) {
+      FAIL(statement, "Identifier in return statement is not const.");
+    }
+
+    return var_info->type();
+  }
+
   FAIL(statement, "Invalid return type expression.");
 }
 
 // 5.4 VariableTypeAnnotations
 // Also used for 5.5 GlobalVariableTypeAnnotations
-AsmType* AsmTyper::VariableTypeAnnotations(Expression* initializer,
-                                           bool global) {
+AsmType* AsmTyper::VariableTypeAnnotations(
+    Expression* initializer, VariableInfo::Mutability mutability_type) {
   if (auto* literal = initializer->AsLiteral()) {
     if (literal->raw_value()->ContainsDot()) {
       SetTypeOf(initializer, AsmType::Double());
@@ -2698,24 +2735,50 @@ AsmType* AsmTyper::VariableTypeAnnotations(Expression* initializer,
     }
     int32_t i32;
     uint32_t u32;
+
+    AsmType* initializer_type = nullptr;
     if (literal->value()->ToUint32(&u32)) {
       if (u32 > LargestFixNum) {
-        SetTypeOf(initializer, AsmType::Unsigned());
+        initializer_type = AsmType::Unsigned();
+        SetTypeOf(initializer, initializer_type);
       } else {
-        SetTypeOf(initializer, AsmType::FixNum());
+        initializer_type = AsmType::FixNum();
+        SetTypeOf(initializer, initializer_type);
+        initializer_type = AsmType::Signed();
       }
     } else if (literal->value()->ToInt32(&i32)) {
-      SetTypeOf(initializer, AsmType::Signed());
+      initializer_type = AsmType::Signed();
+      SetTypeOf(initializer, initializer_type);
     } else {
       FAIL(initializer, "Invalid type annotation - forbidden literal.");
     }
-    return AsmType::Int();
+    if (mutability_type != VariableInfo::kConstGlobal) {
+      return AsmType::Int();
+    }
+    return initializer_type;
+  }
+
+  if (auto* proxy = initializer->AsVariableProxy()) {
+    auto* var_info = Lookup(proxy->var());
+
+    if (var_info == nullptr) {
+      FAIL(initializer,
+           "Undeclared identifier in variable declaration initializer.");
+    }
+
+    if (var_info->mutability() != VariableInfo::kConstGlobal) {
+      FAIL(initializer,
+           "Identifier in variable declaration initializer must be const.");
+    }
+
+    SetTypeOf(initializer, var_info->type());
+    return var_info->type();
   }
 
   auto* call = initializer->AsCall();
   if (call == nullptr) {
     FAIL(initializer,
-         "Invalid variable initialization - it should be a literal, or "
+         "Invalid variable initialization - it should be a literal, const, or "
          "fround(literal).");
   }
 
@@ -2732,7 +2795,7 @@ AsmType* AsmTyper::VariableTypeAnnotations(Expression* initializer,
   }
 
   // Float constants must contain dots in local, but not in globals.
-  if (!global) {
+  if (mutability_type == VariableInfo::kLocal) {
     if (!src_expr->raw_value()->ContainsDot()) {
       FAIL(initializer,
            "Invalid float type annotation - expected literal argument to be a "

src/asmjs/asm-typer.h

@@ -102,6 +102,13 @@ class AsmTyper final {
       kInvalidMutability,
       kLocal,
       kMutableGlobal,
+      // *VIOLATION* We support const variables in asm.js, as per the
+      //
+      // https://discourse.wicg.io/t/allow-const-global-variables/684
+      //
+      // Global const variables are treated as if they were numeric literals,
+      // and can be used anywhere a literal can be used.
+      kConstGlobal,
       kImmutableGlobal,
     };
 
@@ -114,7 +121,8 @@ class AsmTyper final {
     }
 
     bool IsGlobal() const {
-      return mutability_ == kImmutableGlobal || mutability_ == kMutableGlobal;
+      return mutability_ == kImmutableGlobal || mutability_ == kConstGlobal ||
+             mutability_ == kMutableGlobal;
     }
 
     bool IsStdlib() const { return standard_member_ == kStdlib; }
@@ -307,8 +315,9 @@ class AsmTyper final {
   AsmType* ReturnTypeAnnotations(ReturnStatement* statement);
   // 5.4 VariableTypeAnnotations
   // 5.5 GlobalVariableTypeAnnotations
-  AsmType* VariableTypeAnnotations(Expression* initializer,
-                                   bool global = false);
+  AsmType* VariableTypeAnnotations(
+      Expression* initializer,
+      VariableInfo::Mutability global = VariableInfo::kLocal);
   AsmType* ImportExpression(Property* import);
   AsmType* NewHeapView(CallNew* new_heap_view);
 

test/cctest/asmjs/test-asm-typer.cc

@@ -505,14 +505,15 @@ TEST(ErrorsInGlobalVariableDefinition) {
     const char* error_message;
   } kTests[] = {
       {"var v;", "Global variable missing initializer"},
-      {"var v = uninitialized;", "Invalid global variable initializer"},
+      {"var v = uninitialized;", "Undeclared identifier in global"},
       {"var v = 'use asm';", "type annotation - forbidden literal"},
       {"var v = 4294967296;", " - forbidden literal"},
-      {"var v = not_fround;", "Invalid global variable initializer"},
+      {"var v = not_fround;", "initialize a global must be a const"},
       {"var v = not_fround(1);", "expected call fround(literal)"},
       {"var v = __fround__(1.0);", "expected call fround(literal)"},
       {"var v = fround(1.0, 1.0);", "expected call fround(literal)"},
       {"var v = fround(not_fround);", "literal argument for call to fround"},
+      {"var v = i?0:1;", "Invalid global variable initializer"},
       {"var v = stdlib.nan", "Invalid import"},
       {"var v = stdlib.Math.nan", "Invalid import"},
       {"var v = stdlib.Mathh.E", "Invalid import"},
@@ -788,6 +789,19 @@ TEST(ErrorsInFunction) {
        "  var c = 0;\n"
        "}\n",
        "Local variable missing initializer in asm.js module"},
+      {"function f(a) {\n"
+       "  a = a|0;\n"
+       "  var x = a;\n"
+       "}\n",
+       "variable declaration initializer must be const"},
+      {"function f() {\n"
+       "  var x = 1+i;\n"
+       "}\n",
+       "should be a literal, const, or fround(literal"},
+      {"function f() {\n"
+       "  var x = a;\n"
+       "}\n",
+       "Undeclared identifier in variable declaration initializer"},
       {"function f() {\n"
        "  function ff() {}\n"
        "}\n",
@@ -812,6 +826,19 @@ TEST(ErrorsInFunction) {
        " return 2147483648;\n"
        "}\n",
        "Invalid literal in return statement"},
+      {"function f(a) {\n"
+       "  a = a|0;\n"
+       "  return a;\n"
+       "}\n",
+       "in return statement is not const"},
+      {"function f() {\n"
+       "  return a;\n"
+       "}\n",
+       "Undeclared identifier in return statement"},
+      {"function f() {\n"
+       "  return i?0:1;\n"
+       "}\n",
+       "Invalid return type expression"},
       {"function f() {\n"
        "  return stdlib.Math.E;"
        "}\n",

test/mjsunit/asm/asm-validation.js

@@ -8,6 +8,76 @@ function assertValidAsm(func) {
   assertTrue(%IsAsmWasmCode(func));
 }
 
+(function TestConst() {
+  function Module(s) {
+    "use asm";
+    var fround = s.Math.fround;
+    // Global constants. These are treated just like numeric literals.
+    const fConst = fround(-3.0);
+    const dConst = -3.0;
+    const iConst = -3;
+
+    // consts can be used to initialize other consts.
+    const fPrime = fConst;
+
+    // The following methods verify that return statements with global constants
+    // do not need type annotations.
+    function f() {
+      return fPrime;
+    }
+    function d() {
+      return dConst;
+    }
+    function i() {
+      return iConst;
+    }
+
+    // The following methods verify that locals initialized with global
+    // constants do not need type annotations.
+    function fVar() {
+      var v = fPrime;
+      return fround(v);
+    }
+    function iVar() {
+      var v = iConst;
+      return v|0;
+    }
+    function dVar() {
+      var v = dConst;
+      return +v;
+    }
+
+    return {
+      f: f, d: d, i: i,
+      fVar: fVar, dVar: dVar, iVar: iVar,
+    };
+  }
+
+  function DisallowAssignToConstGlobal() {
+    const constant = 0;
+    function invalid(i) {
+      i = i|0;
+      constant = i;
+      return constant;
+    }
+    return invalid;
+  }
+
+  var m = Module(this);
+  assertValidAsm(Module);
+
+  assertEquals(-3, m.i());
+  assertEquals(-3.0, m.d());
+  assertEquals(Math.fround(-3.0), m.f());
+
+  assertEquals(-3, m.iVar());
+  assertEquals(-3.0, m.dVar());
+  assertEquals(Math.fround(-3.0), m.fVar());
+
+  var m = DisallowAssignToConstGlobal();
+  assertTrue(%IsNotAsmWasmCode(DisallowAssignToConstGlobal));
+})();
+
 (function TestModuleArgs() {
   function Module1(stdlib) {
     "use asm";