[asm.js] Fix and test assignment to function imports.

This tests and fixes validation failures caused by assignments to
variables holding functions references (which are all considered
immutable). Such references can come from "stdlib" or "foreign".

R=clemensh@chromium.org
TEST=mjsunit/asm/global-imports
BUG=chromium:719382

Change-Id: Ic02be765e0773a6cc74a54e11a09d42ffb683cb8
Reviewed-on: https://chromium-review.googlesource.com/500188Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45189}

src/asmjs/asm-parser.cc

@@ -546,6 +546,7 @@ void AsmJsParser::ValidateModuleVarImport(VarInfo* info,
       info->kind = VarKind::kImportedFunction;
       info->import = new (zone()->New(sizeof(FunctionImportInfo)))
           FunctionImportInfo({name, WasmModuleBuilder::SignatureMap(zone())});
+      info->mutable_variable = false;
     }
   }
 }
@@ -1484,6 +1485,9 @@ AsmType* AsmJsParser::AssignmentExpression() {
       if (info->kind == VarKind::kUnused) {
         FAILn("Undeclared assignment target");
       }
+      if (!info->mutable_variable) {
+        FAILn("Expected mutable variable in assignment");
+      }
       DCHECK(is_local ? info->kind == VarKind::kLocal
                       : info->kind == VarKind::kGlobal);
       AsmType* value;
@@ -1494,9 +1498,6 @@ AsmType* AsmJsParser::AssignmentExpression() {
       if (info->kind == VarKind::kLocal) {
         current_function_builder_->EmitTeeLocal(info->index);
       } else if (info->kind == VarKind::kGlobal) {
-        if (!info->mutable_variable) {
-          FAILn("Expected mutable variable in assignment");
-        }
         current_function_builder_->EmitWithU32V(kExprSetGlobal, VarIndex(info));
         current_function_builder_->EmitWithU32V(kExprGetGlobal, VarIndex(info));
       } else {

test/mjsunit/asm/global-imports.js

@@ -21,6 +21,7 @@ var throws = {};
 var test_count = 0;
 const stdlib = this;
 const buffer = new ArrayBuffer(1024);
+function p(x) { return x * x; }
 
 function assertThrowsOrEquals(result, fun) {
   if (result === throws) {
@@ -52,6 +53,7 @@ function Run(imp, exp, imports, result, valid) {
   RunAsmJsTest(src, imports, result, valid);
 }
 
+// Imports of values from foreign.
 Run("var x = foreign.x | 0",   "(x + int) | 0", {x:12},  13,     true);
 Run("var x = foreign.x | 0",   "(x = int) | 0", {x:12},  1,      true);
 Run("var x = foreign.x | 0",   "+(x + dbl)",    {x:12},  16.2,   false);
@@ -64,3 +66,9 @@ Run("const x = foreign.x | 0", "+(x + dbl)",    {x:12},  16.2,   false);
 Run("const x = +foreign.x",    "+(x + dbl)",    {x:1.2}, 5.4,    true);
 Run("const x = +foreign.x",    "+(x = dbl)",    {x:1.2}, throws, false);
 Run("const x = +foreign.x",    "(x + int) | 0", {x:1.2}, 2,      false);
+
+// Imports of functions and values from stdlib and foreign.
+Run("var x = foreign.x",        "x(dbl) | 0",               { x:p }, 17, true);
+Run("var x = foreign.x",        "(x = fround, x(dbl)) | 0", { x:p }, 4,  false);
+Run("var x = stdlib.Math.E",    "(x = 3.1415, 1) | 0",      {},      1,  false);
+Run("var x = stdlib.Math.imul", "(x = fround, 1) | 0",      {},      1,  false);