[wasm] ASM-2-WASM. Enforces switch default clause appearing last.

BUG= https://bugs.chromium.org/p/chromium/issues/detail?id=628803
BUG= https://bugs.chromium.org/p/v8/issues/detail?id=4203
TEST= cctest/asmjs/test-asm-typer.cc
LOG= N

Review-Url: https://codereview.chromium.org/2172603002
Cr-Commit-Position: refs/heads/master@{#37954}

src/asmjs/asm-typer.cc

@@ -1347,17 +1347,23 @@ AsmType* AsmTyper::ValidateSwitchStatement(SwitchStatement* stmt) {
     FAIL(stmt, "Switch tag must be signed.");
   }
 
-  bool has_default = false;
-
+  int default_pos = kNoSourcePosition;
+  int last_case_pos = kNoSourcePosition;
   ZoneSet<int32_t> cases_seen(zone_);
   for (auto* a_case : *stmt->cases()) {
     if (a_case->is_default()) {
-      CHECK(!has_default);
+      CHECK(default_pos == kNoSourcePosition);
       RECURSE(ValidateDefault(a_case));
-      has_default = true;
+      default_pos = a_case->position();
       continue;
     }
 
+    if (last_case_pos == kNoSourcePosition) {
+      last_case_pos = a_case->position();
+    } else {
+      last_case_pos = std::max(last_case_pos, a_case->position());
+    }
+
     int32_t case_lbl;
     RECURSE(ValidateCase(a_case, &case_lbl));
     auto case_lbl_pos = cases_seen.find(case_lbl);
@@ -1375,6 +1381,11 @@ AsmType* AsmTyper::ValidateSwitchStatement(SwitchStatement* stmt) {
     }
   }
 
+  if (last_case_pos != kNoSourcePosition && default_pos != kNoSourcePosition &&
+      default_pos < last_case_pos) {
+    FAIL(stmt, "Switch default must appear last.");
+  }
+
   return AsmType::Void();
 }
 

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

@@ -859,6 +859,8 @@ TEST(ErrorsInStatement) {
       {"do {} while (fround(1));", "Do {} While condition must be type int"},
       {"for (;fround(1););", "For condition must be type int"},
       {"switch(flocal){ case 0: return 0; }", "Switch tag must be signed"},
+      {"switch(slocal){ default: case 0: return 0; }",
+       "Switch default must appear last"},
       {"switch(slocal){ case 1: case 1: return 0; }", "Duplicated case label"},
       {"switch(slocal){ case 1: case 0: break; case 1: return 0; }",
        "Duplicated case label"},