Fix crash in ctor-eval when a tag is imported

src/ir/import-name.h

@@ -19,15 +19,32 @@
 
 #include <ostream>
 
+#include "support/hash.h"
 #include "support/name.h"
 
 namespace wasm {
 
 struct ImportNames {
   Name module;
   Name name;
+
+  bool operator==(const ImportNames& other) const {
+    return module == other.module && name == other.name;
+  }
 };
 
 } // namespace wasm
 
+namespace std {
+
+template<> struct hash<wasm::ImportNames> {
+  std::size_t operator()(const wasm::ImportNames& importNames) const noexcept {
+    size_t val = hash<wasm::Name>{}(importNames.module);
+    wasm::rehash(val, importNames.name);
+    return val;
+  }
+};
+
+} // namespace std
+
 #endif // wasm_ir_import_name_h

src/tools/wasm-ctor-eval.cpp

@@ -84,14 +84,22 @@ class EvallingImportResolver : public ImportResolver {
     throw FailToEvalException{"Imported table access."};
   }
 
+  // We assume that each tag import is distinct. This is wrong if the same tag
+  // instantiation is imported twice with different import names.
   Tag* getTagOrNull(ImportNames name,
                     const Signature& signature) const override {
-    Fatal() << "getTagOrNull not implemented in ctor-eval.";
-    return nullptr;
+    auto [it, inserted] = importedTags.try_emplace(name, Tag{});
+    if (inserted) {
+      auto& tag = it->second;
+      tag.type = HeapType(signature);
+    }
+
+    return &it->second;
   }
 
 private:
   mutable Literals stubLiteral;
+  mutable std::unordered_map<ImportNames, Tag> importedTags;
 };
 
 class EvallingRuntimeTable : public RuntimeTable {

test/ctor-eval/tag-import.wast

@@ -0,0 +1,16 @@
+(module
+  ;; an imported tag that isn't accessed doesn't stop us from optimizing
+  (import "import" "tag" (tag $imported))
+  (global $g (mut i32) (i32.const 0))
+  (func $setg (export "setg")
+    (drop (i32.const 1))
+    (global.set $g
+      (i32.add (i32.const 1) (i32.const 2))
+    )
+  )
+
+  (func $keepalive (export "keepalive") (result i32)
+    ;; Keep the global alive so we can see its value.
+    (global.get $g)
+  )
+)

test/ctor-eval/tag-import.wast.ctors

@@ -0,0 +1 @@
+setg

test/ctor-eval/tag-import.wast.out

@@ -0,0 +1,8 @@
+(module
+ (type $0 (func (result i32)))
+ (global $g (mut i32) (i32.const 3))
+ (export "keepalive" (func $keepalive))
+ (func $keepalive (type $0) (result i32)
+  (global.get $g)
+ )
+)