[Strings] Erase the strings section after StringLifting (#7546)

After we lift the strings section into stringref, we don't need the section any
more, and leaving it around could cause problems with repeated lifting/
lowering operations (which would be very much possible with #7540). In
particular, without erasing it, we'd accumulate such sections over time.

Author: kripken

src/passes/StringLifting.cpp

@@ -79,41 +79,45 @@ struct StringLifting : public Pass {
     }
 
     // Imported strings may also be found in the string section.
-    for (auto& section : module->customSections) {
-      if (section.name == "string.consts") {
-        // We found the string consts section. Parse it.
-        auto copy = section.data;
-        json::Value array;
-        array.parse(copy.data(), json::Value::WTF16);
-        if (!array.isArray()) {
+    auto stringSectionIter = std::find_if(
+      module->customSections.begin(),
+      module->customSections.end(),
+      [&](CustomSection& section) { return section.name == "string.consts"; });
+    if (stringSectionIter != module->customSections.end()) {
+      // We found the string consts section. Parse it.
+      auto& section = *stringSectionIter;
+      auto copy = section.data;
+      json::Value array;
+      array.parse(copy.data(), json::Value::WTF16);
+      if (!array.isArray()) {
+        Fatal() << "StringLifting: string.const section should be a JSON array";
+      }
+
+      // We have the array of constants from the section. Find globals that
+      // refer to it.
+      for (auto& global : module->globals) {
+        if (!global->imported() || global->module != "string.const") {
+          continue;
+        }
+        // The index in the array is the basename.
+        Index index = std::stoi(std::string(global->base.str));
+        if (index >= array.size()) {
+          Fatal() << "StringLifting: bad index in string.const section";
+        }
+        auto item = array[index];
+        if (!item->isString()) {
           Fatal()
-            << "StringLifting: string.const section should be a JSON array";
+            << "StringLifting: string.const section entry is not a string";
         }
-
-        // We have the array of constants from the section. Find globals that
-        // refer to it.
-        for (auto& global : module->globals) {
-          if (!global->imported() || global->module != "string.const") {
-            continue;
-          }
-          // The index in the array is the basename.
-          Index index = std::stoi(std::string(global->base.str));
-          if (index >= array.size()) {
-            Fatal() << "StringLifting: bad index in string.const section";
-          }
-          auto item = array[index];
-          if (!item->isString()) {
-            Fatal()
-              << "StringLifting: string.const section entry is not a string";
-          }
-          if (importedStrings.count(global->name)) {
-            Fatal()
-              << "StringLifting: string.const section tramples other const";
-          }
-          importedStrings[global->name] = item->getIString();
+        if (importedStrings.count(global->name)) {
+          Fatal() << "StringLifting: string.const section tramples other const";
         }
-        break;
+        importedStrings[global->name] = item->getIString();
       }
+
+      // Remove the custom section: After lifting it has no purpose (and could
+      // cause problems with repeated lifting/lowering).
+      module->customSections.erase(stringSectionIter);
     }
 
     auto array16 = Type(Array(Field(Field::i16, Mutable)), Nullable);

test/lit/passes/string-lifting-section-erase.wast

@@ -0,0 +1,23 @@
+;; Test that the section vanishes after lifting.
+
+(module
+  (func $strings
+    ;; These strings cannot appear as magic imports, and will definitely go in
+    ;; the strings section.
+    (drop
+      (string.const "unpaired high surrogate \ED\A0\80 ")
+    )
+    (drop
+      (string.const "unpaired low surrogate \ED\BD\88 ")
+    )
+  )
+)
+
+;; Lower into the section. We should see the section.
+;; RUN: wasm-opt %s -all --string-lowering                  -S -o - | filecheck %s --check-prefix=LOWER
+;; LOWER: custom section
+
+;; Also lift. Now no section should appear.
+;; RUN: wasm-opt %s -all --string-lowering --string-lifting -S -o - | filecheck %s --check-prefix=AND_LIFT
+;; AND_LIFT-NOT: custom section
+