Update names field in source maps

src/wasm/wasm-binary.cpp

@@ -16,16 +16,13 @@
 
 #include <algorithm>
 #include <fstream>
-#include <iomanip>
 
-#include "ir/eh-utils.h"
 #include "ir/module-utils.h"
 #include "ir/names.h"
 #include "ir/table-utils.h"
 #include "ir/type-updating.h"
 #include "pass.h"
 #include "support/bits.h"
-#include "support/debug.h"
 #include "support/stdckdint.h"
 #include "support/string.h"
 #include "wasm-annotations.h"
@@ -1242,6 +1239,44 @@ void WasmBinaryWriter::writeSourceMapProlog() {
     }
   }
 
+  // Remove unused function names from 'names' field.
+  if (!wasm->debugInfoSymbolNames.empty()) {
+    std::vector<std::string> newSymbolNames;
+    std::unordered_map<Index, Index> oldToNewIndex;
+
+    // Collect all used symbol name indexes.
+    for (auto& func : wasm->functions) {
+      for (auto& pair : func->debugLocations) {
+        if (pair.second && pair.second->symbolNameIndex) {
+          uint32_t oldIndex = *pair.second->symbolNameIndex;
+          assert(oldIndex < wasm->debugInfoSymbolNames.size());
+          oldToNewIndex[oldIndex] = 0; // placeholder
+        }
+      }
+    }
+
+    // Create the new list of names and the mapping from old to new indices.
+    uint32_t newIndex = 0;
+    for (auto& pair : oldToNewIndex) {
+      // pair.first is the oldIndex
+      newSymbolNames.push_back(wasm->debugInfoSymbolNames[pair.first]);
+      pair.second = newIndex++; // update placeholder to new index
+    }
+
+    // Update all debug locations to point to the new indices.
+    for (auto& func : wasm->functions) {
+      for (auto& pair : func->debugLocations) {
+        if (pair.second && pair.second->symbolNameIndex) {
+          uint32_t oldIndex = *pair.second->symbolNameIndex;
+          pair.second->symbolNameIndex = oldToNewIndex[oldIndex];
+        }
+      }
+    }
+
+    // Replace the old symbol names with the new, pruned list.
+    wasm->debugInfoSymbolNames = std::move(newSymbolNames);
+  }
+
   auto writeOptionalString = [&](const char* name, const std::string& str) {
     if (!str.empty()) {
       *sourceMap << "\"" << name << "\":\"" << str << "\",";
@@ -1269,10 +1304,6 @@ void WasmBinaryWriter::writeSourceMapProlog() {
     writeStringVector("sourcesContent", wasm->debugInfoSourcesContent);
   }
 
-  // TODO: This field is optional; maybe we should omit if it's empty.
-  // TODO: Binaryen actually does not correctly preserve symbol names when it
-  // rewrites the mappings. We should maybe just drop them, or else handle
-  // them correctly.
   writeStringVector("names", wasm->debugInfoSymbolNames);
 
   *sourceMap << "\"mappings\":\"";

test/lit/source-map-names.wast

@@ -0,0 +1,24 @@
+;; RUN: wasm-opt %s --remove-unused-module-elements -o %t.wasm -osm %t.map
+;; RUN: wasm-dis %t.wasm --source-map %t.map | filecheck %s --check-prefix OUT-WAST
+;; RUN: cat %t.map | filecheck %s --check-prefix OUT-MAP
+
+;; After --remove-unused-module-elements, the output source map's 'names' field
+;; should NOT contain 'unused'
+
+;; OUT-MAP: "names":["used"]
+
+(module
+  (export "used" (func $used))
+
+  (func $unused
+    ;;@ src.cpp:1:1:unused
+    (nop)
+  )
+
+  (func $used
+    ;; OUT-WAST:      ;;@ src.cpp:2:1:used
+    ;; OUT-WAST-NEXT: (nop)
+    ;;@ src.cpp:2:1:used
+    (nop)
+  )
+)