wasm-emscripten-finalize: separateDataSegments() fix (#1897)

We should emit a file with only the data segments, starting from the global base, and not starting from zero (the data before is unneeded, and the emscripten loading code assumes it isn't there).

Also fix the auto updater to work properly on .mem test updating.

Author: kripken

auto_update_tests.py

@@ -80,7 +80,7 @@ def update_lld_tests():
     extension_arg_map = {
       '.out': [],
       '.jscall.out': ['--emscripten-reserved-function-pointers=3'],
-      '.mem.out': ['--separate-data-segments', mem_file],
+      '.mem.out': ['--separate-data-segments', mem_file + '.mem'],
     }
     for ext, ext_args in extension_arg_map.items():
       out_path = wast_path + ext

src/tools/wasm-emscripten-finalize.cpp

@@ -224,7 +224,10 @@ int main(int argc, const char *argv[]) {
   // for metadata).
   if (!dataSegmentFile.empty()) {
     Output memInitFile(dataSegmentFile, Flags::Binary, Flags::Release);
-    generator.separateDataSegments(&memInitFile);
+    if (globalBase == INVALID_BASE) {
+      Fatal() << "globalBase must be set";
+    }
+    generator.separateDataSegments(&memInitFile, globalBase);
   }
 
   if (options.debug) {

src/wasm-emscripten.h

@@ -57,7 +57,10 @@ class EmscriptenGlueGenerator {
 
   void fixInvokeFunctionNames();
 
-  void separateDataSegments(Output* outfile);
+  // Emits the data segments to a file. The file contains data from address base
+  // onwards (we must pass in base, as we can't tell it from the wasm - the first
+  // segment may start after a run of zeros, but we need those zeros in the file).
+  void separateDataSegments(Output* outfile, Address base);
 
 private:
   Module& wasm;

src/wasm/wasm-emscripten.cpp

@@ -919,10 +919,11 @@ std::string EmscriptenGlueGenerator::generateEmscriptenMetadata(
   return meta.str();
 }
 
-void EmscriptenGlueGenerator::separateDataSegments(Output* outfile) {
+void EmscriptenGlueGenerator::separateDataSegments(Output* outfile, Address base) {
   size_t lastEnd = 0;
   for (Memory::Segment& seg : wasm.memory.segments) {
     size_t offset = seg.offset->cast<Const>()->value.geti32();
+    offset -= base;
     size_t fill = offset - lastEnd;
     if (fill > 0) {
       std::vector<char> buf(fill);