6 files changed, 32 insertions, 6 deletions
diff --git a/lld/wasm/OutputSections.cpp b/lld/wasm/OutputSections.cpp
index 469a51a0f06..0403f1d76f5 100644
--- a/lld/wasm/OutputSections.cpp
+++ b/lld/wasm/OutputSections.cpp
@@ -128,8 +128,11 @@ void CodeSection::writeRelocations(raw_ostream &os) const {
 
 void DataSection::finalizeContents() {
   raw_string_ostream os(dataSectionHeader);
+  unsigned segmentCount =
+      std::count_if(segments.begin(), segments.end(),
+                    [](OutputSegment *segment) { return !segment->isBss; });
 
-  writeUleb128(os, segments.size(), "data segment count");
+  writeUleb128(os, segmentCount, "data segment count");
   os.flush();
   bodySize = dataSectionHeader.size();
 
@@ -137,6 +140,8 @@ void DataSection::finalizeContents() {
          "Currenly only a single data segment is supported in PIC mode");
 
   for (OutputSegment *segment : segments) {
+    if (segment->isBss)
+      continue;
     raw_string_ostream os(segment->header);
     writeUleb128(os, segment->initFlags, "init flags");
     if (segment->initFlags & WASM_SEGMENT_HAS_MEMINDEX)
@@ -181,6 +186,8 @@ void DataSection::writeTo(uint8_t *buf) {
   memcpy(buf, dataSectionHeader.data(), dataSectionHeader.size());
 
   for (const OutputSegment *segment : segments) {
+    if (segment->isBss)
+      continue;
     // Write data segment header
     uint8_t *segStart = buf + segment->sectionOffset;
     memcpy(segStart, segment->header.data(), segment->header.size());
@@ -205,6 +212,13 @@ void DataSection::writeRelocations(raw_ostream &os) const {
       c->writeRelocations(os);
 }
 
+bool DataSection::isNeeded() const {
+  for (const OutputSegment *seg : segments)
+    if (!seg->isBss)
+      return true;
+  return false;
+}
+
 void CustomSection::finalizeContents() {
   raw_string_ostream os(nameData);
   encodeULEB128(name.size(), os);
diff --git a/lld/wasm/OutputSections.h b/lld/wasm/OutputSections.h
index 6b1c2a2eba2..1fcb5723df9 100644
--- a/lld/wasm/OutputSections.h
+++ b/lld/wasm/OutputSections.h
@@ -82,7 +82,7 @@ public:
   void writeTo(uint8_t *buf) override;
   uint32_t getNumRelocations() const override;
   void writeRelocations(raw_ostream &os) const override;
-  bool isNeeded() const override { return segments.size() > 0; }
+  bool isNeeded() const override;
   void finalizeContents() override;
 
 protected:
diff --git a/lld/wasm/OutputSegment.h b/lld/wasm/OutputSegment.h
index 663a9b82282..06f675c379d 100644
--- a/lld/wasm/OutputSegment.h
+++ b/lld/wasm/OutputSegment.h
@@ -32,6 +32,7 @@ public:
   }
 
   StringRef name;
+  bool isBss = false;
   uint32_t index = 0;
   uint32_t initFlags = 0;
   uint32_t sectionOffset = 0;
diff --git a/lld/wasm/SyntheticSections.cpp b/lld/wasm/SyntheticSections.cpp
index 30c1096e16c..ba04543211c 100644
--- a/lld/wasm/SyntheticSections.cpp
+++ b/lld/wasm/SyntheticSections.cpp
@@ -365,6 +365,12 @@ void ElemSection::writeBody() {
   }
 }
 
+DataCountSection::DataCountSection(ArrayRef<OutputSegment *> segments)
+    : SyntheticSection(llvm::wasm::WASM_SEC_DATACOUNT),
+      numSegments(std::count_if(
+          segments.begin(), segments.end(),
+          [](OutputSegment *const segment) { return !segment->isBss; })) {}
+
 void DataCountSection::writeBody() {
   writeUleb128(bodyOutputStream, numSegments, "data count");
 }
diff --git a/lld/wasm/SyntheticSections.h b/lld/wasm/SyntheticSections.h
index 1557ce79d2d..23f9711dec1 100644
--- a/lld/wasm/SyntheticSections.h
+++ b/lld/wasm/SyntheticSections.h
@@ -250,9 +250,7 @@ protected:
 
 class DataCountSection : public SyntheticSection {
 public:
-  DataCountSection(uint32_t numSegments)
-      : SyntheticSection(llvm::wasm::WASM_SEC_DATACOUNT),
-        numSegments(numSegments) {}
+  DataCountSection(ArrayRef<OutputSegment *> segments);
   bool isNeeded() const override;
   void writeBody() override;
 
diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
index 479b44b53fb..fe596257bea 100644
--- a/lld/wasm/Writer.cpp
+++ b/lld/wasm/Writer.cpp
@@ -669,6 +669,13 @@ void Writer::createOutputSegments() {
         s = make<OutputSegment>(name);
         if (config->sharedMemory || name == ".tdata")
           s->initFlags = WASM_SEGMENT_IS_PASSIVE;
+        // Exported memories are guaranteed to be zero-initialized, so no need
+        // to emit data segments for bss sections.
+        // TODO: consider initializing bss sections with memory.fill
+        // instructions when memory is imported and bulk-memory is available.
+        if (!config->importMemory && !config->relocatable &&
+            name.startswith(".bss"))
+          s->isBss = true;
         segments.push_back(s);
       }
       s->addInputSegment(segment);
@@ -961,7 +968,7 @@ void Writer::createSyntheticSections() {
   out.exportSec = make<ExportSection>();
   out.startSec = make<StartSection>(segments.size());
   out.elemSec = make<ElemSection>();
-  out.dataCountSec = make<DataCountSection>(segments.size());
+  out.dataCountSec = make<DataCountSection>(segments);
   out.linkingSec = make<LinkingSection>(initFunctions, segments);
   out.nameSec = make<NameSection>();
   out.producersSec = make<ProducersSection>();