[LLD][COFF] Ensure .bss is merged at the end of a section.

Because it is full of zeros, it is expected that as much of it as
possible is elided from the actual image, and that cannot happen if
there is initialized data in the section after it.

Test this by having large .bss sections in the cygwin-symbols test.

Author: jeremyd2019

lld/COFF/Writer.cpp

@@ -215,6 +215,7 @@ class Writer {
   void appendImportThunks();
   void locateImportTables();
   void createExportTable();
+  void mergeSection(const std::map<StringRef, StringRef>::value_type &p);
   void mergeSections();
   void sortECChunks();
   void appendECImportTables();
@@ -1571,6 +1572,29 @@ void Writer::createSymbolAndStringTable() {
   fileSize = alignTo(fileOff, ctx.config.fileAlign);
 }
 
+void Writer::mergeSection(const std::map<StringRef, StringRef>::value_type &p) {
+  StringRef toName = p.second;
+  if (p.first == toName)
+    return;
+  StringSet<> names;
+  while (true) {
+    if (!names.insert(toName).second)
+      Fatal(ctx) << "/merge: cycle found for section '" << p.first << "'";
+    auto i = ctx.config.merge.find(toName);
+    if (i == ctx.config.merge.end())
+      break;
+    toName = i->second;
+  }
+  OutputSection *from = findSection(p.first);
+  OutputSection *to = findSection(toName);
+  if (!from)
+    return;
+  if (!to) {
+    from->name = toName;
+    return;
+  }
+  to->merge(from);
+}
 void Writer::mergeSections() {
   llvm::TimeTraceScope timeScope("Merge sections");
   if (!pdataSec->chunks.empty()) {
@@ -1599,28 +1623,13 @@ void Writer::mergeSections() {
   }
 
   for (auto &p : ctx.config.merge) {
-    StringRef toName = p.second;
-    if (p.first == toName)
-      continue;
-    StringSet<> names;
-    while (true) {
-      if (!names.insert(toName).second)
-        Fatal(ctx) << "/merge: cycle found for section '" << p.first << "'";
-      auto i = ctx.config.merge.find(toName);
-      if (i == ctx.config.merge.end())
-        break;
-      toName = i->second;
-    }
-    OutputSection *from = findSection(p.first);
-    OutputSection *to = findSection(toName);
-    if (!from)
-      continue;
-    if (!to) {
-      from->name = toName;
-      continue;
-    }
-    to->merge(from);
+    if (p.first != ".bss")
+      mergeSection(p);
   }
+
+  auto it = ctx.config.merge.find(".bss");
+  if (it != ctx.config.merge.end())
+    mergeSection(*it);
 }
 
 // EC targets may have chunks of various architectures mixed together at this

lld/test/COFF/cygwin-symbols.s

@@ -39,8 +39,7 @@ main:
   nop
 
 .bss
-  .quad 0
-  .byte 0
+  .zero 8192
 
 .section .test, "w"
   .quad __data_start__
@@ -63,8 +62,7 @@ main:
   .byte 4
 
 .bss
-  .quad 0
-  .byte 0
+  .zero 8192
 
 .section .test, "w"
   .quad __data_start__
@@ -79,22 +77,23 @@ main:
 # __data_end__   pointing at 0x140003009.
 # DATANOBSS-NEXT: Contents of section .test:
 # DATANOBSS-NEXT: 140004000 00300040 01000000 09300040 01000000
-# DATANOBSS-NEXT: 140004010 0c300040 01000000 0c300040 01000000
+# DATANOBSS-NEXT: 140004010 18300040 01000000 18300040 01000000
 
 # __bss_start__ pointing at 0x140003000 and
-# __bss_end__   pointing at 0x140003009.
+# __bss_end__   pointing at 0x140005000.
+# BSSNODATA-NOT:  Contents of section .data:
 # BSSNODATA:      Contents of section .test:
-# BSSNODATA-NEXT: 140004000 00300040 01000000 00300040 01000000
-# BSSNODATA-NEXT: 140004010 00300040 01000000 09300040 01000000
+# BSSNODATA-NEXT: 140005000 00300040 01000000 00300040 01000000
+# BSSNODATA-NEXT: 140005010 00300040 01000000 00500040 01000000
 
 # DATAANDBSS:      Contents of section .data:
-# DATAANDBSS-NEXT: 140003000 01000000 00000000 02000000 00000000
-# DATAANDBSS-NEXT: 140003010 00000000 00000000 03000000 00000000
-# DATAANDBSS-NEXT: 140003020 04
+# DATAANDBSS-NEXT: 140003000 01000000 00000000 02000000 03000000
+# DATAANDBSS-NEXT: 140003010 00000000 04000000 00000000 00000000
 # __data_start__ pointing at 0x140003000 and
 # __data_end__   pointing at 0x140003009.
-# __bss_start__ pointing at 0x14000300c and
-# __bss_end__   pointing at 0x140003015.
+# __bss_start__ pointing at 0x140003018 and
+# __bss_end__   pointing at 0x140005018.
+# DATAANDBSS:      1400031f0 00000000 00000000 00000000 00000000
 # DATAANDBSS-NEXT: Contents of section .test:
-# DATAANDBSS-NEXT: 140004000 00300040 01000000 09300040 01000000
-# DATAANDBSS-NEXT: 140004010 0c300040 01000000 15300040 01000000
+# DATAANDBSS-NEXT: 140006000 00300040 01000000 09300040 01000000
+# DATAANDBSS-NEXT: 140006010 18300040 01000000 18500040 01000000