[LLD][COFF][NFC] Always align null chunks (#116677)

Currently, null chunks always follow other aligned chunks, so this patch
is NFC. However, it will become observable once support for ARM64X
imports is added. The import tables are shared between the native and EC
views. They are usually very similar, but in cases where they differ,
ARM64X relocations handle the discrepancies. If a DLL is only imported
by EC code, the native view will see it as importing zero functions from
this DLL (with ARM64X relocations replacing those null chunks with
actual imports). In this scenario, the null chunks may appear as the
very first chunks, meaning there is nothing else forcing their
alignment.

Author: cjacek

lld/COFF/DLL.cpp

@@ -131,7 +131,14 @@ class ImportDirectoryChunk : public NonSectionChunk {
 // Contents of this chunk is always null bytes.
 class NullChunk : public NonSectionChunk {
 public:
-  explicit NullChunk(size_t n) : size(n) { hasData = false; }
+  explicit NullChunk(size_t n, uint32_t align) : size(n) {
+    hasData = false;
+    setAlignment(align);
+  }
+  explicit NullChunk(COFFLinkerContext &ctx)
+      : NullChunk(ctx.config.wordsize, ctx.config.wordsize) {}
+  explicit NullChunk(COFFLinkerContext &ctx, size_t n)
+      : NullChunk(n, ctx.config.wordsize) {}
   size_t getSize() const override { return size; }
 
   void writeTo(uint8_t *buf) const override {
@@ -737,11 +744,11 @@ void IdataContents::create(COFFLinkerContext &ctx) {
       }
     }
     // Terminate with null values.
-    lookups.push_back(make<NullChunk>(ctx.config.wordsize));
-    addresses.push_back(make<NullChunk>(ctx.config.wordsize));
+    lookups.push_back(make<NullChunk>(ctx));
+    addresses.push_back(make<NullChunk>(ctx));
     if (ctx.config.machine == ARM64EC) {
-      auxIat.push_back(make<NullChunk>(ctx.config.wordsize));
-      auxIatCopy.push_back(make<NullChunk>(ctx.config.wordsize));
+      auxIat.push_back(make<NullChunk>(ctx));
+      auxIatCopy.push_back(make<NullChunk>(ctx));
     }
 
     for (int i = 0, e = syms.size(); i < e; ++i)
@@ -755,7 +762,7 @@ void IdataContents::create(COFFLinkerContext &ctx) {
     dirs.push_back(dir);
   }
   // Add null terminator.
-  dirs.push_back(make<NullChunk>(sizeof(ImportDirectoryTableEntry)));
+  dirs.push_back(make<NullChunk>(sizeof(ImportDirectoryTableEntry), 4));
 }
 
 std::vector<Chunk *> DelayLoadContents::getChunks() {
@@ -830,17 +837,16 @@ void DelayLoadContents::create(Defined *h) {
         saver().save("__tailMerge_" + syms[0]->getDLLName().lower());
     ctx.symtab.addSynthetic(tmName, tm);
     // Terminate with null values.
-    addresses.push_back(make<NullChunk>(8));
-    names.push_back(make<NullChunk>(8));
+    addresses.push_back(make<NullChunk>(ctx, 8));
+    names.push_back(make<NullChunk>(ctx, 8));
     if (ctx.config.machine == ARM64EC) {
-      auxIat.push_back(make<NullChunk>(8));
-      auxIatCopy.push_back(make<NullChunk>(8));
+      auxIat.push_back(make<NullChunk>(ctx, 8));
+      auxIatCopy.push_back(make<NullChunk>(ctx, 8));
     }
 
     for (int i = 0, e = syms.size(); i < e; ++i)
       syms[i]->setLocation(addresses[base + i]);
-    auto *mh = make<NullChunk>(8);
-    mh->setAlignment(8);
+    auto *mh = make<NullChunk>(8, 8);
     moduleHandles.push_back(mh);
 
     // Fill the delay import table header fields.
@@ -853,7 +859,8 @@ void DelayLoadContents::create(Defined *h) {
   if (unwind)
     unwindinfo.push_back(unwind);
   // Add null terminator.
-  dirs.push_back(make<NullChunk>(sizeof(delay_import_directory_table_entry)));
+  dirs.push_back(
+      make<NullChunk>(sizeof(delay_import_directory_table_entry), 4));
 }
 
 Chunk *DelayLoadContents::newTailMergeChunk(Chunk *dir) {