[lld] [LLD][COFF] Ensure .bss is merged at the end of a section. (PR #137677)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld

Author: None (jeremyd2019)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/137677.diff


2 Files Affected:

- (modified) lld/COFF/Writer.cpp (+31-21) 
- (added) lld/test/COFF/merge-data-bss.test (+92) 


``````````diff
diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index a5582cc8074d1..00e63cb90c2d0 100644
--- a/lld/COFF/Writer.cpp
+++ b/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();
@@ -1566,6 +1567,30 @@ 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()) {
@@ -1594,28 +1619,13 @@ 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
diff --git a/lld/test/COFF/merge-data-bss.test b/lld/test/COFF/merge-data-bss.test
new file mode 100644
index 0000000000000..a821f8d6e9048
--- /dev/null
+++ b/lld/test/COFF/merge-data-bss.test
@@ -0,0 +1,92 @@
+# RUN: yaml2obj %s -o %t.obj
+# RUN: lld-link /out:%t.exe /entry:main /subsystem:console /force \
+# RUN:   /merge:.other=.data %t.obj /debug
+# RUN: llvm-readobj --sections %t.exe | FileCheck %s
+
+# CHECK:      Name: .data
+# CHECK-NEXT: VirtualSize: 0x2018
+# CHECK-NEXT: VirtualAddress: 0x3000
+# CHECK-NEXT: RawDataSize: 512
+# CHECK-NOT: Name: .other
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     '90'
+    SizeOfRawData:   1
+  - Name:            .data
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     '010000000000000002'
+    SizeOfRawData:   9
+  - Name:            .bss
+    Characteristics: [ IMAGE_SCN_CNT_UNINITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     ''
+    SizeOfRawData:   8192
+  - Name:            .other
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     '030000000000000004'
+    SizeOfRawData:   9
+symbols:
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          1
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        4027552580
+      Number:          1
+  - Name:            .data
+    Value:           0
+    SectionNumber:   2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          9
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        4185224559
+      Number:          2
+  - Name:            .bss
+    Value:           0
+    SectionNumber:   3
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          8192
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          3
+  - Name:            .other
+    Value:           0
+    SectionNumber:   4
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          9
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        1054931164
+      Number:          4
+  - Name:            main
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...

``````````

</details>


https://github.com/llvm/llvm-project/pull/137677