[LLD][COFF] Ensure .bss is merged at the end of a section. (#137677)
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.
diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index a5582cc..cf0a92b 100644
--- a/lld/COFF/Writer.cpp
+++ b/lld/COFF/Writer.cpp
@@ -215,6 +215,7 @@
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 @@
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 +1619,16 @@
}
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);
}
+
+ // Because .bss contains all zeros, it should be merged at the end of
+ // whatever section it is being merged into (usually .data) so that the image
+ // need not actually contain all of the zeros.
+ 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 0000000..a821f8d
--- /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
+...
