[lld] [LLD][COFF] Merge .bss at end of .data section. (PR #137677)
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llvm-commits at lists.llvm.org
Mon Apr 28 11:15:42 PDT 2025
https://github.com/jeremyd2019 updated https://github.com/llvm/llvm-project/pull/137677
>From 6cefe3882eb8d04fd51d31e6ea8529712b4bb41a Mon Sep 17 00:00:00 2001
From: Jeremy Drake <github at jdrake.com>
Date: Mon, 28 Apr 2025 10:46:47 -0700
Subject: [PATCH] [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.
---
lld/COFF/Writer.cpp | 52 ++++++++++-------
lld/test/COFF/merge-data-bss.test | 92 +++++++++++++++++++++++++++++++
2 files changed, 123 insertions(+), 21 deletions(-)
create mode 100644 lld/test/COFF/merge-data-bss.test
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
+...
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