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[LLD][COFF] Add support for custom section layout (#152779)
MS link.exe provides the `/sectionlayout:@` option to specify the order of output sections at the granularity of individual sections. LLD/COFF currently does not have capability for user-controlled ordering of one or more output sections (as LLD/COFF does not support linker scripts), and this PR adds the option to align with MS link.exe. The option accepts only a file that specifies the order of sections, one per line. For example, `mylayout.txt` could emit the `.text` section after all other sections while preserving the original relative order of the remaining sections. ``` .data .rdata .pdata .rsrc .reloc .text ``` ```bash echo 'int main() { return 0; }' > main.c cl main.c /link /entry:main /sectionlayout:@mylayout.txt llvm-readobj --sections main.exe ```
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‎lld/COFF/Config.h‎

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@@ -212,6 +212,8 @@ struct Configuration {
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// Used for /section=.name,{DEKPRSW} to set section attributes.
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std::map<StringRef, uint32_t> section;
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// Used for /sectionlayout: to layout sections in specified order.
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std::map<std::string, int> sectionOrder;
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// Options for manifest files.
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ManifestKind manifest = Default;

‎lld/COFF/Driver.cpp‎

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@@ -2049,6 +2049,9 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
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// Handle /section
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for (auto *arg : args.filtered(OPT_section))
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parseSection(arg->getValue());
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// Handle /sectionlayout
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if (auto *arg = args.getLastArg(OPT_sectionlayout))
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parseSectionLayout(arg->getValue());
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// Handle /align
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if (auto *arg = args.getLastArg(OPT_align)) {

‎lld/COFF/Driver.h‎

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@@ -213,6 +213,7 @@ class LinkerDriver {
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void parseMerge(StringRef);
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void parsePDBPageSize(StringRef);
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void parseSection(StringRef);
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void parseSectionLayout(StringRef);
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void parseSameAddress(StringRef);
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‎lld/COFF/DriverUtils.cpp‎

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@@ -214,6 +214,43 @@ void LinkerDriver::parseSection(StringRef s) {
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ctx.config.section[name] = parseSectionAttributes(ctx, attrs);
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}
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// Parses /sectionlayout: option argument.
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void LinkerDriver::parseSectionLayout(StringRef path) {
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if (path.starts_with("@"))
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path = path.substr(1);
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std::unique_ptr<MemoryBuffer> layoutFile =
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CHECK(MemoryBuffer::getFile(path), "could not open " + path);
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StringRef content = layoutFile->getBuffer();
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int index = 0;
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while (!content.empty()) {
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size_t pos = content.find_first_of("\r\n");
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StringRef line;
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if (pos == StringRef::npos) {
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line = content;
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content = StringRef();
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} else {
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line = content.substr(0, pos);
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content = content.substr(pos).ltrim("\r\n");
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}
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line = line.trim();
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if (line.empty())
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continue;
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StringRef sectionName = line.split(' ').first;
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if (ctx.config.sectionOrder.count(sectionName.str())) {
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Warn(ctx) << "duplicate section '" << sectionName.str()
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<< "' in section layout file, ignoring";
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continue;
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}
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ctx.config.sectionOrder[sectionName.str()] = index++;
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}
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}
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void LinkerDriver::parseDosStub(StringRef path) {
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std::unique_ptr<MemoryBuffer> stub =
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CHECK(MemoryBuffer::getFile(path), "could not open " + path);

‎lld/COFF/Options.td‎

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@@ -102,6 +102,7 @@ def pdbstream : Joined<["/", "-", "/?", "-?"], "pdbstream:">,
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MetaVarName<"<name>=<file>">,
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HelpText<"Embed the contents of <file> in the PDB as named stream <name>">;
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def section : P<"section", "Specify section attributes">;
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def sectionlayout : P<"sectionlayout", "Specifies the layout strategy for output sections">;
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def stack : P<"stack", "Size of the stack">;
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def stub : P<"stub", "Specify DOS stub file">;
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def subsystem : P<"subsystem", "Specify subsystem">;

‎lld/COFF/Writer.cpp‎

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@@ -219,6 +219,7 @@ class Writer {
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void sortECChunks();
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void appendECImportTables();
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void removeUnusedSections();
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void layoutSections();
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void assignAddresses();
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bool isInRange(uint16_t relType, uint64_t s, uint64_t p, int margin,
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MachineTypes machine);
@@ -783,6 +784,7 @@ void Writer::run() {
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appendECImportTables();
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createDynamicRelocs();
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removeUnusedSections();
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layoutSections();
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finalizeAddresses();
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removeEmptySections();
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assignOutputSectionIndices();
@@ -1413,6 +1415,33 @@ void Writer::removeUnusedSections() {
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llvm::erase_if(ctx.outputSections, isUnused);
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}
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void Writer::layoutSections() {
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llvm::TimeTraceScope timeScope("Layout sections");
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if (ctx.config.sectionOrder.empty())
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return;
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llvm::stable_sort(ctx.outputSections,
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[this](const OutputSection *a, const OutputSection *b) {
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auto itA = ctx.config.sectionOrder.find(a->name.str());
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auto itB = ctx.config.sectionOrder.find(b->name.str());
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bool aInOrder = itA != ctx.config.sectionOrder.end();
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bool bInOrder = itB != ctx.config.sectionOrder.end();
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// Put unspecified sections after all specified sections
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if (aInOrder && bInOrder) {
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return itA->second < itB->second;
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} else if (aInOrder && !bInOrder) {
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return true; // ordered sections come before unordered
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} else {
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// (!aInOrder && bInOrder): unordered comes after
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// ordered
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// (!aInOrder && !bInOrder): both unspecified, preserve
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// the original order
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return false;
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}
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});
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}
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// The Windows loader doesn't seem to like empty sections,
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// so we remove them if any.
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void Writer::removeEmptySections() {

‎lld/test/COFF/Inputs/sectionlayout.yaml‎

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@@ -0,0 +1,25 @@
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--- !COFF
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header:
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Machine: IMAGE_FILE_MACHINE_AMD64
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Characteristics: []
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sections:
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- Name: '.text1'
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Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
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Alignment: 16
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SectionData: B82A000000C3
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- Name: '.text2'
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Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
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Alignment: 16
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SectionData: CC
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- Name: '.text3'
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Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
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Alignment: 16
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SectionData: CC
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symbols:
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- Name: main
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Value: 0
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SectionNumber: 1
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SimpleType: IMAGE_SYM_TYPE_NULL
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ComplexType: IMAGE_SYM_DTYPE_FUNCTION
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StorageClass: IMAGE_SYM_CLASS_EXTERNAL
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...

‎lld/test/COFF/sectionlayout.test‎

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RUN: yaml2obj %p/Inputs/sectionlayout.yaml -o %t.obj
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## Error on non-exist input layout file
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RUN: not lld-link /entry:main /sectionlayout:doesnotexist.txt %t.obj
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## Order in 1 -> 3 -> 2
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RUN: echo ".text1" > %t.layout.txt
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RUN: echo ".text3" >> %t.layout.txt
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RUN: echo ".text2" >> %t.layout.txt
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK1 %s
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## While /sectionlayout:abc is valid, /sectionlayout:@abc is also accepted (to align with MS link.exe)
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:@%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK1 %s
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## Ensure tokens after section name is ignored (for now, to align with MS link.exe)
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RUN: echo ".text1 ALIGN=1" > %t.layout.txt
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RUN: echo ".text3" >> %t.layout.txt
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RUN: echo ".text2" >> %t.layout.txt
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:@%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK1 %s
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CHECK1: Sections [
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CHECK1: Section {
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CHECK1: Number: 1
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CHECK1: Name: .text1
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CHECK1: }
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CHECK1: Section {
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CHECK1: Number: 2
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CHECK1: Name: .text3
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CHECK1: }
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CHECK1: Section {
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CHECK1: Number: 3
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CHECK1: Name: .text2
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CHECK1: }
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CHECK1: ]
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## Order in 3 -> 2 -> 1
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RUN: echo ".text3" > %t.layout.txt
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RUN: echo ".text2" >> %t.layout.txt
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RUN: echo ".text1" >> %t.layout.txt
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK2 %s
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CHECK2: Sections [
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CHECK2: Section {
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CHECK2: Number: 1
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CHECK2: Name: .text3
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CHECK2: }
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CHECK2: Section {
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CHECK2: Number: 2
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CHECK2: Name: .text2
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CHECK2: }
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CHECK2: Section {
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CHECK2: Number: 3
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CHECK2: Name: .text1
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CHECK2: }
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CHECK2: ]
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## Put non-exisist section in layout file has no effect; original order is respected
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RUN: echo "notexist" > %t.layout.txt
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK3 %s
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## Empty layout file has no effect
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RUN: echo "" > %t.layout.txt
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK3 %s
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## Empty layout file has no effect
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RUN: echo " " > %t.layout.txt
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RUN: echo " " >> %t.layout.txt
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK3 %s
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CHECK3: Sections [
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CHECK3: Section {
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CHECK3: Number: 1
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CHECK3: Name: .text1
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CHECK3: }
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CHECK3: Section {
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CHECK3: Number: 2
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CHECK3: Name: .text2
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CHECK3: }
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CHECK3: Section {
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CHECK3: Number: 3
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CHECK3: Name: .text3
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CHECK3: }
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CHECK3: ]
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## Order in 3 -> 1, but 2 remains unspecified
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RUN: echo ".text3" > %t.layout.txt
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RUN: echo ".text1" >> %t.layout.txt
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK4 %s
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CHECK4: Sections [
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CHECK4: Section {
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CHECK4: Number: 1
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CHECK4: Name: .text3
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CHECK4: }
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CHECK4: Section {
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CHECK4: Number: 2
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CHECK4: Name: .text1
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CHECK4: }
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CHECK4: Section {
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CHECK4: Number: 3
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CHECK4: Name: .text2
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CHECK4: }
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CHECK4: ]
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## Order in 3 -> 2, but 1 remains unspecified.
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## 1 should be the first, as the original order (1 -> 2 -> 3) is respected
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RUN: echo ".text3" > %t.layout.txt
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RUN: echo ".text2" >> %t.layout.txt
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK2 %s
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## Order in 3 -> 2 -> 1, multiple specification has no effect (the first one is used)
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RUN: echo ".text3" > %t.layout.txt
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RUN: echo ".text3" >> %t.layout.txt
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RUN: echo ".text3" >> %t.layout.txt
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RUN: echo ".text2" >> %t.layout.txt
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RUN: echo ".text2" >> %t.layout.txt
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RUN: echo ".text1" >> %t.layout.txt
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RUN: echo ".text3" >> %t.layout.txt
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RUN: lld-link /out:%t.exe /entry:main /sectionlayout:%t.layout.txt %t.obj
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RUN: llvm-readobj --sections %t.exe | FileCheck -check-prefix=CHECK2 %s

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