Always emit symbol table when dwarf section exists in COFF

[HaohaiWen]

Summary

This also fixes check prefix NO which is pointless in symtab.test

[mstorsjo]

This also fixes check prefix NO which never worked in symtab.test

I disagree with this statement; the check worked (otherwise we would have seen test failures?), but I agree that the test was pointless before the fix in this patch.

lld/test/COFF/symtab.test

151

Shouldn't we leave out the .debug_info section? Otherwise it kinda makes the existing tests, that test /debug:symtab and /debug:dwarf, kinda not actually test the options themselves?

[HaohaiWen]

Fix tests

[HaohaiWen]

In D149235#4298643, @mstorsjo wrote:

This also fixes check prefix NO which never worked in symtab.test

I disagree with this statement; the check worked (otherwise we would have seen test failures?), but I agree that the test was pointless before the fix in this patch.

Thanks for pointing it out.

[mstorsjo]

I think this sounds reasonable, so LGTM.

lld/COFF/Writer.cpp

void Writer::createSymbolAndStringTable() {
   // PE/COFF images are limited to 8 byte section names. Longer names can be
   // supported by writing a non-standard string table, but this string table is
   // not mapped at runtime and the long names will therefore be inaccessible.
   // link.exe always truncates section names to 8 bytes, whereas binutils always
   // preserves long section names via the string table. LLD adopts a hybrid
   // solution where discardable sections have long names preserved and
   // non-discardable sections have their names truncated, to ensure that any
   // section which is mapped at runtime also has its name mapped at runtime.
+  bool HasDwarfSection = false;
   for (OutputSection *sec : ctx.outputSections) {
+    HasDwarfSection |= sec->name.startswith(".debug_");
     if (sec->name.size() <= COFF::NameSize)
       continue;
     if ((sec->header.Characteristics & IMAGE_SCN_MEM_DISCARDABLE) == 0)
       continue;
     if (ctx.config.warnLongSectionNames) {
       warn("section name " + sec->name +
            " is longer than 8 characters and will use a non-standard string "
            "table");
     }
     sec->setStringTableOff(addEntryToStringTable(sec->name));
   }
 
-  if (ctx.config.debugDwarf || ctx.config.debugSymtab) {
+  if (ctx.config.debugDwarf || ctx.config.debugSymtab || HasDwarfSection) {
     for (ObjFile *file : ctx.objFileInstances) {
       for (Symbol *b : file->getSymbols()) {
         auto *d = dyn_cast_or_null<Defined>(b);
         if (!d || d->writtenToSymtab)
           continue;
         d->writtenToSymtab = true;
         if (auto *dc = dyn_cast_or_null<DefinedCOFF>(d)) {
           COFFSymbolRef symRef = dc->getCOFFSymbol();

lld/test/COFF/symtab.test

 # RUN: yaml2obj %s -o %t.obj
 # RUN: lld-link /debug:dwarf /out:%t.exe /entry:main %t.obj %p/Inputs/std64.lib
 # RUN: llvm-readobj --symbols %t.exe | FileCheck %s
 # RUN: lld-link /debug:dwarf /opt:noref /out:%t.exe /entry:main %t.obj %p/Inputs/std64.lib
 # RUN: llvm-readobj --symbols %t.exe | FileCheck %s
 # RUN: lld-link /debug:symtab /opt:noref /out:%t.exe /entry:main %t.obj %p/Inputs/std64.lib
 # RUN: llvm-readobj --symbols %t.exe | FileCheck %s
 
 # RUN: lld-link /debug /out:%t.exe /entry:main %t.obj %p/Inputs/std64.lib
 # RUN: llvm-readobj --symbols %t.exe | FileCheck -check-prefix=NO %s
+# RUN: lld-link /debug /merge:.blank=.debug_info /out:%t.exe /entry:main %t.obj %p/Inputs/std64.lib
+# RUN: llvm-readobj --symbols %t.exe | FileCheck %s
 
 # CHECK:      Symbols [
 # CHECK-NEXT:   Symbol {
 # CHECK-NEXT:     Name: MessageBoxA
 # CHECK-NEXT:     Value: 80
 # CHECK-NEXT:     Section: .text (1)
 # CHECK-NEXT:     BaseType: Null (0x0)
 # CHECK-NEXT:     ComplexType: Function (0x2)

 # CHECK-NEXT:     Section: IMAGE_SYM_ABSOLUTE (-1)
 # CHECK-NEXT:     BaseType: Null (0x0)
 # CHECK-NEXT:     ComplexType: Null (0x0)
 # CHECK-NEXT:     StorageClass: External (0x2)
 # CHECK-NEXT:     AuxSymbolCount: 0
 # CHECK-NEXT:   }
 # CHECK-NEXT: ]
 
-# NO: Symbols [
+# NO:       Symbols [
+# NO-NEXT:  ]
 
 --- !COFF
 header:
   Machine:         IMAGE_FILE_MACHINE_AMD64
   Characteristics: []
 sections:
   - Name:            .text
     Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
Relocations:
         Type:            IMAGE_REL_AMD64_REL32
       - VirtualAddress:  30
         SymbolName:      __ImageBase
         Type:            IMAGE_REL_AMD64_ADDR64
   - Name:            .data
     Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
     Alignment:       4
     SectionData:     48656C6C6F0048656C6C6F20576F726C6400
+  - Name:            .blank
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     0000
 symbols:
   - Name:            "@comp.id"
     Value:           10394907
     SectionNumber:   65535
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_NULL
     StorageClass:    IMAGE_SYM_CLASS_STATIC
   - Name:            .text
     Value:           0