[LLD] [COFF] Write the debug directory and build id to a separate section for MinGW

[mstorsjo]

Summary

For dwarf debug info, an executable normally either contains the debug info, or it is stripped out. To reduce the storage needed (slightly) for the debug info kept separately from the released, stripped binaries, one can choose to only copy the debug data from the original executable (essentially the reverse of the strip operation), producing a file with only debug info.

When copying the debug data from an executable with GNU objcopy, the build id and debug directory need to reside in a separate section, as this will be kept while the rest of the .rdata section is removed.

[rnk]

lgtm

[pcc]

COFF/Writer.cpp

425 ↗

(On Diff #155597)

You could create the section unconditionally; we'll end up discarding empty sections later.

[mstorsjo]

COFF/Writer.cpp

425 ↗

(On Diff #155597)

Thanks, will change that before committing.

lld/trunk/COFF/Writer.cpp

private:
 
   uint64_t FileSize;
   uint32_t PointerToSymbolTable = 0;
   uint64_t SizeOfImage;
   uint64_t SizeOfHeaders;
 
   OutputSection *TextSec;
   OutputSection *RdataSec;
+  OutputSection *BuildidSec;
   OutputSection *DataSec;
   OutputSection *PdataSec;
   OutputSection *IdataSec;
   OutputSection *EdataSec;
   OutputSection *DidatSec;
   OutputSection *RsrcSec;
   OutputSection *RelocSec;
 
auto CreateSection = [&](StringRef Name, uint32_t OutChars) {
     }
     return Sec;
   };
 
   // Try to match the section order used by link.exe.
   TextSec = CreateSection(".text", CODE | R | X);
   CreateSection(".bss", BSS | R | W);
   RdataSec = CreateSection(".rdata", DATA | R);
+  BuildidSec = CreateSection(".buildid", DATA | R);
   DataSec = CreateSection(".data", DATA | R | W);
   PdataSec = CreateSection(".pdata", DATA | R);
   IdataSec = CreateSection(".idata", DATA | R);
   EdataSec = CreateSection(".edata", DATA | R);
   DidatSec = CreateSection(".didat", DATA | R);
   RsrcSec = CreateSection(".rsrc", DATA | R);
   RelocSec = CreateSection(".reloc", DATA | DISCARDABLE | R);
 
if (!Symtab->LocalImportChunks.empty()) {
     for (Chunk *C : Symtab->LocalImportChunks)
       RdataSec->addChunk(C);
   }
 
   // Create Debug Information Chunks
   if (Config->Debug) {
     DebugDirectory = make<DebugDirectoryChunk>(DebugRecords);
 
+    OutputSection *DebugInfoSec = Config->MinGW ? BuildidSec : RdataSec;
+
     // Make a CVDebugRecordChunk even when /DEBUG:CV is not specified.  We
     // output a PDB no matter what, and this chunk provides the only means of
     // allowing a debugger to match a PDB and an executable.  So we need it even
     // if we're ultimately not going to write CodeView data to the PDB.
     auto *CVChunk = make<CVDebugRecordChunk>();
     BuildId = CVChunk;
     DebugRecords.push_back(CVChunk);
 
-    RdataSec->addChunk(DebugDirectory);
+    DebugInfoSec->addChunk(DebugDirectory);
     for (Chunk *C : DebugRecords)
-      RdataSec->addChunk(C);
+      DebugInfoSec->addChunk(C);
   }
 
   // Create SEH table. x86-only.
   if (Config->Machine == I386)
     createSEHTable();
 
   // Create /guard:cf tables if requested.
   if (Config->GuardCF != GuardCFLevel::Off)

lld/trunk/test/COFF/debug-reloc.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t.obj
 
 # RUN: lld-link -lldmingw -debug:dwarf -out:%t.exe -entry:mainfunc -subsystem:console %t.obj
 # RUN: llvm-readobj -sections %t.exe | FileCheck %s -check-prefix SECTIONS
 # RUN: llvm-readobj -coff-basereloc %t.exe | FileCheck %s -check-prefix RELOCS
+# RUN: llvm-readobj -file-headers %t.exe | FileCheck %s -check-prefix HEADERS
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck %s -check-prefix DEBUG
 
+# SECTIONS:         Number: 2
+# SECTIONS-NEXT:    Name: .buildid (2E 62 75 69 6C 64 69 64)
+# SECTIONS-NEXT:    VirtualSize: 0x35
+# SECTIONS-NEXT:    VirtualAddress: 0x2000
 # SECTIONS:         Number: 3
 # SECTIONS-NEXT:    Name: .data (2E 64 61 74 61 00 00 00)
 # SECTIONS-NEXT:    VirtualSize: 0x8
 # SECTIONS-NEXT:    VirtualAddress: 0x3000
 
 # RELOCS:      BaseReloc [
 # RELOCS-NEXT:   Entry {
 # RELOCS-NEXT:     Type: DIR64
 # RELOCS-NEXT:     Address: 0x3000
 # RELOCS-NEXT:   }
 # RELOCS-NEXT:   Entry {
 # RELOCS-NEXT:     Type: ABSOLUTE
 # RELOCS-NEXT:     Address: 0x3000
 # RELOCS-NEXT:   }
 # RELOCS-NEXT: ]
 
+# HEADERS:     DebugRVA: 0x2000
+# HEADERS:     DebugSize: 0x1C
+
+# DEBUG: DebugDirectory [
+# DEBUG:   DebugEntry {
+# DEBUG:     Type: CodeView (0x2)
+# DEBUG:     SizeOfData: 0x19
+# DEBUG:     AddressOfRawData: 0x201C
+# DEBUG:     PointerToRawData: 0x61C
+
   .text
   .def   mainfunc;
   .scl  2;
   .type 32;
   .endef
   .globl  mainfunc
 mainfunc:
 .Lfunc_begin0: