[LLD] [COFF] Order .debug_* sections at the end, to avoid leaving gaps if stripped

So far, we sort all discardable sections at the end, with only some
extra logic to make sure that the .reloc section is at the start
of that group of sections. But if there are other discardable
sections, other than .reloc, they must also be ordered before
.debug_* sections, to avoid leaving gaps if the executable is
stripped.

(Stripping executables doesn't remove all discardable sections,
only the ones named .debug_*).

Rust binaries seem to include a .rmeta section, which is marked
discardable. This fixes stripping such binaries if built with
dwarf debug info included.

This fixes issues observed in MSYS2 in
msys2/MINGW-packages#10555.

Differential Revision: https://reviews.llvm.org/D120805

Author: mstorsjo

lld/COFF/Writer.cpp

@@ -928,8 +928,14 @@ void Writer::createSections() {
     // Move DISCARDABLE (or non-memory-mapped) sections to the end of file
     // because the loader cannot handle holes. Stripping can remove other
     // discardable ones than .reloc, which is first of them (created early).
-    if (s->header.Characteristics & IMAGE_SCN_MEM_DISCARDABLE)
+    if (s->header.Characteristics & IMAGE_SCN_MEM_DISCARDABLE) {
+      // Move discardable sections named .debug_ to the end, after other
+      // discardable sections. Stripping only removes the sections named
+      // .debug_* - thus try to avoid leaving holes after stripping.
+      if (s->name.startswith(".debug_"))
+        return 3;
       return 2;
+    }
     // .rsrc should come at the end of the non-discardable sections because its
     // size may change by the Win32 UpdateResources() function, causing
     // subsequent sections to move (see https://crbug.com/827082).

lld/test/COFF/sort-debug.test

@@ -10,6 +10,7 @@
 
 # CHECK: Name: .text
 # CHECK: Name: .reloc
+# CHECK: Name: .rmeta
 # CHECK: Name: .debug_abbrev
 # CHECK: Name: .debug_info
 # CHECK: Name: .debug_line
@@ -18,6 +19,7 @@
 
 # NODEBUG: Name: .text
 # NODEBUG: Name: .reloc
+# NODEBUG: Name: .rmeta
 # NODEBUG-NOT: Name: .debug_abbrev
 # NODEBUG-NOT: Name: .debug_info
 # NODEBUG-NOT: Name: .debug_line
@@ -183,6 +185,10 @@ sections:
       - VirtualAddress:  43
         SymbolName:      .text
         Type:            IMAGE_REL_I386_DIR32
+  - Name:            .rmeta
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     00112233
 symbols:
   - Name:            .text
     Value:           0