[COFF] Don't treat DWARF sections as GC roots

DWARF sections are typically live and not COMDAT, so they would be
treated as GC roots. Enabling DWARF would essentially keep all code with
debug info alive, preventing any section GC.

Fixes PR45273

Reviewed By: mstorsjo, MaskRay

Differential Revision: https://reviews.llvm.org/D76935

Author: rnk

lld/COFF/MarkLive.cpp

@@ -28,10 +28,12 @@ void markLive(ArrayRef<Chunk *> chunks) {
   // as we push, so sections never appear twice in the list.
   SmallVector<SectionChunk *, 256> worklist;
 
-  // COMDAT section chunks are dead by default. Add non-COMDAT chunks.
+  // COMDAT section chunks are dead by default. Add non-COMDAT chunks. Do not
+  // traverse DWARF sections. They are live, but they should not keep other
+  // sections alive.
   for (Chunk *c : chunks)
     if (auto *sc = dyn_cast<SectionChunk>(c))
-      if (sc->live)
+      if (sc->live && !sc->isDWARF())
         worklist.push_back(sc);
 
   auto enqueue = [&](SectionChunk *c) {

lld/test/COFF/gc-dwarf.s

@@ -0,0 +1,60 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -triple=x86_64-windows-msvc %s -filetype=obj -o %t.obj
+# RUN: lld-link -lldmap:%t.map -out:%t.exe -opt:ref -entry:main %t.obj -verbose 2>&1 | FileCheck %s
+# RUN: FileCheck %s --check-prefix=MAP --input-file=%t.map
+
+# CHECK:      Discarded unused1
+# CHECK-NEXT: Discarded unused2
+# CHECK-NOT: Discarded
+
+# MAP: In Symbol
+# MAP: gc-dwarf.s.tmp.obj:(.text)
+# MAP: {{ main$}}
+# MAP: gc-dwarf.s.tmp.obj:(.text)
+# MAP: {{ used$}}
+
+	.def	 @feat.00; .scl	3; .type	0; .endef
+	.globl	@feat.00
+.set @feat.00, 0
+
+	.def	 main; .scl	2; .type	32; .endef
+	.section	.text,"xr",one_only,main
+	.globl	main
+main:
+	callq used
+	xorl	%eax, %eax
+	retq
+
+	.def	 used; .scl	2; .type	32; .endef
+	.section	.text,"xr",one_only,used
+	.globl	used
+used:
+	retq
+
+
+	.def	 unused1; .scl	2; .type	32; .endef
+	.section	.text,"xr",one_only,unused1
+	.globl	unused1
+unused1:
+	retq
+
+	.def	 unused2; .scl	2; .type	32; .endef
+	.section	.text,"xr",one_only,unused2
+	.globl	unused2
+unused2:
+	retq
+
+# This isn't valid DWARF, but LLD doesn't care. Make up some data that
+# references the functions above.
+.section .debug_info,"r"
+.long main@IMGREL
+.long unused1@IMGREL
+.long unused2@IMGREL
+
+# Similarly, .eh_frame unwind info should not keep functions alive. Again, this
+# is not valid unwind info, but it doesn't matter for testing purposes.
+.section .eh_frame,"r"
+.long main@IMGREL
+.long unused1@IMGREL
+.long unused2@IMGREL