[LLD] [COFF] Don't produce base relocs for discardable sections

[mstorsjo]

Summary

Dwarf debug info contains some data that otherwise would end up containing absolute addresses. Since these sections are discardable and aren't loaded at runtime, there's no point in adding base relocations for them.

This makes sure that after stripping out dwarf debug info, there are no base relocations that point to nonexistent sections.

[pcc]

LGTM

lld/trunk/COFF/Writer.cpp

 }
 
 // Add base relocations to .reloc section.
 void Writer::addBaserels() {
   if (!Config->Relocatable)
     return;
   std::vector<Baserel> V;
   for (OutputSection *Sec : OutputSections) {
-    if (Sec == RelocSec)
+    if (Sec->Header.Characteristics & IMAGE_SCN_MEM_DISCARDABLE)
       continue;
     // Collect all locations for base relocations.
     for (Chunk *C : Sec->getChunks())
       C->getBaserels(&V);
     // Add the addresses to .reloc section.
     if (!V.empty())
       addBaserelBlocks(V);
     V.clear();

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
+
+# 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: ]
+
+  .text
+  .def   mainfunc;
+  .scl  2;
+  .type 32;
+  .endef
+  .globl  mainfunc
+mainfunc:
+.Lfunc_begin0:
+  xorl  %eax, %eax
+  retq
+
+  .data
+  .globl  ptr
+ptr:
+  .quad mainfunc
+
+  .section  .debug_info,"dr"
+  .quad .Lfunc_begin0