Eliminate .debug_gnu_pub{names,types} if -gdb-index is given.

[ruiu]

Summary

This patch is to ignore .debug_gnu_pub{names,types} sections if the
-gdb-index option was given.

[dblaikie]

Looks great - does exactly what I was wanting/expecting.

First run clang startup time drops from 4.4 seconds to 2.5 seconds (PR32718 discusses the issue with first run startup/binary size on (some?) linux systems), binary size drops from 837MiB to 466MiB.

Thanks a bunch!

lld/trunk/ELF/InputFiles.cpp

if (Name == ".note.GNU-split-stack") {
     error(toString(this) +
           ": object file compiled with -fsplit-stack is not supported");
     return &InputSection::Discarded;
   }
 
   if (Config->Strip != StripPolicy::None && Name.startswith(".debug"))
     return &InputSection::Discarded;
 
+  // If -gdb-index is given, LLD creates .gdb_index section, and that
+  // section serves the same purpose as .debug_gnu_pub{names,types} sections.
+  // If that's the case, we want to eliminate .debug_gnu_pub{names,types}
+  // because they are redundant and can waste large amount of disk space
+  // (for example, they are about 400 MiB in total for a clang debug build.)
+  if (Config->GdbIndex &&
+      (Name == ".debug_gnu_pubnames" || Name == ".debug_gnu_pubtypes"))
+    return &InputSection::Discarded;
+
   // The linkonce feature is a sort of proto-comdat. Some glibc i386 object
   // files contain definitions of symbol "__x86.get_pc_thunk.bx" in linkonce
   // sections. Drop those sections to avoid duplicate symbol errors.
   // FIXME: This is glibc PR20543, we should remove this hack once that has been
   // fixed for a while.
   if (Name.startswith(".gnu.linkonce."))
     return &InputSection::Discarded;
 

lld/trunk/test/ELF/debug-gnu-pubnames.s

+# REQUIRES: x86
+# RUN: ld.lld -e main %p/Inputs/gdb-index-a.elf %p/Inputs/gdb-index-b.elf -o %t1.exe
+# RUN: llvm-readobj -sections %t1.exe | FileCheck -check-prefix=CHECK1 %s
+# CHECK1: Name: .debug_gnu_pubnames
+# CHECK1: Name: .debug_gnu_pubtypes
+
+# RUN: ld.lld -gdb-index -e main %p/Inputs/gdb-index-a.elf %p/Inputs/gdb-index-b.elf -o %t2.exe
+# RUN: llvm-readobj -sections %t2.exe | FileCheck -check-prefix=CHECK2 %s
+# CHECK2-NOT: Name: .debug_gnu_pubnames
+# CHECK2-NOT: Name: .debug_gnu_pubtypes