[ELF, PR27016] - Do not keep undefined locals in .symtab

[grimar]

Summary

While looking into PR27016 I found that both gold and bfd just do not
include the undefined locals in symtab.

We can support that for sure, but do anybody see reasons to do that then ?
(I can probably implement that if so).

Now that is pretty simple patch to fix PR27016 and also looks even have benefits,
helps to "get rid of undefined ppc64 "dot-symbols"
(https://sourceware.org/ml/binutils/2015-02/msg00237.html).

[grimar]

• Removed useless arg for llvm-readobj in testcase

[• rafael]

Lgtm

[• rafael]

Wait, won't this also drop global undefined?

[grimar]

In D18554#385664, @rafael wrote:

Wait, won't this also drop global undefined?

No, shouldKeepInSymtab() is executed only from copyLocalSymbols()

[emaste]

Should be a continue in copyLocalSymbols instead?

[grimar]

In D18554#385680, @emaste wrote:

Should be a continue in copyLocalSymbols instead?

I think both ways are ok, but I prefer mine. That is exactly what we do - just do not put them in symtab and for me It is ok check it in shouldKeepInSymtab.

[grimar]

So I guess lets wait for possible other opinions about that then.

[llvm-commits]

• msg-15314-33.txt162 BDownload

[emaste]

In D18554#385688, @grimar wrote:

for me It is ok check it in shouldKeepInSymtab.

OK, either way is fine with me.

[ruiu]

LGTM

ELF/Writer.cpp

static bool shouldKeepInSymtab(const elf::ObjectFile<ELFT> &File,
                                const typename ELFT::Sym &Sym) {
   if (Sym.getType() == STT_FILE)
     return false;
 
   // We keep sections in symtab for relocatable output.
   if (Sym.getType() == STT_SECTION)
     return Config->Relocatable;
 
+  // No reason to keep local undefined symbol in symtab.
+  if (Sym.st_shndx == SHN_UNDEF)
+    return false;
+
   InputSectionBase<ELFT> *Sec = File.getSection(Sym);
   // If sym references a section in a discarded group, don't keep it.
   if (Sec == InputSection<ELFT>::Discarded)
     return false;
 
   if (Config->DiscardNone)
     return true;
 

test/ELF/local-undefined-symbol.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t -o %t1
+# RUN: llvm-readobj -t %t1 | FileCheck %s
+
+# CHECK:     Symbols [
+# CHECK-NOT:  Name: foo
+
+.global _start
+_start:
+ jmp foo
+
+.local foo