[JITLink][ELF] Don't skip sections of size 0

[steven_wu]

Summary

Size 0 sections can have symbols that have size 0. Build those sections
and symbols into the LinkGraph so they can be used properly if needed.

[lhames]

LGTM.

llvm/lib/ExecutionEngine/JITLink/ELFLinkGraphBuilder.h

302–303

I wish I could remember what "undefined" sections referred to here -- we'll no longer be skipping them.

This is still the right way to go though -- we can put the proper check in if/when we hit issues with this.

308

Is this needed?

[steven_wu]

llvm/lib/ExecutionEngine/JITLink/ELFLinkGraphBuilder.h

308

Some of those are rebase errors. Let me clean it up.

[steven_wu]

Address review feedback

llvm/lib/ExecutionEngine/JITLink/ELFLinkGraphBuilder.h

for (ELFSectionIndex SecIndex = 0; SecIndex != Sections.size(); ++SecIndex) {
 
     // Get the section's memory protection flags.
     MemProt Prot;
     if (Sec.sh_flags & ELF::SHF_EXECINSTR)
       Prot = MemProt::Read | MemProt::Exec;
     else
       Prot = MemProt::Read | MemProt::Write;
 
-    // For now we just use this to skip the "undefined" section, probably need
-    // to revist.
-    if (Sec.sh_size == 0)
-      continue;
+    // Look for existing sections first.
+    auto *GraphSec = G->findSectionByName(*Name);
+    if (!GraphSec)
+      GraphSec = &G->createSection(*Name, Prot);
+    assert(GraphSec->getMemProt() == Prot && "MemProt should match");
 
-    auto &GraphSec = G->createSection(*Name, Prot);
+    Block *B = nullptr;
     if (Sec.sh_type != ELF::SHT_NOBITS) {
       auto Data = Obj.template getSectionContentsAsArray<char>(Sec);
       if (!Data)
         return Data.takeError();
 
       G->createContentBlock(GraphSec, *Data, Sec.sh_addr, Sec.sh_addralign, 0);
     } else
       G->createZeroFillBlock(GraphSec, Sec.sh_size, Sec.sh_addr,

llvm/test/ExecutionEngine/JITLink/X86/ELF_empty_section.s

+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o %t
+// RUN: llvm-jitlink -noexec %t
+
+.section  .foo,"ax"
+.globl  zero
+zero:
+
+
+.text
+.globl main
+main:
+nop