[lld] r315658 - Slightly simplify code and add comment.

[Rui Ueyama via llvm-commits]

Author: ruiu
Date: Thu Oct 12 18:19:10 2017
New Revision: 315658

URL: http://llvm.org/viewvc/llvm-project?rev=315658&view=rev
Log:
Slightly simplify code and add comment.

This is not a mechanical transformation. Even though I believe this
patch is correct, I'm not 100% sure if lld with this patch behaves
exactly the same way as before on all edge cases. At least all tests
still pass.

I'm submitting this patch because it took almost a day to understand
this function, and I don't want to lose it.

Modified:
    lld/trunk/ELF/Relocations.cpp

Modified: lld/trunk/ELF/Relocations.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Relocations.cpp?rev=315658&r1=315657&r2=315658&view=diff
==============================================================================
--- lld/trunk/ELF/Relocations.cpp (original)
+++ lld/trunk/ELF/Relocations.cpp Thu Oct 12 18:19:10 2017
@@ -815,13 +815,31 @@ static void addGotEntry(SymbolBody &Sym,
     DynType = Target->GotRel;
   }
 
-  bool Constant = !Preemptible && (!Config->Pic || isAbsolute(Sym));
-  if (!Constant)
+  // If a GOT slot value can be calculated at link-time, which is now,
+  // we can just fill that out.
+  //
+  // (We don't actually write a value to a GOT slot right now, but we
+  // add a static relocation to a Relocations vector so that
+  // InputSection::relocate will do the work for us. We may be able
+  // to just write a value now, but it is a TODO.)
+  bool IsLinkTimeConstant = !Preemptible && (!Config->Pic || isAbsolute(Sym));
+  if (IsLinkTimeConstant) {
+    InX::Got->Relocations.push_back({Expr, DynType, Off, 0, &Sym});
+  } else {
+    // Otherwise, we emit a dynamic relocation to .rel[a].dyn so that
+    // the GOT slot will be fixed at load-time.
     In<ELFT>::RelaDyn->addReloc(
         {DynType, InX::Got, Off, !Preemptible, &Sym, 0});
 
-  if (Constant || (!Config->IsRela && !Preemptible))
-    InX::Got->Relocations.push_back({Expr, DynType, Off, 0, &Sym});
+    // REL type relocations don't have addend fields unlike RELAs, and
+    // their addends are stored to the section to which they are applied.
+    // So, store addends if we need to.
+    //
+    // This is ugly -- the difference between REL and RELA should be
+    // handled in a better way. It's a TODO.
+    if (!Config->IsRela)
+      InX::Got->Relocations.push_back({R_ABS, Target->GotRel, Off, 0, &Sym});
+  }
 }
 
 // The reason we have to do this early scan is as follows