Simplify gp0 handling

[• rafael]

Summary

If gp0 cannot point to merge sections (or if -r takes care of it) this is a simplification.

If gp0 can point to a merge section this is either a fix or a regression. In either case it would be awesome if you could provide a testcase :-)
Currently all tests pass with this.

[atanasyan]

LGTM

I cannot imagine the case when R_MIPS_GPRELxxx points to a merge section. Anyway now LLD does not support relocations pointing to SHF_MERGE for any targets. So let's consider the patch as a simplification.

[atanasyan]

Closed by commit rL266888.

ELF/InputSection.h

enum RelExpr {
   R_GOT,
   R_GOTONLY_PC,
   R_GOTREL,
   R_GOT_FROM_END,
   R_GOT_PAGE_PC,
   R_GOT_PC,
   R_MIPS_GOT,
   R_MIPS_GOT_LOCAL,
-  R_MIPS_GP0,
   R_NEG_TLS,
   R_PAGE_PC,
   R_PC,
   R_PLT,
   R_PLT_PC,
   R_PPC_OPD,
   R_PPC_PLT_OPD,
   R_PPC_TOC,

ELF/InputSection.cpp

case R_TLS:
     return Body.getVA<ELFT>(A) - Out<ELFT>::TlsPhdr->p_memsz;
   case R_NEG_TLS:
     return Out<ELF32LE>::TlsPhdr->p_memsz - Body.getVA<ELFT>(A);
   case R_ABS:
   case R_RELAX_TLS_GD_TO_LE:
   case R_RELAX_TLS_IE_TO_LE:
   case R_RELAX_TLS_LD_TO_LE:
     return Body.getVA<ELFT>(A);
-  case R_MIPS_GP0:
-    // We need to adjust SymVA value in case of R_MIPS_GPREL16/32
-    // relocations because they use the following expression to calculate
-    // the relocation's result for local symbol: S + A + GP0 - G.
-    return Body.getVA<ELFT>(A) + File.getMipsGp0();
   case R_MIPS_GOT_LOCAL:
     // If relocation against MIPS local symbol requires GOT entry, this entry
     // should be initialized by 'page address'. This address is high 16-bits
     // of sum the symbol's value and the addend.
     return Out<ELFT>::Got->getMipsLocalPageAddr(Body.getVA<ELFT>(A));
   case R_MIPS_GOT:
     // For non-local symbols GOT entries should contain their full
     // addresses. But if such symbol cannot be preempted, we do not

ELF/Writer.cpp

for (auto I = Rels.begin(), E = Rels.end(); I != E; ++I) {
     // If the relocation depends on the symbol value (not the size or distances
     // in the output), we still need some help from the dynamic linker.
     // We can however do better than just copying the incoming relocation. We
     // can process some of it and and just ask the dynamic linker to add the
     // load address.
     if (!Config->Pic || Target->isRelRelative(Type) || Expr == R_PC ||
         Expr == R_SIZE || isAbsolute<ELFT>(Body)) {
       if (Config->EMachine == EM_MIPS && Body.isLocal() &&
-          (Type == R_MIPS_GPREL16 || Type == R_MIPS_GPREL32))
-        Expr = R_MIPS_GP0;
+          (Type == R_MIPS_GPREL16 || Type == R_MIPS_GPREL32)) {
+        Expr = R_ABS;
+        Addend += File.getMipsGp0();
+      }
       C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
       continue;
     }
 
     AddDyn({Target->RelativeRel, C.OutSec, Offset, true, &Body, Addend});
     C.Relocations.push_back({R_ABS, Type, Offset, Addend, &Body});
   }