[ELF][MIPS] Revert r264761 and add test case to demonstrate the problem

[atanasyan]

Summary

If we make R_MIPS_LO16 a relative relocation, linker:

• never creates R_MIPS_COPY relocation for it
• attempts to create R_MIPS_REL32 dynamic relocation if R_MIPS_LO16's target is a preemptible symbol

[• rafael]

LGTM.

lld/trunk/ELF/Target.cpp

 bool MipsTargetInfo<ELFT>::isRelRelative(uint32_t Type) const {
   switch (Type) {
   default:
     return true;
   case R_MIPS_26:
   case R_MIPS_32:
   case R_MIPS_64:
   case R_MIPS_HI16:
+  case R_MIPS_LO16:
   case R_MIPS_TLS_DTPREL_HI16:
   case R_MIPS_TLS_DTPREL_LO16:
   case R_MIPS_TLS_TPREL_HI16:
   case R_MIPS_TLS_TPREL_LO16:
     return false;
   }
 }
 

lld/trunk/ELF/Writer.cpp

if (Target->needsGot(Type, Body)) {
           DynType = Target->RelativeRel;
         Out<ELFT>::RelaDyn->addReloc(
             {DynType, DynamicReloc<ELFT>::Off_Got, !Preemptible, &Body});
       }
       continue;
     }
 
     if (Config->EMachine == EM_MIPS) {
+      if (Type == R_MIPS_LO16)
+        // Ignore R_MIPS_LO16 relocation. If it is a pair for R_MIPS_GOT16 we
+        // already completed all required action (GOT entry allocation) when
+        // handle R_MIPS_GOT16. If it is a pair for R_MIPS_HI16 against
+        // _gp_disp it does not require dynamic relocation. If its a pair for
+        // R_MIPS_HI16 against a regular symbol it does not require dynamic
+        // relocation too because that case is possible for executable file
+        // linking only.
+        continue;
       if (&Body == Config->MipsGpDisp || &Body == Config->MipsLocalGp)
         // MIPS _gp_disp designates offset between start of function and 'gp'
         // pointer into GOT. __gnu_local_gp is equal to the current value of
         // the 'gp'. Therefore any relocations against them do not require
         // dynamic relocation.
         continue;
     }
 

lld/trunk/test/ELF/mips-lo16-not-relative.s

+# Check that R_MIPS_LO16 relocation is handled as non-relative,
+# and if a target symbol is a DSO data symbol, LLD create a copy
+# relocation.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux \
+# RUN:         %S/Inputs/mips-dynamic.s -o %t.so.o
+# RUN: ld.lld %t.so.o -shared -o %t.so
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o %t.so -o %t.exe
+# RUN: llvm-readobj -r %t.exe | FileCheck %s
+
+# REQUIRES: mips
+
+# CHECK:      Relocations [
+# CHECK-NEXT:   Section (7) .rel.dyn {
+# CHECK-NEXT:     0x{{[0-9A-F]+}} R_MIPS_COPY data0 0x0
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]
+
+  .text
+  .global __start
+__start:
+  addi   $t0, $t0, %lo(data0)