[ELF2/AArch64] Comment R_AARCH64_ADD_ABS_LO12_NC relocation

[davide]

Summary

Quick sanity (and english) check.

[• rafael]

LGTM. If there are more of these we might want to add write12
functions, but for now this should do it.

lld/trunk/ELF/Target.cpp

case R_AARCH64_ABS32:
     write32le(L, SA);
     break;
   case R_AARCH64_ABS64:
     // No overflow check needed.
     write64le(L, SA);
     break;
   case R_AARCH64_ADD_ABS_LO12_NC:
     // No overflow check needed.
+    // This relocation stores 12 bits and there's no instruction
+    // to do it. Instead, we do a 32 bits store of the value
+    // of r_addend bitwise-or'ed L. This assumes that the addend
+    // bits in L are zero.
     or32le(L, (SA & 0xFFF) << 10);
     break;
   case R_AARCH64_ADR_PREL_LO21: {
     uint64_t X = SA - P;
     if (!isInt<21>(X))
       error("Relocation R_AARCH64_ADR_PREL_LO21 out of range");
     updateAArch64Adr(L, X & 0x1FFFFF);
     break;