[ELF][ARM] Represent R_ARM_LDO32 as R_DTPREL instead of R_ABS

[MaskRay]

Summary

Follow-up to D82899. Note, we need to disable R_DTPREL relaxation
because ARM psABI does not define TLS relaxation.

[psmith]

Thanks, will take a look over the weekend. Would like to make sure all is well running some TLS test cases on a native Arm machine, fully expect it to be OK.

[psmith]

LGTM, it looks like it is difficult to generate local dynamic from clang, although it is possible with GCC. I was able to make a test application, that also had the advantage of working for shared libraries and PIE as R_ABS does not in that case.

[grimar]

lld/ELF/Relocations.cpp

241

I've noticed that canRelax is always used with && !config->shared now.
So can it be:

bool canRelax = !config->shared && config->emachine != EM_ARM &&
                config->emachine != EM_HEXAGON &&
                config->emachine != EM_RISCV;

[grimar]

LGTM too (perhaps, with the nit I've mentioned. Up to you).

[MaskRay]

lld/ELF/Relocations.cpp

241

I agree. canRelax does not capture the meaning precisely now. It actually means whether we can transit a TLS model for shared objects (general/local dynamic) to a TLS model for executables (initial/local exec). If we are going to rename the variable, that does not belong this change. I'll do that separately.

lld/ELF/Arch/ARM.cpp

if (config->target2 == Target2Policy::Rel)
       return R_PC;
     if (config->target2 == Target2Policy::Abs)
       return R_ABS;
     return R_GOT_PC;
   case R_ARM_TLS_GD32:
     return R_TLSGD_PC;
   case R_ARM_TLS_LDM32:
     return R_TLSLD_PC;
+  case R_ARM_TLS_LDO32:
+    return R_DTPREL;
   case R_ARM_BASE_PREL:
     // B(S) + A - P
     // FIXME: currently B(S) assumed to be .got, this may not hold for all
     // platforms.
     return R_GOTONLY_PC;
   case R_ARM_MOVW_PREL_NC:
   case R_ARM_MOVT_PREL:
   case R_ARM_REL32:

lld/ELF/Relocations.cpp

if (in.got->addTlsIndex()) {
         mainPart->relaDyn->addReloc(target->tlsModuleIndexRel, in.got,
                                 in.got->getTlsIndexOff(), nullptr);
     }
     c.relocations.push_back({expr, type, offset, addend, &sym});
     return 1;
   }
 
   // Local-Dynamic relocs can be relaxed to Local-Exec.
-  if (expr == R_DTPREL && !config->shared) {
+  if (expr == R_DTPREL && canRelax && !config->shared) {
     c.relocations.push_back(
         {target->adjustRelaxExpr(type, nullptr, R_RELAX_TLS_LD_TO_LE), type,
          offset, addend, &sym});
     return 1;
   }
 
   // Local-Dynamic sequence where offset of tls variable relative to dynamic
   // thread pointer is stored in the got. This cannot be relaxed to Local-Exec.

lld/test/ELF/debug-dead-reloc-tls-arm.s

 # REQUIRES: arm
 ## Test we resolve relocations referencing TLS symbols in .debug_* sections to
 ## a tombstone value if the referenced TLS symbol is discarded.
 
 # RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
 # RUN: ld.lld --gc-sections %t.o -o %t
 # RUN: llvm-objdump -s %t | FileCheck %s
 
 # CHECK:      Contents of section .debug_info:
-## FIXME: Use ffffffff
-# CHECK-NEXT:  0000 00000000
+# CHECK-NEXT:  0000 ffffffff
 
 .globl _start
 _start:
   bx lr
 
 .section .tbss,"awT",%nobits
 .globl tls
   .long 0
 
 .section .debug_info
 ## R_ARM_TLS_LDO32
   .long tls(tlsldo)