[ARM] Add support for tlsldm operator to llvm-mc

[peter.smith]

Summary

The standard local dynamic model for TLS on ARM systems needs two relocations for each data access:

• R_ARM_TLS_LDM32 (module idx)
• R_ARM_TLS_LDO32 (offset of object from origin of module TLS block)

In GNU style assembler we use symbol(tlsldm) and symbol(tlsldo) to produce these relocations.

llvm-mc for ARM supports symbol(tlsldo) but does not support symbol(tlsldm). This patch wires up the existing symbol(tlsldm) support in the assembler to the R_ARM_TLS_LDM32 relocation.

This change is for llvm-mc only, it does not implement lowering of generated TLS code to local-dynamic (-mtls-model=local-dynamic).

TLS for ARM is defined in Addenda to, and Errata in, the ABI for the ARM Architecture

[rengolin]

Simple test comment, otherwise, LGTM. Thanks!

test/MC/ARM/symbol-variants.s

93

why not just put this as the last test and avoid the change to the got_prel part?

[peter.smith]

Putting TLSLDM before the GOT_PREL makes all the relocations pair up in source file order. The ldr r3, =f31(GOT_PREL) creates a literal pool entry with a relocation, the literal pool entry will always be at the end so if GOT_PREL isn't at the end of the source file the .word and the literal pool entry R_ARM_GOT_PREL relocations won't appear as a pair like all the others.

I chose to insert before the GOT_PREL to neaten the expected output.

[rengolin]

ok, LGTM.

[peter.smith]

Thanks for the review, committed 275977. Will close when bots are ok.

lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp

case FK_Data_4:
         Type = ELF::R_ARM_TLS_LDO32;
         break;
       case MCSymbolRefExpr::VK_TLSCALL:
         Type = ELF::R_ARM_TLS_CALL;
         break;
       case MCSymbolRefExpr::VK_TLSDESC:
         Type = ELF::R_ARM_TLS_GOTDESC;
         break;
+      case MCSymbolRefExpr::VK_TLSLDM:
+        Type = ELF::R_ARM_TLS_LDM32;
+        break;
       case MCSymbolRefExpr::VK_ARM_TLSDESCSEQ:
         Type = ELF::R_ARM_TLS_DESCSEQ;
         break;
       }
       break;
     case ARM::fixup_arm_ldst_pcrel_12:
     case ARM::fixup_arm_pcrel_10:
     case ARM::fixup_arm_adr_pcrel_12:

test/MC/ARM/symbol-variants.s

 @ CHECK: 64 R_ARM_TLS_GOTDESC f25
 
 @ prel31 (relative)
 .word f26(PREL31)-.
 .word f27(prel31)-.
 @CHECK: 68 R_ARM_PREL31 f26
 @CHECK: 6c R_ARM_PREL31 f27
 
+@ tlsldm
+.word f28(TLSLDM)
+.word f29(tlsldm)
+@CHECK: 70 R_ARM_TLS_LDM32 f28
+@CHECK: 74 R_ARM_TLS_LDM32 f29
+
 @ got_prel
-.word  f28(GOT_PREL) + (. - .Lsym)
-  ldr r3, =f29(GOT_PREL)
-@ CHECK: 70 R_ARM_GOT_PREL f28
-@ CHECK: 78 R_ARM_GOT_PREL f29
+.word  f30(GOT_PREL) + (. - .Lsym)
+  ldr r3, =f31(GOT_PREL)
+@ CHECK: 78 R_ARM_GOT_PREL f30
+@ CHECK: 80 R_ARM_GOT_PREL f31