Fix fatal linker error on R_MIPS_JALR against a local TLS symbol

[arichardson]

Summary

In order to determine whether R_MIPS_JALR should use R_PC or R_NONE, the
MIPS code checks whether the target is preemptible and not microMIPS.
To check the later condition we look at the low bit of the virtual address.
However, for TLS symbols calling Symbol::getVA() will report a fatal error
until Out::tlsPhdr has been initialized (which only happens later).

It is unclear whether clang should be emitting these relocations for TLS
symbols in the first place, but we should be able to handle this without
failing in LLD even if clang is later change to not emit the relocation.

[arichardson]

Check objdump output

[arichardson]

Add .set noreorder to avoid excessive nops and make test more readable

[jrtc27]

R_MIPS_JALR really should not be being emitted here; its semantics are a hint to say that you are making a direct call to the referenced symbol (albeit by indirecting through a loaded GOT entry), but this is an indirect call. Interestingly, Local Dynamic is the only case where LLVM wants to emit the relocation; the three other models all leave it out, as does a normal global. This is because it happens to have the same structure as a direct call in the SelectionDAG representation, with the second operand for the instruction defining $t9 being a GlobalAddressSDNode, just this time an MO_DTPREL_LO rather than the expected MO_GOT_CALL (%call16)/MO_CALL_LO16 (%call_lo).

[arichardson]

This change is no longer necessary after https://reviews.llvm.org/D70406. I wonder if the testcase is still useful?

[arichardson]

No longer required after integrating test into D70406

lld/ELF/Arch/Mips.cpp

RelExpr MIPS<ELFT>::getRelExpr(RelType type, const Symbol &s,
   if (ELFT::Is64Bits || config->mipsN32Abi)
     type &= 0xff;
 
   switch (type) {
   case R_MIPS_JALR:
     // If the target symbol is not preemptible and is not microMIPS,
     // it might be possible to replace jalr/jr instruction by bal/b.
     // It depends on the target symbol's offset.
-    if (!s.isPreemptible && !(s.getVA() & 0x1))
+    // Additionally, we can't optimize references to TLS variables (and
+    // s.getVA() will generate a fatal error for TLS symbols since Out::tlsPhdr
+    // will not have been initialized yet).
+    if (!s.isPreemptible && !s.isTls() && !(s.getVA() & 0x1))
       return R_PC;
     return R_NONE;
   case R_MICROMIPS_JALR:
     return R_NONE;
   case R_MIPS_GPREL16:
   case R_MIPS_GPREL32:
   case R_MICROMIPS_GPREL16:
   case R_MICROMIPS_GPREL7_S2:

lld/test/ELF/tls-r-mips-jalr.s

+# REQUIRES: mips
+# RUN: llvm-mc -filetype=obj -triple=mips64-none-freebsd %s -o %t.o
+# RUN: ld.lld -shared %t.o -o %t.so
+# RUN: llvm-objdump -d %t.so | FileCheck %s
+## Minimized test case create from the following C source code
+## typedef int (*callback_f)(void);
+## static __thread callback_f callback;
+## void set_callback(callback_f f) {
+##   callback = f;
+## }
+## int test(void) {
+##   return callback();
+## }
+
+  .set  noat
+  .set  noreorder
+  .globl  test                    # -- Begin function test
+  .p2align  3
+  .type test,@function
+  .ent  test
+test:                                   # @test
+# %bb.0:                                # %entry
+  ld  $t9, %call16(__tls_get_addr)($gp)
+  .reloc .Ltmp1, R_MIPS_JALR, __tls_get_addr
+.Ltmp1:
+  jalr  $t9
+  nop
+  lui $1, %dtprel_hi(callback)
+  daddu $1, $1, $2
+  ld  $t9, %dtprel_lo(callback)($1)
+## The following line (creating a R_MIPS_JALR against a TLS symbol)
+## previously caused a linker error:
+## tls-r-mips-jalr.s.tmp.o has an STT_TLS symbol but doesn't have an SHF_TLS section
+  .reloc .Ltmp2, R_MIPS_JALR, callback
+.Ltmp2:
+  jr  $t9
+  nop
+  .end  test
+.Lfunc_end1:
+  .size test, .Lfunc_end1-test
+
+
+  .type callback,@object        # @callback
+  .section  .tbss,"awT",@nobits
+  .p2align  3
+callback:
+  .8byte  0
+  .size callback, 8
+
+## Check that the R_MIPS_JALR relocations did not change jalr to bal:
+# CHECK-LABEL: Disassembly of section .text:
+# CHECK-EMPTY:
+# CHECK-NEXT: test:
+# CHECK-NEXT:    103d0: df 99 80 20                   ld  $25, -32736($gp)
+# CHECK-NEXT:    103d4: 03 20 f8 09                   jalr  $25
+# CHECK-NEXT:    103d8: 00 00 00 00                   nop
+# CHECK-NEXT:    103dc: 3c 01 00 00                   lui $1, 0
+# CHECK-NEXT:    103e0: 00 22 08 2d                   daddu $1, $1, $2
+# CHECK-NEXT:    103e4: dc 39 80 00                   ld  $25, -32768($1)
+# CHECK-NEXT:    103e8: 03 20 00 08                   jr  $25
+# CHECK-NEXT:    103ec: 00 00 00 00                   nop
+