[lld/ELF] Avoid unnecessary TPOFF relocations in GOT for -pie (llvm#8…

…1739)

With the new SystemZ port we noticed that -pie executables generated
from files containing R_390_TLS_IEENT relocations will have unnecessary
relocations in their GOT:

                        9e8d8: R_390_TLS_TPOFF  *ABS*+0x18

This is caused by the config->isPic conditon in addTpOffsetGotEntry:

 static void addTpOffsetGotEntry(Symbol &sym) {
   in.got->addEntry(sym);
   uint64_t off = sym.getGotOffset();
   if (!sym.isPreemptible && !config->isPic) {
     in.got->addConstant({R_TPREL, target->symbolicRel, off, 0, &sym});
     return;
   }

It is correct that we need to retain a TPOFF relocation if the target
symbol is preemptible or if we're building a shared library. But when
building a -pie executable, those values are fixed at link time and
there's no need for any remaining dynamic relocation.

Note that the equivalent MIPS-specific code in MipsGotSection::build
checks for config->shared instead of config->isPic; we should use the
same check here. (Note also that on many other platforms we're not even
using addTpOffsetGotEntry in this case as an IE->LE relaxation is
applied before; we don't have this type of relaxation on SystemZ.)

(cherry picked from commit 6f90773)

lld/ELF/Relocations.cpp

@@ -940,7 +940,7 @@ void elf::addGotEntry(Symbol &sym) {
 static void addTpOffsetGotEntry(Symbol &sym) {
   in.got->addEntry(sym);
   uint64_t off = sym.getGotOffset();
-  if (!sym.isPreemptible && !config->isPic) {
+  if (!sym.isPreemptible && !config->shared) {
     in.got->addConstant({R_TPREL, target->symbolicRel, off, 0, &sym});
     return;
   }

lld/test/ELF/systemz-tls-ie.s

@@ -12,6 +12,14 @@
 # RUN: llvm-objdump --section .data --full-contents %t | FileCheck --check-prefix=LE-DATA %s
 # RUN: llvm-objdump --section .got --full-contents %t | FileCheck --check-prefix=LE-GOT %s
 
+## With -pie we still have the R_390_RELATIVE for the data element, but all GOT
+## entries should be fully resolved without any remaining R_390_TLS_TPOFF.
+# RUN: ld.lld -pie %t.o -o %t.pie
+# RUN: llvm-readelf -r %t.pie | FileCheck --check-prefix=PIE-REL %s
+# RUN: llvm-objdump -d --no-show-raw-insn %t.pie | FileCheck --check-prefix=PIE %s
+# RUN: llvm-objdump --section .data --full-contents %t.pie | FileCheck --check-prefix=PIE-DATA %s
+# RUN: llvm-objdump --section .got --full-contents %t.pie | FileCheck --check-prefix=PIE-GOT %s
+
 # IE-REL: Relocation section '.rela.dyn' at offset {{.*}} contains 4 entries:
 # IE-REL: 0000000000003478 000000000000000c R_390_RELATIVE 2460
 # IE-REL: 0000000000002460 0000000100000038 R_390_TLS_TPOFF 0000000000000008 a + 0
@@ -58,6 +66,32 @@
 # LE-GOT: 1002248 00000000 00000000 ffffffff fffffff8
 # LE-GOT: 1002258 ffffffff fffffffc 00000000 00000000
 
+# PIE-REL: Relocation section '.rela.dyn' at offset {{.*}} contains 1 entries:
+# PIE-REL: 00000000000033d0 000000000000000c R_390_RELATIVE 23b8
+
+## TP offset for a is at 0x23b8
+# PIE:      lgrl    %r1, 0x23b8
+# PIE-NEXT: lgf     %r1, 0(%r1,%r7)
+
+## TP offset for b is at 0x23c0
+# PIE-NEXT: lgrl    %r1, 0x23c0
+# PIE-NEXT: lgf     %r1, 0(%r1,%r7)
+
+## TP offset for c is at 0x23c8
+# PIE-NEXT: lgrl    %r1, 0x23c8
+# PIE-NEXT: lgf     %r1, 0(%r1,%r7)
+
+## Data element: TP offset for a is at 0x23b8 (relocated via R_390_RELATIVE above)
+# PIE-DATA: 33d0 00000000 00000000
+
+## TP offsets in GOT:
+# a: -8
+# b: -4
+# c: 0
+# PIE-GOT: 23a0 00000000 000022d0 00000000 00000000
+# PIE-GOT: 23b0 00000000 00000000 ffffffff fffffff8
+# PIE-GOT: 23c0 ffffffff fffffffc 00000000 00000000
+
 ear     %r7,%a0
 sllg    %r7,%r1,32
 ear     %r7,%a1