[lld] [lld] Add target support for SystemZ (s390x) (PR #75643)

[Ulrich Weigand via llvm-commits]

================
@@ -1374,8 +1379,8 @@ static unsigned handleTlsRelocation(RelType type, Symbol &sym,
             R_LOONGARCH_GOT_PAGE_PC, R_GOT_OFF, R_TLSIE_HINT>(expr)) {
     ctx.hasTlsIe.store(true, std::memory_order_relaxed);
     // Initial-Exec relocs can be optimized to Local-Exec if the symbol is
-    // locally defined.
-    if (execOptimize && isLocalInExecutable) {
+    // locally defined.  This is not supported on SystemZ.
+    if (execOptimize && isLocalInExecutable && config->emachine != EM_S390) {
----------------
uweigand wrote:

Yes, exactly.  Both Initial-Exec and Local-Exec sequences end up being LGRL+LGF, so any rewrite wouldn't actually be an optimization here.  Also, to rewrite into local-exec we'd have to allocate a constant pool entry slot somewhere, which is difficult for the linker.  Easier to just keep using the (already allocated) GOT slot.

Now, if we could use a more efficient Local-Exec sequence, this might be a different story.  Back when the ABI was defined, the architecture didn't yet have LGFI, so there wasn't really a way to load even a 32-bit constant without constant pool.  (B.t.w. are we certain that TLS offsets on a 64-bit platform can be restricted to 32 bits?)

https://github.com/llvm/llvm-project/pull/75643