[lld][Hexagon] Test undefined weak branches (#186613)

Undefined weak branches do not needsThunk().

Add a test case to cover undef weak.

lld/ELF/Arch/Hexagon.cpp

@@ -370,7 +370,10 @@ bool Hexagon::inBranchRange(RelType type, uint64_t src, uint64_t dst) const {
 bool Hexagon::needsThunk(RelExpr expr, RelType type, const InputFile *file,
                          uint64_t branchAddr, const Symbol &s,
                          int64_t a) const {
-  // Only check branch range for supported branch relocation types
+  // Undefined weak symbols without PLT entries resolve to address zero.
+  // Thunks are not needed since the branch target is fixed.
+  if (s.isUndefined() && !s.isInPlt(ctx))
+    return false;
   switch (type) {
   case R_HEX_B22_PCREL:
   case R_HEX_PLT_B22_PCREL:

lld/test/ELF/hexagon-undefined-weak.s

@@ -0,0 +1,28 @@
+# REQUIRES: hexagon
+# RUN: llvm-mc -filetype=obj -triple=hexagon-unknown-elf %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s
+
+## Undefined weak branch targets resolve to address zero.  Verify that
+## no thunks are created and that branches encode without error.
+
+.weak undefined_weak
+.globl _start
+.type _start, @function
+_start:
+  ## Simple call -- single-word packet.
+  call undefined_weak
+
+  ## Call in a two-word packet with an ALU op.
+  { r0 = #0
+    call undefined_weak }
+
+  jumpr r31
+
+## All branches target address zero.
+# CHECK:       <_start>:
+# CHECK-NEXT:  { call 0x0 <undefined_weak> }
+# CHECK-NEXT:  { call 0x0 <undefined_weak>
+# CHECK-NEXT:    r0 = #0x0 }
+# CHECK-NEXT:  { jumpr r31 }
+# CHECK-NOT:   __hexagon_thunk