[LLD][ARM] Do not create interworking thunks for undefined weak references.

[peter.smith]

Summary

An undefined weak reference is given an address of 0 this will incorrectly trigger the creation of a Thumb to ARM interworking Thunk
if there is a Thumb branch instruction to the symbol. This results in a linker error as Thunks only make sense to defined or shared symbols.

We prevent this by detecting an undefined symbol and not creating a thunk for it.

[ruiu]

LGTM

lld/trunk/ELF/Target.cpp

void ARMTargetInfo::writePlt(uint8_t *Buf, uint64_t GotEntryAddr,
   memcpy(Buf, PltData, sizeof(PltData));
   uint64_t L1 = PltEntryAddr + 4;
   write32le(Buf + 12, GotEntryAddr - L1 - 8);
 }
 
 RelExpr ARMTargetInfo::getThunkExpr(RelExpr Expr, uint32_t RelocType,
                                     const InputFile &File,
                                     const SymbolBody &S) const {
+  // If S is an undefined weak symbol we don't need a Thunk
+  if (S.isUndefined())
+    return Expr;
   // A state change from ARM to Thumb and vice versa must go through an
   // interworking thunk if the relocation type is not R_ARM_CALL or
   // R_ARM_THM_CALL.
   switch (RelocType) {
   case R_ARM_PC24:
   case R_ARM_PLT32:
   case R_ARM_JUMP24:
     // Source is ARM, all PLT entries are ARM so no interworking required.

lld/trunk/test/ELF/arm-thumb-no-undefined-thunk.s

+// RUN: llvm-mc -filetype=obj -triple=thumbv7a-none-linux-gnueabi %s -o %t
+// RUN: ld.lld %t -o %t2 2>&1
+// RUN: llvm-objdump -triple=thumbv7a-none-linux-gnueabi -d %t2 | FileCheck %s
+// REQUIRES: arm
+
+// Check that no thunks are created for an undefined weak symbol
+ .syntax unified
+
+.weak target
+
+.section .text.thumb, "ax", %progbits
+ .thumb
+ .global
+_start:
+ bl target
+ b target
+ b.w target
+
+// CHECK: Disassembly of section .text:
+// CHECK-NEXT: _start:
+// CHECK-NEXT:    11000:        ee f7 fe ef     blx     #-69636
+// CHECK-NEXT:    11004:        ee f7 fc bf     b.w     #-69640
+// CHECK-NEXT:    11008:        ee f7 fa bf     b.w     #-69644