[ELF][ARM] Refine check for when undefined weak needs a Thunk

[peter.smith]

Summary

When an undefined weak reference has a PLT entry we must generate a range extension thunk for any B or BL that can't reach the PLT entry.

This change explicitly looks for whether a PLT entry exists rather than assuming that weak references never need PLT entries unless Config->Shared is in operation. This covers the case where we are linking an executable with dynamic linking, hence a PLT entry will be needed for undefined weak references. This case comes up in real programs over 32 Mb in size as there is a B to a weak reference gmonstart__ in the Arm crti.o for glibc.

Discovered while linking chrome on arm, error message: /usr/lib/../lib/crti.o:(.text+0x18): relocation R_ARM_JUMP24 out of range

lld/trunk/ELF/Arch/ARM.cpp

 void ARM::addPltSymbols(InputSectionBase *ISD, uint64_t Off) const {
   auto *IS = cast<InputSection>(ISD);
   addSyntheticLocal("$a", STT_NOTYPE, Off, 0, IS);
   addSyntheticLocal("$d", STT_NOTYPE, Off + 12, 0, IS);
 }
 
 bool ARM::needsThunk(RelExpr Expr, RelType Type, const InputFile *File,
                      uint64_t BranchAddr, const Symbol &S) const {
-  // If S is an undefined weak symbol in an executable we don't need a Thunk.
-  // In a DSO calls to undefined symbols, including weak ones get PLT entries
-  // which may need a thunk.
-  if (S.isUndefWeak() && !Config->Shared)
+  // If S is an undefined weak symbol and does not have a PLT entry then it
+  // will be resolved as a branch to the next instruction.
+  if (S.isUndefWeak() && !S.isInPlt())
     return false;
   // 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 (Type) {
   case R_ARM_PC24:
   case R_ARM_PLT32:
   case R_ARM_JUMP24:

lld/trunk/test/ELF/arm-branch-undef-weak-plt-thunk.s

+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-shared.s -o %t
+// RUN: ld.lld %t --shared -o %t.so
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t2
+// RUN: ld.lld %t2 %t.so -o %t3
+// RUN: llvm-objdump -d -triple=armv7a-none-linux-gnueabi -start-address=69632 -stop-address=69664 %t3 | FileCheck %s
+// REQUIRES: arm
+
+// When we are dynamic linking, undefined weak references have a PLT entry so
+// we must create a thunk for the branch to the PLT entry.
+
+ .text
+ .globl bar2
+ .weak undefined_weak_we_expect_a_plt_entry_for
+_start:
+ .globl _start
+ .type _start, %function
+ b undefined_weak_we_expect_a_plt_entry_for
+ bl bar2
+// Create 32 Mb gap between the call to the weak reference and the PLT so that
+// the b and bl need a range-extension thunk.
+ .section .text.1, "ax", %progbits
+ .space 32 * 1024 * 1024
+
+// CHECK: Disassembly of section .text:
+// CHECK-NEXT: _start:
+// CHECK-NEXT:    11000:       00 00 00 ea     b       #0 <__ARMv7ABSLongThunk_undefined_weak_we_expect_a_plt_entry_for>
+// CHECK-NEXT:    11004:       02 00 00 eb     bl      #8 <__ARMv7ABSLongThunk_bar2>
+// CHECK: __ARMv7ABSLongThunk_undefined_weak_we_expect_a_plt_entry_for:
+// CHECK-NEXT:    11008:       34 c0 01 e3     movw    r12, #4148
+// CHECK-NEXT:    1100c:       01 c2 40 e3     movt    r12, #513
+// CHECK-NEXT:    11010:       1c ff 2f e1     bx      r12
+// CHECK: __ARMv7ABSLongThunk_bar2:
+// CHECK-NEXT:    11014:       44 c0 01 e3     movw    r12, #4164
+// CHECK-NEXT:    11018:       01 c2 40 e3     movt    r12, #513
+// CHECK-NEXT:    1101c:       1c ff 2f e1     bx      r12