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

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
an 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.

Differential Revision: https://reviews.llvm.org/D26239

llvm-svn: 285896

lld/ELF/Target.cpp

@@ -1652,6 +1652,9 @@ void ARMTargetInfo::writePlt(uint8_t *Buf, uint64_t GotEntryAddr,
 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.

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

@@ -0,0 +1,23 @@
+// 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