[PATCH] D26239: [LLD][ARM] Do not create interworking thunks for undefined weak references.

[Peter Smith via llvm-commits]

peter.smith created this revision.
peter.smith added reviewers: ruiu, rafael.
peter.smith added a subscriber: llvm-commits.
Herald added subscribers: rengolin, aemerson.

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.


https://reviews.llvm.org/D26239

Files:
  ELF/Target.cpp
  test/ELF/arm-thumb-no-undefined-thunk.s


Index: test/ELF/arm-thumb-no-undefined-thunk.s
===================================================================
--- /dev/null
+++ 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
Index: ELF/Target.cpp
===================================================================
--- ELF/Target.cpp
+++ ELF/Target.cpp
@@ -1652,6 +1652,9 @@
 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.


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