[PATCH] D117734: [ELF] Fix the branch range computation when reusing a thunk

[Peter Smith via Phabricator via llvm-commits]

peter.smith added a comment.

Here's a modification of your test case for AArch64 that will fail for Arm

  # REQUIRES: arm
  # RUN: rm -rf %t && split-file %s %t
  # RUN: llvm-mc -filetype=obj -triple=armv7-a-none-eabi --arm-add-build-attributes %t/a.s -o %t/a.o
  # RUN: ld.lld -pie -T %t/lds %t/a.o -o %t/a
  # RUN: llvm-objdump -d --no-show-raw-insn %t/a | FileCheck %s
  
  ## We create a thunk for dest.
  # CHECK-LABEL: <mid>:
  # CHECK-NEXT: 2010004:       b       0x2010008 <__ARMV7PILongThunk_dest> @ imm = #-4
  # CHECK-EMPTY:
  # CHECK-NEXT: 02010008 <__ARMV7PILongThunk_dest>:
  # CHECK-NEXT:                movw    r12, #65516
  # CHECK-NEXT:                movt    r12, #65023
  # CHECK-NEXT:                add     r12, r12, pc
  # CHECK-NEXT:                bx      r12
  
  ## The first instruction can reuse the thunk but the second can't.
  ## If we reuse the thunk for b, we will get an "out of range" error.
  # CHECK-LABEL: <high>:
  # CHECK-NEXT: 4010000:       bl      0x2010008 <__ARMV7PILongThunk_dest> @ imm = #-33554432
  # CHECK-NEXT:                b       0x4010008 <__ARMV7PILongThunk_dest> @ imm = #-4
  # CHECK-EMPTY:
  # CHECK-NEXT: 04010008 <__ARMV7PILongThunk_dest>:
  # CHECK-NEXT:                movw    r12, #65516
  # CHECK-NEXT:                movt    r12, #64511
  # CHECK-NEXT:                add     r12, r12, pc
  # CHECK-NEXT:                bx      r12
  
  #--- a.s
  .section .text_low, "ax", %progbits
  
  .globl _start
  _start:
    nop
  dest:
    bx lr
  
  .section .text_mid, "ax", %progbits
  mid:
    b dest
  
  .section .text_high, "ax", %progbits
  high:
    bl dest
    b dest
  
  #--- lds
  SECTIONS {
    .text_low 0x10000: { *(.text_low) }
    .text_mid 0x2010004 : { *(.text_mid) }
    .text_high 0x4010000 : { *(.text_high) }
  }

Both will pass with `target->inBranchRange(rel.type, src, t->getThunkTargetSym()->getVA(-getPCBias(rel.type))))`

In theory there is scope for another test for Thumb2 to cover the case of PC BIAS of 4, the test is for Arm with PC BIAS of 8. However if anyone changes the PC BIAS for Thumb other tests will fail.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D117734/new/

https://reviews.llvm.org/D117734