[lld] r319020 - [ELF][ARM] Refine check for when undefined weak needs a Thunk

[Peter Smith via llvm-commits]

It should do, if there is a PLT entry for the symbol the thunk will
use it. I'm not sure if on Arm we can ever get a range-extension thunk
to a NeedsPltAddr Symbol as all branches on Arm that need Thunks are
pc-relative. It may be possible on Mips, but I'll have to go check as
I'm not that familiar with Mips.

Peter

On 27 November 2017 at 17:04, Rafael Avila de Espindola
<rafael.espindola at gmail.com> wrote:
> BTW, do we correctly handle NeedsPltAddr? I think we do with this patch,
> but it is probably a good thing to test.
>
> Cheers,
> Rafael
>
> Peter Smith via llvm-commits <llvm-commits at lists.llvm.org> writes:
>
>> Author: psmith
>> Date: Mon Nov 27 03:49:18 2017
>> New Revision: 319020
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=319020&view=rev
>> Log:
>> [ELF][ARM] Refine check for when undefined weak needs a Thunk
>>
>> 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 __gmon__start__ in the Arm crti.o for glibc.
>>
>> Differential Revision: https://reviews.llvm.org/D40248
>>
>>
>> Added:
>>     lld/trunk/test/ELF/arm-branch-undef-weak-plt-thunk.s
>> Modified:
>>     lld/trunk/ELF/Arch/ARM.cpp
>>
>> Modified: lld/trunk/ELF/Arch/ARM.cpp
>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Arch/ARM.cpp?rev=319020&r1=319019&r2=319020&view=diff
>> ==============================================================================
>> --- lld/trunk/ELF/Arch/ARM.cpp (original)
>> +++ lld/trunk/ELF/Arch/ARM.cpp Mon Nov 27 03:49:18 2017
>> @@ -229,10 +229,9 @@ void ARM::addPltSymbols(InputSectionBase
>>
>>  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
>>
>> Added: lld/trunk/test/ELF/arm-branch-undef-weak-plt-thunk.s
>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/arm-branch-undef-weak-plt-thunk.s?rev=319020&view=auto
>> ==============================================================================
>> --- lld/trunk/test/ELF/arm-branch-undef-weak-plt-thunk.s (added)
>> +++ lld/trunk/test/ELF/arm-branch-undef-weak-plt-thunk.s Mon Nov 27 03:49:18 2017
>> @@ -0,0 +1,35 @@
>> +// 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
>>
>>
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