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

[Rafael Avila de Espindola via llvm-commits]

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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