[lld] r286353 - [ELF] ARM and AArch64 undefined weak reference values
Peter Smith via llvm-commits
llvm-commits at lists.llvm.org
Wed Nov 9 07:17:00 PST 2016
>From C Code the recommended way is with inline assembler asm(".global
reference"). ARM's proprietary compiler armcc has a #pragma import
<symbol> [1] which will embed a reference directly, the asm(".global
reference") is the recommended replacement.
You raise an interesting point about ADRP and the
R_AARCH64_ADR_PREL_PG_HI21 as while it is relative it can't
necessarily return the address of the place either. I'll need to do
some more thinking as I have my suspicions that neither the old or the
new way is quite right. I think that what should happen is the linker
always creates a GOT slot for the weak reference that can ADRP +
follow up instruction can always guarantee to access, if the weak
reference is unresolved the GOT slot contains 0.
[1] http://infocenter.arm.com/help/topic/com.arm.doc.dui0472m/chr1359124987739.html
On 9 November 2016 at 14:22, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
> This is quite interesting. How are these references produced? Directly
> writing assembly? I wonder if
> AArch64Subtarget::ClassifyGlobalReference should be changed in any
> way:
>
> // The small code mode's direct accesses use ADRP, which cannot necessarily
> // produce the value 0 (if the code is above 4GB).
> if (TM.getCodeModel() == CodeModel::Small && GV->hasExternalWeakLinkage())
> return AArch64II::MO_GOT;
>
> Cheers,
> Rafael
>
>
>
> On 9 November 2016 at 05:22, Peter Smith via llvm-commits
> <llvm-commits at lists.llvm.org> wrote:
>> Author: psmith
>> Date: Wed Nov 9 04:22:29 2016
>> New Revision: 286353
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=286353&view=rev
>> Log:
>> [ELF] ARM and AArch64 undefined weak reference values
>>
>> The ARM 32 and 64-bit ABI does not use 0 for undefined weak references
>> that are used in PC relative relocations. In particular:
>> - A branch relocation to an undefined weak resolves to the next
>> instruction. Effectively making the branch a no-op
>> - In all other cases the symbol resolves to the place so that S + A - P
>> resolves to A.
>>
>> Differential Revision: https://reviews.llvm.org/D26240
>>
>>
>> Added:
>> lld/trunk/test/ELF/aarch64-undefined-weak.s (with props)
>> lld/trunk/test/ELF/arm-thumb-undefined-weak.s (with props)
>> lld/trunk/test/ELF/arm-undefined-weak.s (with props)
>> Modified:
>> lld/trunk/ELF/InputSection.cpp
>> lld/trunk/test/ELF/arm-thumb-no-undefined-thunk.s
>>
>> Modified: lld/trunk/ELF/InputSection.cpp
>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputSection.cpp?rev=286353&r1=286352&r2=286353&view=diff
>> ==============================================================================
>> --- lld/trunk/ELF/InputSection.cpp (original)
>> +++ lld/trunk/ELF/InputSection.cpp Wed Nov 9 04:22:29 2016
>> @@ -258,6 +258,42 @@ static uint64_t getAArch64Page(uint64_t
>> return Expr & (~static_cast<uint64_t>(0xFFF));
>> }
>>
>> +static uint32_t getARMUndefinedRelativeWeakVA(uint32_t Type,
>> + uint32_t A,
>> + uint32_t P) {
>> + switch (Type) {
>> + case R_ARM_THM_JUMP11:
>> + return P + 2;
>> + case R_ARM_CALL:
>> + case R_ARM_JUMP24:
>> + case R_ARM_PC24:
>> + case R_ARM_PLT32:
>> + case R_ARM_PREL31:
>> + case R_ARM_THM_JUMP19:
>> + case R_ARM_THM_JUMP24:
>> + return P + 4;
>> + case R_ARM_THM_CALL:
>> + // We don't want an interworking BLX to ARM
>> + return P + 5;
>> + default:
>> + return A;
>> + }
>> +}
>> +
>> +static uint64_t getAArch64UndefinedRelativeWeakVA(uint64_t Type,
>> + uint64_t A,
>> + uint64_t P) {
>> + switch (Type) {
>> + case R_AARCH64_CALL26:
>> + case R_AARCH64_CONDBR19:
>> + case R_AARCH64_JUMP26:
>> + case R_AARCH64_TSTBR14:
>> + return P + 4;
>> + default:
>> + return A;
>> + }
>> +}
>> +
>> template <class ELFT>
>> static typename ELFT::uint getSymVA(uint32_t Type, typename ELFT::uint A,
>> typename ELFT::uint P,
>> @@ -373,10 +409,20 @@ static typename ELFT::uint getSymVA(uint
>> return SymVA - P;
>> }
>> case R_PC:
>> + if (Body.isUndefined() && !Body.isLocal() && Body.symbol()->isWeak()) {
>> + // On ARM and AArch64 a branch to an undefined weak resolves to the
>> + // next instruction, otherwise the place.
>> + if (Config->EMachine == EM_ARM)
>> + return getARMUndefinedRelativeWeakVA(Type, A, P);
>> + if (Config->EMachine == EM_AARCH64)
>> + return getAArch64UndefinedRelativeWeakVA(Type, A, P);
>> + }
>> case R_RELAX_GOT_PC:
>> return Body.getVA<ELFT>(A) - P;
>> case R_PLT_PAGE_PC:
>> case R_PAGE_PC:
>> + if (Body.isUndefined() && !Body.isLocal() && Body.symbol()->isWeak())
>> + return getAArch64Page(A);
>> return getAArch64Page(Body.getVA<ELFT>(A)) - getAArch64Page(P);
>> }
>> llvm_unreachable("Invalid expression");
>>
>> Added: lld/trunk/test/ELF/aarch64-undefined-weak.s
>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/aarch64-undefined-weak.s?rev=286353&view=auto
>> ==============================================================================
>> --- lld/trunk/test/ELF/aarch64-undefined-weak.s (added)
>> +++ lld/trunk/test/ELF/aarch64-undefined-weak.s Wed Nov 9 04:22:29 2016
>> @@ -0,0 +1,45 @@
>> +// RUN: llvm-mc -filetype=obj -triple=aarch64-none-linux %s -o %t
>> +// RUN: ld.lld %t -o %t2 2>&1
>> +// RUN: llvm-objdump -triple=aarch64-none-linux -d %t2 | FileCheck %s
>> +// REQUIRES: aarch64
>> +
>> +// Check that the ARM 64-bit ABI rules for undefined weak symbols are applied.
>> +// Branch instructions are resolved to the next instruction. Undefined
>> +// Symbols in relative are resolved to the place so S - P + A = A.
>> +
>> + .weak target
>> +
>> + .text
>> + .global _start
>> +_start:
>> +// R_AARCH64_JUMP26
>> + b target
>> +// R_AARCH64_CALL26
>> + bl target
>> +// R_AARCH64_CONDBR19
>> + b.eq target
>> +// R_AARCH64_TSTBR14
>> + cbz x1, target
>> +// R_AARCH64_ADR_PREL_LO21
>> + adr x0, target
>> +// R_AARCH64_ADR_PREL_PG_HI21
>> + adrp x0, target
>> +// R_AARCH64_PREL32
>> + .word target - .
>> +// R_AARCH64_PREL64
>> + .xword target - .
>> +// R_AARCH64_PREL16
>> + .hword target - .
>> +
>> +// CHECK: Disassembly of section .text:
>> +// 131076 = 0x20004
>> +// CHECK: 20000: 01 80 00 14 b #131076
>> +// CHECK-NEXT: 20004: 02 80 00 94 bl #131080
>> +// CHECK-NEXT: 20008: 60 00 10 54 b.eq #131084
>> +// CHECK-NEXT: 2000c: 81 00 10 b4 cbz x1, #131088
>> +// CHECK-NEXT: 20010: 00 00 00 10 adr x0, #0
>> +// CHECK-NEXT: 20014: 00 00 00 90 adrp x0, #0
>> +// CHECK: 20018: 00 00 00 00 .word 0x00000000
>> +// CHECK-NEXT: 2001c: 00 00 00 00 .word 0x00000000
>> +// CHECK-NEXT: 20020: 00 00 00 00 .word 0x00000000
>> +// CHECK-NEXT: 20024: 00 00 .short 0x0000
>>
>> Propchange: lld/trunk/test/ELF/aarch64-undefined-weak.s
>> ------------------------------------------------------------------------------
>> svn:eol-style = native
>>
>> Propchange: lld/trunk/test/ELF/aarch64-undefined-weak.s
>> ------------------------------------------------------------------------------
>> svn:keywords = Rev Date Author URL Id
>>
>> Modified: lld/trunk/test/ELF/arm-thumb-no-undefined-thunk.s
>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/arm-thumb-no-undefined-thunk.s?rev=286353&r1=286352&r2=286353&view=diff
>> ==============================================================================
>> --- lld/trunk/test/ELF/arm-thumb-no-undefined-thunk.s (original)
>> +++ lld/trunk/test/ELF/arm-thumb-no-undefined-thunk.s Wed Nov 9 04:22:29 2016
>> @@ -18,6 +18,7 @@ _start:
>>
>> // 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
>> +// 69636 = 0x11004 = next instruction
>> +// CHECK: 11000: 11 f0 02 f8 bl #69636
>> +// CHECK-NEXT: 11004: 11 f0 04 b8 b.w #69640
>> +// CHECK-NEXT: 11008: 11 f0 06 b8 b.w #69644
>>
>> Added: lld/trunk/test/ELF/arm-thumb-undefined-weak.s
>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/arm-thumb-undefined-weak.s?rev=286353&view=auto
>> ==============================================================================
>> --- lld/trunk/test/ELF/arm-thumb-undefined-weak.s (added)
>> +++ lld/trunk/test/ELF/arm-thumb-undefined-weak.s Wed Nov 9 04:22:29 2016
>> @@ -0,0 +1,38 @@
>> +// 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 the ARM ABI rules for undefined weak symbols are applied.
>> +// Branch instructions are resolved to the next instruction. Relative
>> +// relocations are resolved to the place.
>> +
>> + .syntax unified
>> +
>> + .weak target
>> +
>> + .text
>> + .global _start
>> +_start:
>> +// R_ARM_THM_JUMP19
>> + beq.w target
>> +// R_ARM_THM_JUMP24
>> + b.w target
>> +// R_ARM_THM_CALL
>> + bl target
>> +// R_ARM_THM_CALL with exchange
>> + blx target
>> +// R_ARM_THM_MOVT_PREL
>> + movt r0, :upper16:target - .
>> +// R_ARM_THM_MOVW_PREL_NC
>> + movw r0, :lower16:target - .
>> +
>> +// CHECK: Disassembly of section .text:
>> +// 69636 = 0x11004
>> +// CHECK: 11000: 11 f0 02 80 beq.w #69636
>> +// CHECK-NEXT: 11004: 11 f0 04 b8 b.w #69640
>> +// CHECK-NEXT: 11008: 11 f0 06 f8 bl #69644
>> +// blx is transformed into bl so we don't change state
>> +// CHECK-NEXT: 1100c: 11 f0 08 f8 bl #69648
>> +// CHECK-NEXT: 11010: c0 f2 00 00 movt r0, #0
>> +// CHECK-NEXT: 11014: 40 f2 00 00 movw r0, #0
>>
>> Propchange: lld/trunk/test/ELF/arm-thumb-undefined-weak.s
>> ------------------------------------------------------------------------------
>> svn:eol-style = native
>>
>> Propchange: lld/trunk/test/ELF/arm-thumb-undefined-weak.s
>> ------------------------------------------------------------------------------
>> svn:keywords = Rev Date Author URL Id
>>
>> Added: lld/trunk/test/ELF/arm-undefined-weak.s
>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/arm-undefined-weak.s?rev=286353&view=auto
>> ==============================================================================
>> --- lld/trunk/test/ELF/arm-undefined-weak.s (added)
>> +++ lld/trunk/test/ELF/arm-undefined-weak.s Wed Nov 9 04:22:29 2016
>> @@ -0,0 +1,39 @@
>> +// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t
>> +// RUN: ld.lld %t -o %t2 2>&1
>> +// RUN: llvm-objdump -triple=armv7a-none-linux-gnueabi -d %t2 | FileCheck %s
>> +// REQUIRES: arm
>> +
>> +// Check that the ARM ABI rules for undefined weak symbols are applied.
>> +// Branch instructions are resolved to the next instruction. Undefined
>> +// Symbols in relative are resolved to the place so S - P + A = A.
>> +
>> + .syntax unified
>> +
>> + .weak target
>> +
>> + .text
>> + .global _start
>> +_start:
>> +// R_ARM_JUMP24
>> + b target
>> +// R_ARM_CALL
>> + bl target
>> +// R_ARM_CALL with exchange
>> + blx target
>> +// R_ARM_MOVT_PREL
>> + movt r0, :upper16:target - .
>> +// R_ARM_MOVW_PREL_NC
>> + movw r0, :lower16:target - .
>> +// R_ARM_REL32
>> + .word target - .
>> +
>> +// CHECK: Disassembly of section .text:
>> +// 69636 = 0x11004
>> +// CHECK: 11000: 01 44 00 ea b #69636
>> +// CHECK-NEXT: 11004: 02 44 00 eb bl #69640
>> +// blx is transformed into bl so we don't change state
>> +// CHECK-NEXT: 11008: 03 44 00 eb bl #69644
>> +// CHECK-NEXT: 1100c: 00 00 40 e3 movt r0, #0
>> +// CHECK-NEXT: 11010: 00 00 00 e3 movw r0, #0
>> +// CHECK: 11014: 00 00 00 00 .word 0x00000000
>> +
>>
>> Propchange: lld/trunk/test/ELF/arm-undefined-weak.s
>> ------------------------------------------------------------------------------
>> svn:eol-style = native
>>
>> Propchange: lld/trunk/test/ELF/arm-undefined-weak.s
>> ------------------------------------------------------------------------------
>> svn:keywords = Rev Date Author URL Id
>>
>>
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