[llvm-dev] Do I need to modify the AddrLoc of LLD for ARC target?

Thu Sep 14 21:33:12 PDT 2017


在 2017年09月14日 17:36, Peter Smith 写道:
> Hello Leslie,
>
> I think we are going to need to know a bit more about the ELF ABI for
> what looks like the ArcCompact before we can help you.
>
> LLD's calculation of P (the place to be relocated) is as it is in the
> generic ELF specification. The Rel.Offset corresponds to the ELF
> r_offset field. This is covered by: "For a relocatable file, the value
> is the byte offset from the beginning of the section to the storage
> unit affected by the relocation."
>
> For LLD we are calculating the virtual address (VA) of P, as I
> understand it this is equivalent to the vma used in BFD. Assuming that
> the relocation is relocating a regular InputSection from the
> basic-arc.o object then the LLD calculation of P =
> getOutputSection()->Addr + getOffset(Rel.Offset); translates to: (VA
> of OutputSection) + (Offset of InputSection within OutputSection) +
> (Offset within InputSection given by r_offset)
>
> The BFD linker seems to be doing the equivalent calculation with an
> extra modification of the (Offset within InputSection given by
> r_offset) and is rounding down the result to the nearest 4-byte
> boundary. This looks unfamiliar to me, and could well be specific to
> ArcCompact. I think that you will need to refer to the ELF ABI
> documentation as this should tell you if there are any processor
> specific modifications to generic ELF that you have to follow.
>
> The other thing that you should do is try and work out why the VA
> (vma) is 6 in LD and 8 in LLD and whether this is actually a problem.
> The VA of the OutputSection is not guaranteed to be the same between
> different linkers so it may have just been that differences in order
> of InputSections or alignment has caused a different VA. I would check
> the output of the linker map file to see where it placed the Output
> and Input Sections to see what the answer should be.

VMA in LD and LLD is the same for AVR target https://reviews.llvm.org/D37615

$ avr-gcc -mmcu=atmega328p -o basic-avr.o -c basic-avr.s
$ avr-ld -o basic-avr basic-avr.o -Ttext=6 /*with-different-high-address*/

DEBUG: avr-ld: R_AVR_CALL: VMA: 6 Output Offset: 0 Reloc Offset: 0
DEBUG: avr-ld: R_AVR_16_PM: VMA: 6 Output Offset: 0 Reloc Offset: 4
DEBUG: avr-ld: R_AVR_8: VMA: 6 Output Offset: 0 Reloc Offset: 6
DEBUG: avr-ld: R_AVR_8_LO8: VMA: 6 Output Offset: 0 Reloc Offset: 7
DEBUG: avr-ld: R_AVR_8_HI8: VMA: 6 Output Offset: 0 Reloc Offset: 8
DEBUG: avr-ld: R_AVR_8_HLO8: VMA: 6 Output Offset: 0 Reloc Offset: 9
DEBUG: avr-ld: R_AVR_LO8_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 10
DEBUG: avr-ld: R_AVR_HI8_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 12
DEBUG: avr-ld: R_AVR_HH8_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 14
DEBUG: avr-ld: R_AVR_MS8_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 16
DEBUG: avr-ld: R_AVR_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 18
DEBUG: avr-ld: R_AVR_LO8_LDI_NEG: VMA: 6 Output Offset: 0 Reloc Offset: 20
DEBUG: avr-ld: R_AVR_HI8_LDI_NEG: VMA: 6 Output Offset: 0 Reloc Offset: 22
DEBUG: avr-ld: R_AVR_HH8_LDI_NEG: VMA: 6 Output Offset: 0 Reloc Offset: 24
DEBUG: avr-ld: R_AVR_MS8_LDI_NEG: VMA: 6 Output Offset: 0 Reloc Offset: 26
DEBUG: avr-ld: R_AVR_LO8_LDI_PM: VMA: 6 Output Offset: 0 Reloc Offset: 28
DEBUG: avr-ld: R_AVR_HI8_LDI_PM: VMA: 6 Output Offset: 0 Reloc Offset: 30
DEBUG: avr-ld: R_AVR_HH8_LDI_PM: VMA: 6 Output Offset: 0 Reloc Offset: 32
DEBUG: avr-ld: R_AVR_LO8_LDI_PM_NEG: VMA: 6 Output Offset: 0 Reloc 
Offset: 34
DEBUG: avr-ld: R_AVR_HI8_LDI_PM_NEG: VMA: 6 Output Offset: 0 Reloc 
Offset: 36
DEBUG: avr-ld: R_AVR_HH8_LDI_PM_NEG: VMA: 6 Output Offset: 0 Reloc 
Offset: 38
DEBUG: avr-ld: R_AVR_7_PCREL: VMA: 6 Output Offset: 0 Reloc Offset: 40
DEBUG: avr-ld: R_AVR_13_PCREL: VMA: 6 Output Offset: 0 Reloc Offset: 42
DEBUG: avr-ld: R_AVR_6: VMA: 6 Output Offset: 0 Reloc Offset: 52
DEBUG: avr-ld: R_AVR_16: VMA: 6 Output Offset: 0 Reloc Offset: 56
DEBUG: avr-ld: R_AVR_LO8_LDI_GS: VMA: 6 Output Offset: 0 Reloc Offset: 58
DEBUG: avr-ld: R_AVR_HI8_LDI_GS: VMA: 6 Output Offset: 0 Reloc Offset: 60
DEBUG: avr-ld: R_AVR_PORT6: VMA: 6 Output Offset: 0 Reloc Offset: 62
DEBUG: avr-ld: R_AVR_PORT5: VMA: 6 Output Offset: 0 Reloc Offset: 64
DEBUG: avr-ld: R_AVR_6_ADIW: VMA: 6 Output Offset: 0 Reloc Offset: 66

$ llvm/build/bin/ld.lld -o basic-avr-lld basic-avr.o -Ttext=6 
/*with-different-high-address*/

DEBUG: lld: R_AVR_CALL TargetVA: 50 A: 44 P: 6 VMA: 6 Output Offset: 0 
Reloc Offset: 0
DEBUG: lld: R_AVR_16_PM TargetVA: 50 A: 44 P: 10 VMA: 6 Output Offset: 0 
Reloc Offset: 4
DEBUG: lld: R_AVR_8 TargetVA: 50 A: 44 P: 12 VMA: 6 Output Offset: 0 
Reloc Offset: 6
DEBUG: lld: R_AVR_8_LO8 TargetVA: 50 A: 44 P: 13 VMA: 6 Output Offset: 0 
Reloc Offset: 7
DEBUG: lld: R_AVR_8_HI8 TargetVA: 50 A: 44 P: 14 VMA: 6 Output Offset: 0 
Reloc Offset: 8
DEBUG: lld: R_AVR_8_HLO8 TargetVA: 50 A: 44 P: 15 VMA: 6 Output Offset: 
0 Reloc Offset: 9
DEBUG: lld: R_AVR_LO8_LDI TargetVA: 50 A: 44 P: 16 VMA: 6 Output Offset: 
0 Reloc Offset: 10
DEBUG: lld: R_AVR_HI8_LDI TargetVA: 50 A: 44 P: 18 VMA: 6 Output Offset: 
0 Reloc Offset: 12
DEBUG: lld: R_AVR_HH8_LDI TargetVA: 50 A: 44 P: 20 VMA: 6 Output Offset: 
0 Reloc Offset: 14
DEBUG: lld: R_AVR_MS8_LDI TargetVA: 50 A: 44 P: 22 VMA: 6 Output Offset: 
0 Reloc Offset: 16
DEBUG: lld: R_AVR_LDI TargetVA: 50 A: 44 P: 24 VMA: 6 Output Offset: 0 
Reloc Offset: 18
DEBUG: lld: R_AVR_LO8_LDI_NEG TargetVA: 50 A: 44 P: 26 VMA: 6 Output 
Offset: 0 Reloc Offset: 20
DEBUG: lld: R_AVR_HI8_LDI_NEG TargetVA: 50 A: 44 P: 28 VMA: 6 Output 
Offset: 0 Reloc Offset: 22
DEBUG: lld: R_AVR_HH8_LDI_NEG TargetVA: 50 A: 44 P: 30 VMA: 6 Output 
Offset: 0 Reloc Offset: 24
DEBUG: lld: R_AVR_MS8_LDI_NEG TargetVA: 50 A: 44 P: 32 VMA: 6 Output 
Offset: 0 Reloc Offset: 26
DEBUG: lld: R_AVR_LO8_LDI_PM TargetVA: 50 A: 44 P: 34 VMA: 6 Output 
Offset: 0 Reloc Offset: 28
DEBUG: lld: R_AVR_HI8_LDI_PM TargetVA: 50 A: 44 P: 36 VMA: 6 Output 
Offset: 0 Reloc Offset: 30
DEBUG: lld: R_AVR_HH8_LDI_PM TargetVA: 50 A: 44 P: 38 VMA: 6 Output 
Offset: 0 Reloc Offset: 32
DEBUG: lld: R_AVR_LO8_LDI_PM_NEG TargetVA: 50 A: 44 P: 40 VMA: 6 Output 
Offset: 0 Reloc Offset: 34
DEBUG: lld: R_AVR_HI8_LDI_PM_NEG TargetVA: 50 A: 44 P: 42 VMA: 6 Output 
Offset: 0 Reloc Offset: 36
DEBUG: lld: R_AVR_HH8_LDI_PM_NEG TargetVA: 50 A: 44 P: 44 VMA: 6 Output 
Offset: 0 Reloc Offset: 38
DEBUG: lld: R_AVR_7_PCREL TargetVA: 4 A: 44 P: 46 VMA: 6 Output Offset: 
0 Reloc Offset: 40
DEBUG: lld: R_AVR_13_PCREL TargetVA: 2 A: 44 P: 48 VMA: 6 Output Offset: 
0 Reloc Offset: 42
DEBUG: lld: R_AVR_6 TargetVA: 50 A: 44 P: 58 VMA: 6 Output Offset: 0 
Reloc Offset: 52
DEBUG: lld: R_AVR_16 TargetVA: 50 A: 44 P: 62 VMA: 6 Output Offset: 0 
Reloc Offset: 56
DEBUG: lld: R_AVR_LO8_LDI_GS TargetVA: 50 A: 44 P: 64 VMA: 6 Output 
Offset: 0 Reloc Offset: 58
DEBUG: lld: R_AVR_HI8_LDI_GS TargetVA: 50 A: 44 P: 66 VMA: 6 Output 
Offset: 0 Reloc Offset: 60
DEBUG: lld: R_AVR_PORT6 TargetVA: 51 A: 45 P: 68 VMA: 6 Output Offset: 0 
Reloc Offset: 62
DEBUG: lld: R_AVR_PORT5 TargetVA: 6 A: 0 P: 70 VMA: 6 Output Offset: 0 
Reloc Offset: 64
DEBUG: lld: R_AVR_6_ADIW TargetVA: 50 A: 44 P: 72 VMA: 6 Output Offset: 
0 Reloc Offset: 66

interesting :)

>
> In summary:
> It looks like there are some Arc specific things that might need to be
> done. Unfortunately I don't have any experience with Arc, and I'm not
> sure the other people that work on LLD do either. I suggest looking at
> the public ABI documentation and making any arguments for changes
> based on that documentation, it is worth assuming that we know nothing
> about Arc, don't have the documentation to hand and don't know where
> to find it!
>
> Hope that is of some help, with a bit more context I might be able to
> help a bit more, unfortunately I can't spend a lot of time learning
> about Arc.
>
> Peter
>
>
> On 14 September 2017 at 07:16, Leslie Zhai via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Hi LLVM developers,
>>
>> basic-arc.s:
>>
>> main:
>>    bl memset
>>
>> $ arc-elf32-gcc -mcpu=arc600 -o basic-arc.o -c
>>
>> $ arc-elf32-readelf -r basic-arc.o
>>
>> Relocation section '.rela.text' at offset 0xd4 contains 1 entries:
>>   Offset     Info    Type            Sym.Value  Sym. Name + Addend
>> 00000000  00000611 R_ARC_S25W_PCREL  00000000   memset + 0
>>
>> High address: 0x0
>>
>> $ arc-elf32-ld -o basic-arc basic-arc.o
>> -L/opt/arc-gnu/lib/gcc/arc-elf32/7.1.1/arc600
>> -L/opt/arc-gnu/lib/gcc/arc-elf32/7.1.1/../../../../arc-elf32/lib/arc600
>> -L/opt/arc-gnu/lib/gcc/arc-elf32/7.1.1
>> -L/opt/arc-gnu/lib/gcc/arc-elf32/7.1.1/../../../../arc-elf32/lib
>> --start-group -lgcc -lc -lnosys --end-group -Ttext=0
>>
>> DEBUG: arc-ld: R_ARC_S25W_PCREL relocation: 1 S: 4 A: 0 P: 0 = (vma: 0 +
>> output_offset: 0 + reloc_offset: 0 - 0) & ~0x3
>> DEBUG: arc-ld: type: R_ARC_S25W_PCREL insn: 2054
>>
>> $ ld.lld -o basic-arc-lld basic-arc.o $ARC_LINKER_LIB -Ttext=0
>>
>> DEBUG: lld: R_ARC_S25W_PCREL TargetVA: 4 A: 0 P: 0 <-- same P as arc-ld
>> DEBUG: lld: R_ARC_S25W_PCREL: Insn: 2050 Rel: 1
>> DEBUG: lld: R_ARC_S25W_PCREL: Insn: 2054 <-- same relocation value as arc-ld
>>
>> But with several different high address *not* 0x0, such as 0x6:
>>
>> DEBUG: arc-ld: R_ARC_S25W_PCREL relocation: 2 S: 12 A: 0 P: 4 = (vma: 6 +
>> output_offset: 0 + reloc_offset: 0 - 0) & ~0x3
>> DEBUG: arc-ld: type: R_ARC_S25W_PCREL insn: 2058
>>
>> DEBUG: lld: R_ARC_S25W_PCREL TargetVA: 4 A: 0 P: 8 <-- different P?
>> DEBUG: lld: R_ARC_S25W_PCREL: Insn: 2050 Rel: 1
>> DEBUG: lld: R_ARC_S25W_PCREL: Insn: 2054 <-- different relocation value
>>
>> How arc-ld calculates P?
>>
>> P = ((reloc_data.input_section->output_section ?
>> reloc_data.input_section->output_section->vma : 0) +
>> reloc_data.input_section->output_offset + (reloc_data.reloc_offset -
>> (reloc_data.bitsize >= 32 ? 4 : 0))) & ~0x3;
>>
>> for example, R_ARC_S25W_PCREL's bitsize < 32, P = (6 + 0 + 0 - 0) & ~0x3 =
>> 4, when vma is 6, output and reloc offset is 0.
>>
>> How LLD calculates P (AddrLoc)?
>>
>> P = getOutputSection()->Addr + getOffset(Rel.Offset);
>>
>> for example, the same high address 0x6, LLD's P is 8, different with arc-ld?
>> so do I need to modify the value of P for R_PC case in the getRelocTargetVA?
>> please give me some hints, thanks a lot!
>>
>>
>> PS: arc-ld R_ARC_S25W_PCREL's FORMULA is: ( S + A ) - P ) >> 2, and it needs
>> middle endian convert, so:
>>
>> Insn = middleEndianConvert (insn, TRUE);
>>
>> Insn = replaceDisp25w(Insn, ( S + A ) - P ) >> 2);
>>
>> Insn = middleEndianConvert (insn, TRUE);
>>
>> write32le(Loc, Insn);
>>
>> --
>> Regards,
>> Leslie Zhai - https://reviews.llvm.org/p/xiangzhai/
>>
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-- 
Regards,
Leslie Zhai - https://reviews.llvm.org/p/xiangzhai/