Peter Smith via llvm-dev
2017-Sep-18 12:44 UTC
[llvm-dev] Do I need to modify the AddrLoc of LLD for ARC target?
Hello Leslie, I don't know quite what to say as I don't know precisely what your question is? If I am not being precise enough please can you put some explicit questions in? From what I can see in the output, here are some comments.>From your arc mapfiles it looks like that in the output both linker'shave given the .text output section the correct base address given the alignment restrictions as the alignment requirement of .text from lib_a-memset-bs.o is 4, therefor the alignment requirement of the OutputSection .text should be 4: LLD: Address Size Alignment 00000000 00000080 4 .text 00000000 00000004 1 basic-arc.o:(.text) 00000000 00000000 0 main 00000004 0000007c 4 ... (lib_a-memset-bs.o):(.text) LD .text 0x0000000000000000 0x80 *(.text .stub .text.* .gnu.linkonce.t.*) .text 0x0000000000000000 0x4 basic-arc.o .text 0x0000000000000004 0x7c ... libc.a(lib_a-memset-bs.o) 0x0000000000000004 memset 0x0000000000000060 __strncpy_bzero I'm not entirely sure where the Arm example has come from, but it does show an interesting difference. It looks like the linker's are handling the -ttext <address> option slightly differently when the <address> of the OutputSection is not 0 modulo OutputSection alignment.>From the map file we can see that lld is aligning the OutputSection tothe nearest 4-byte boundary, GNU-ld is placing the OutputSection on the requested address, but is adding padding before the .text section to make sure that in the final executable the InputSection is aligned. LLD Address Size Align Out In Symbol 00011008 00000018 4 .text 00011008 00000018 4 arm-thumb-undefined-weak.o:(.text) 00011008 00000000 0 $t.0 00011008 00000000 0 _start LD .text 0x0000000000011006 0x1a ... *fill* 0x0000000000011006 0x2 .text 0x0000000000011008 0x18 arm-thumb-undefined-weak.o The *fill* is visible as a nop in the disassembly for the LD produced image. Strictly speaking I think LD is producing a file that doesn't strictly conform to ELF here as the sh_addr of the .text OutputSection is 0 modulo sh_addralign (4). In practice it probably wouldn't make much difference. My preference is for LLD's behaviour here. Peter On 18 September 2017 at 03:28, Leslie Zhai <lesliezhai at llvm.org.cn> wrote:> Hi Peter, > > Map file about LD for ARC target > https://drive.google.com/open?id=0ByE8c-y74l_uRWpQdUh2c0VXZ1k > > LLD for ARC https://drive.google.com/open?id=0ByE8c-y74l_ueGVuYkR0a3RSWjQ > > > arm-thumb-undefined-weak.s > https://github.com/llvm-mirror/lld/blob/master/test/ELF/arm-thumb-undefined-weak.s > > $ llvm/build/bin/llvm-mc -filetype=obj -triple=thumbv7a-none-linux-gnueabi > arm-thumb-undefined-weak.s -o arm-thumb-undefined-weak.o > $ llvm/build/bin/ld.lld -o arm-thumb-undefined-weak-lld > arm-thumb-undefined-weak.o -Ttext=11006 > $ arm-linux-gnu-ld -o arm-thumb-undefined-weak-ld arm-thumb-undefined-weak.o > -Ttext=11006 > > $ arm-linux-gnu-readelf -r arm-thumb-undefined-weak.o > > Relocation section '.rel.text' at offset 0x8c contains 6 entries: > Offset Info Type Sym.Value Sym. Name > 00000000 00000333 R_ARM_THM_JUMP19 00000000 target > 00000004 0000031e R_ARM_THM_JUMP24 00000000 target > 00000008 0000030a R_ARM_THM_CALL 00000000 target > 0000000c 0000030a R_ARM_THM_CALL 00000000 target > 00000010 00000332 R_ARM_THM_MOVT_PR 00000000 target > 00000014 00000331 R_ARM_THM_MOVW_PR 00000000 target > > > DEBUG: lld: R_ARM_THM_JUMP19 TargetVA: 0 A: -4 P: 69640 Align: 4 VMA: 69640 > Output Offset: 0 Reloc Offset: 0 > DEBUG: lld: R_ARM_THM_JUMP24 TargetVA: 0 A: -4 P: 69644 Align: 4 VMA: 69640 > Output Offset: 0 Reloc Offset: 4 > DEBUG: lld: R_ARM_THM_CALL TargetVA: 1 A: -4 P: 69648 Align: 4 VMA: 69640 > Output Offset: 0 Reloc Offset: 8 > DEBUG: lld: R_ARM_THM_CALL TargetVA: 1 A: -4 P: 69652 Align: 4 VMA: 69640 > Output Offset: 0 Reloc Offset: 12 > DEBUG: lld: R_ARM_THM_MOVT_PREL TargetVA: 0 A: 0 P: 69656 Align: 4 VMA: > 69640 Output Offset: 0 Reloc Offset: 16 > DEBUG: lld: R_ARM_THM_MOVW_PREL_NC TargetVA: 0 A: 0 P: 69660 Align: 4 VMA: > 69640 Output Offset: 0 Reloc Offset: 20 > > DEBUG: arm-linux-gnu-ld: R_ARM_THM_JUMP19: VMA: 69638 Output Offset: 2 Reloc > Offset: 0 > DEBUG: arm-linux-gnu-ld: R_ARM_THM_JUMP24: VMA: 69638 Output Offset: 2 Reloc > Offset: 4 > DEBUG: arm-linux-gnu-ld: R_ARM_THM_CALL: VMA: 69638 Output Offset: 2 Reloc > Offset: 8 > DEBUG: arm-linux-gnu-ld: R_ARM_THM_CALL: VMA: 69638 Output Offset: 2 Reloc > Offset: 12 > DEBUG: arm-linux-gnu-ld: R_ARM_THM_MOVT_PREL: VMA: 69638 Output Offset: 2 > Reloc Offset: 16 > DEBUG: arm-linux-gnu-ld: R_ARM_THM_MOVW_PREL_NC: VMA: 69638 Output Offset: 2 > Reloc Offset: 20 > > > $ llvm/build/bin/llvm-objdump -triple=thumbv7a-none-linux-gnueabi -d > arm-thumb-undefined-weak-lld > > arm-thumb-undefined-weak-lld: file format ELF32-arm-little > > Disassembly of section .text: > _start: > 11008: 00 f0 00 80 beq.w #0 <_start+0x4> > 1100c: 00 f0 00 b8 b.w #0 <_start+0x8> > 11010: 00 f0 00 f8 bl #0 > 11014: 00 f0 00 f8 bl #0 > 11018: c0 f2 00 00 movt r0, #0 > 1101c: 40 f2 00 00 movw r0, #0 > > > $ llvm/build/bin/llvm-objdump -triple=thumbv7a-none-linux-gnueabi -d > arm-thumb-undefined-weak-ld > > arm-thumb-undefined-weak-ld: file format ELF32-arm-little > > Disassembly of section .text: > .text: > 11006: 00 00 movs r0, r0 > > _start: > 11008: 2e f4 fa af beq.w #-69644 > 1100c: 00 e0 b #0 <_start+0x8> > 1100e: 00 bf nop > 11010: 00 e0 b #0 <_start+0xC> > 11012: 00 bf nop > 11014: 00 e0 b #0 <_start+0x10> > 11016: 00 bf nop > 11018: cf f6 fe 70 movt r0, #65534 > 1101c: 4e f6 e4 70 movw r0, #61412 > > > > > 在 2017年09月15日 20:49, Peter Smith 写道: >> >> Just a thought I had about the calculation of P. I think that >> following the ld approach too closely may be a mistake. >> >> I'm speculating that the reason for this change in the value of P is >> similar to the situation in Arm for a Thumb BLX immediate instruction >> (Branch Link and Exchange with the immediate an offset from the PC). >> When calculating the target address the immediate is added to >> Align(PC, 4) where Align rounds down to nearest 4-byte boundary. The >> linker needs to account for this when resolving the relocation >> R_ARM_THM_CALL. >> >> To handle the alignment difference for this one special case in lld I >> accounted for the alignment difference in relocateOne. You may be able >> to use a similar method for Arc rather than writing modifyARCAddrLoc. >> Again I know nothing about Arc so you'll need to look at the >> Architecture reference manual to understand what the instruction the >> relocation applies to works. >> >> Peter >> >> >> On 15 September 2017 at 04:19, Leslie Zhai <lesliezhai at llvm.org.cn> wrote: >>> >>> Hi Peter, >>> >>> Thanks for your kind response! >>> >>> >>> 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. >>> >>> https://github.com/foss-for-synopsys-dwc-arc-processors/arc-ABI-manual >>> >>> But I prefer to read bfd linker's source code about ARC instead: >>> 1. Specific e_flags >>> >>> https://github.com/foss-for-synopsys-dwc-arc-processors/binutils-gdb/blob/arc-2017.09/include/elf/arc.h >>> 2. Relocation define >>> >>> https://github.com/foss-for-synopsys-dwc-arc-processors/binutils-gdb/blob/arc-2017.09/include/elf/arc-reloc.def >>> 3. Relocation replace function >>> >>> https://github.com/foss-for-synopsys-dwc-arc-processors/binutils-gdb/blob/arc-2017.09/include/opcode/arc-func.h >>> 4. Calculation of S, A, P, PDATA, GOT, etc. >>> >>> https://github.com/foss-for-synopsys-dwc-arc-processors/binutils-gdb/blob/arc-2017.09/bfd/elf32-arc.c#L1156 >>> >>> >>>> 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. >>> >>> I implemented the MOD P for ARC: >>> >>> static void modifyARCAddrLoc(uint64_t &AddrLoc, const uint16_t EMachine, >>> RelExpr Expr, uint32_t Type, uint64_t VMA, >>> uint64_t OutSecOff, uint64_t RelOff) { >>> if (EMachine != EM_ARC_COMPACT || EMachine != EM_ARC_COMPACT2 || >>> Expr != R_PC || Expr != R_GOT_PC) { >>> return; >>> } >>> >>> uint64_t M = 0; >>> if (Type == R_ARC_32_PCREL || Type == R_ARC_PC32 || Type =>>> R_ARC_GOTPC32 >>> || >>> Type == R_ARC_GOTPC) { >>> M = 4; // bitsize >= 32 ? 4 : 0 >>> } >>> AddrLoc = (VMA + OutSecOff + RelOff - M) & ~0x3; >>> } >>> >>> modifyARCAddrLoc(AddrLoc, Config->EMachine, Expr, Type, >>> getOutputSection()->Addr, <-- VMA is important! >>> cast<InputSection>(this)->OutSecOff, Rel.Offset); >>> >>> >>>> 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. >>> >>> LLD's getOutputSection()->Addr >>> https://github.com/llvm-mirror/lld/blob/master/ELF/LinkerScript.cpp#L530 >>> >>> >>> >>>> 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/ >>> >>> >>> > > -- > Regards, > Leslie Zhai - https://reviews.llvm.org/p/xiangzhai/ > > >
Leslie Zhai via llvm-dev
2017-Sep-19 03:28 UTC
[llvm-dev] Do I need to modify the AddrLoc of LLD for ARC target?
Hi Peter, Thanks for your kind response! 在 2017年09月18日 20:44, Peter Smith 写道:> Hello Leslie, > > I don't know quite what to say as I don't know precisely what your > question is? If I am not being precise enough please can you put some > explicit questions in? From what I can see in the output, here are > some comments. > > From your arc mapfiles it looks like that in the output both linker's > have given the .text output section the correct base address given the > alignment restrictions as the alignment requirement of .text from > lib_a-memset-bs.o is 4, therefor the alignment requirement of the > OutputSection .text should be 4: > LLD: > Address Size Alignment > 00000000 00000080 4 .text > 00000000 00000004 1 basic-arc.o:(.text) > 00000000 00000000 0 main > 00000004 0000007c 4 ... (lib_a-memset-bs.o):(.text) > > LD > .text 0x0000000000000000 0x80 > *(.text .stub .text.* .gnu.linkonce.t.*) > .text 0x0000000000000000 0x4 basic-arc.o > .text 0x0000000000000004 0x7c ... libc.a(lib_a-memset-bs.o) > 0x0000000000000004 memset > 0x0000000000000060 __strncpy_bzeroReloc type=R_ARC_S25W_PCREL, should_relocate = true offset = 0x0, addend = 0x0 Symbol: value = 0x00000000 Symbol Section: section name = .text, output_offset 0x00000006, output_section->vma = 0x00000006 file: lib_a-memset-bs.o Input_section: section name = .text, output_offset 0x00000000, output_section->vma = 0x00000006 changed_address = 0x00000006 file: basic-arc.o RELOC_TYPE = ARC_S25W_PCREL FORMULA = ( ME ( ( ( ( S + A ) - P ) >> 2 ) ) ) S = 0xc A = 0 L = c symbol_section->vma = 0xc symbol_section->vma = 0x6 PCL = 0x4 P = 0x4 G = 0 SDA_OFFSET = 0x2188 SDA_SET = 1 GOT_OFFSET = 0 relocation = 0x000002 before = 0x000802 data = 00000002 (2) (2) after = 0x0000080a then I need to investigate how LD calculate reloc_data.input_section->output_section->vma, it might different with LLD even the same Alignment https://github.com/llvm-mirror/lld/blob/master/ELF/LinkerScript.cpp#L485> > I'm not entirely sure where the Arm example has come from, but it does > show an interesting difference. It looks like the linker's are > handling the -ttext <address> option slightly differently when the > <address> of the OutputSection is not 0 modulo OutputSection > alignment. > > From the map file we can see that lld is aligning the OutputSection to > the nearest 4-byte boundary, GNU-ld is placing the OutputSection on > the requested address, but is adding padding before the .text section > to make sure that in the final executable the InputSection is aligned. > > LLD > Address Size Align Out In Symbol > 00011008 00000018 4 .text > 00011008 00000018 4 arm-thumb-undefined-weak.o:(.text) > 00011008 00000000 0 $t.0 > 00011008 00000000 0 _start > > LD > .text 0x0000000000011006 0x1a > ... > *fill* 0x0000000000011006 0x2 > .text 0x0000000000011008 0x18 arm-thumb-undefined-weak.o > > The *fill* is visible as a nop in the disassembly for the LD produced image. > > Strictly speaking I think LD is producing a file that doesn't strictly > conform to ELF here as the sh_addr of the .text OutputSection is 0 > modulo sh_addralign (4). In practice it probably wouldn't make much > difference. My preference is for LLD's behaviour here.It might be arm-linux-gnu toolchain's issue: $ arm-linux-gnu-gcc -o arm-thumb-undefined-weak-ld.o -c arm-thumb-undefined-weak.s arm-thumb-undefined-weak.s: Assembler messages: arm-thumb-undefined-weak.s:18: Error: width suffixes are invalid in ARM mode -- `beq.w target' arm-thumb-undefined-weak.s:20: Error: width suffixes are invalid in ARM mode -- `b.w target' then arm-linux-gnu-ld might wrongly relocated R_ARM_THM_CALL for arm-thumb-undefined-weak-lld.o generated by llvm-mc.> > Peter > > > > On 18 September 2017 at 03:28, Leslie Zhai <lesliezhai at llvm.org.cn> wrote: >> Hi Peter, >> >> Map file about LD for ARC target >> https://drive.google.com/open?id=0ByE8c-y74l_uRWpQdUh2c0VXZ1k >> >> LLD for ARC https://drive.google.com/open?id=0ByE8c-y74l_ueGVuYkR0a3RSWjQ >> >> >> arm-thumb-undefined-weak.s >> https://github.com/llvm-mirror/lld/blob/master/test/ELF/arm-thumb-undefined-weak.s >> >> $ llvm/build/bin/llvm-mc -filetype=obj -triple=thumbv7a-none-linux-gnueabi >> arm-thumb-undefined-weak.s -o arm-thumb-undefined-weak.o >> $ llvm/build/bin/ld.lld -o arm-thumb-undefined-weak-lld >> arm-thumb-undefined-weak.o -Ttext=11006 >> $ arm-linux-gnu-ld -o arm-thumb-undefined-weak-ld arm-thumb-undefined-weak.o >> -Ttext=11006 >> >> $ arm-linux-gnu-readelf -r arm-thumb-undefined-weak.o >> >> Relocation section '.rel.text' at offset 0x8c contains 6 entries: >> Offset Info Type Sym.Value Sym. Name >> 00000000 00000333 R_ARM_THM_JUMP19 00000000 target >> 00000004 0000031e R_ARM_THM_JUMP24 00000000 target >> 00000008 0000030a R_ARM_THM_CALL 00000000 target >> 0000000c 0000030a R_ARM_THM_CALL 00000000 target >> 00000010 00000332 R_ARM_THM_MOVT_PR 00000000 target >> 00000014 00000331 R_ARM_THM_MOVW_PR 00000000 target >> >> >> DEBUG: lld: R_ARM_THM_JUMP19 TargetVA: 0 A: -4 P: 69640 Align: 4 VMA: 69640 >> Output Offset: 0 Reloc Offset: 0 >> DEBUG: lld: R_ARM_THM_JUMP24 TargetVA: 0 A: -4 P: 69644 Align: 4 VMA: 69640 >> Output Offset: 0 Reloc Offset: 4 >> DEBUG: lld: R_ARM_THM_CALL TargetVA: 1 A: -4 P: 69648 Align: 4 VMA: 69640 >> Output Offset: 0 Reloc Offset: 8 >> DEBUG: lld: R_ARM_THM_CALL TargetVA: 1 A: -4 P: 69652 Align: 4 VMA: 69640 >> Output Offset: 0 Reloc Offset: 12 >> DEBUG: lld: R_ARM_THM_MOVT_PREL TargetVA: 0 A: 0 P: 69656 Align: 4 VMA: >> 69640 Output Offset: 0 Reloc Offset: 16 >> DEBUG: lld: R_ARM_THM_MOVW_PREL_NC TargetVA: 0 A: 0 P: 69660 Align: 4 VMA: >> 69640 Output Offset: 0 Reloc Offset: 20 >> >> DEBUG: arm-linux-gnu-ld: R_ARM_THM_JUMP19: VMA: 69638 Output Offset: 2 Reloc >> Offset: 0 >> DEBUG: arm-linux-gnu-ld: R_ARM_THM_JUMP24: VMA: 69638 Output Offset: 2 Reloc >> Offset: 4 >> DEBUG: arm-linux-gnu-ld: R_ARM_THM_CALL: VMA: 69638 Output Offset: 2 Reloc >> Offset: 8 >> DEBUG: arm-linux-gnu-ld: R_ARM_THM_CALL: VMA: 69638 Output Offset: 2 Reloc >> Offset: 12 >> DEBUG: arm-linux-gnu-ld: R_ARM_THM_MOVT_PREL: VMA: 69638 Output Offset: 2 >> Reloc Offset: 16 >> DEBUG: arm-linux-gnu-ld: R_ARM_THM_MOVW_PREL_NC: VMA: 69638 Output Offset: 2 >> Reloc Offset: 20 >> >> >> $ llvm/build/bin/llvm-objdump -triple=thumbv7a-none-linux-gnueabi -d >> arm-thumb-undefined-weak-lld >> >> arm-thumb-undefined-weak-lld: file format ELF32-arm-little >> >> Disassembly of section .text: >> _start: >> 11008: 00 f0 00 80 beq.w #0 <_start+0x4> >> 1100c: 00 f0 00 b8 b.w #0 <_start+0x8> >> 11010: 00 f0 00 f8 bl #0 >> 11014: 00 f0 00 f8 bl #0 >> 11018: c0 f2 00 00 movt r0, #0 >> 1101c: 40 f2 00 00 movw r0, #0My question: why LD's relocation is different from LLD? and thanks for your explanation :)>> >> >> $ llvm/build/bin/llvm-objdump -triple=thumbv7a-none-linux-gnueabi -d >> arm-thumb-undefined-weak-ld >> >> arm-thumb-undefined-weak-ld: file format ELF32-arm-little >> >> Disassembly of section .text: >> .text: >> 11006: 00 00 movs r0, r0 >> >> _start: >> 11008: 2e f4 fa af beq.w #-69644 >> 1100c: 00 e0 b #0 <_start+0x8> >> 1100e: 00 bf nop >> 11010: 00 e0 b #0 <_start+0xC> >> 11012: 00 bf nop >> 11014: 00 e0 b #0 <_start+0x10> >> 11016: 00 bf nop >> 11018: cf f6 fe 70 movt r0, #65534 >> 1101c: 4e f6 e4 70 movw r0, #61412 >> >> >> >> >> 在 2017年09月15日 20:49, Peter Smith 写道: >>> Just a thought I had about the calculation of P. I think that >>> following the ld approach too closely may be a mistake. >>> >>> I'm speculating that the reason for this change in the value of P is >>> similar to the situation in Arm for a Thumb BLX immediate instruction >>> (Branch Link and Exchange with the immediate an offset from the PC). >>> When calculating the target address the immediate is added to >>> Align(PC, 4) where Align rounds down to nearest 4-byte boundary. The >>> linker needs to account for this when resolving the relocation >>> R_ARM_THM_CALL. >>> >>> To handle the alignment difference for this one special case in lld I >>> accounted for the alignment difference in relocateOne. You may be able >>> to use a similar method for Arc rather than writing modifyARCAddrLoc. >>> Again I know nothing about Arc so you'll need to look at the >>> Architecture reference manual to understand what the instruction the >>> relocation applies to works. >>> >>> Peter >>> >>> >>> On 15 September 2017 at 04:19, Leslie Zhai <lesliezhai at llvm.org.cn> wrote: >>>> Hi Peter, >>>> >>>> Thanks for your kind response! >>>> >>>> >>>> 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. >>>> https://github.com/foss-for-synopsys-dwc-arc-processors/arc-ABI-manual >>>> >>>> But I prefer to read bfd linker's source code about ARC instead: >>>> 1. Specific e_flags >>>> >>>> https://github.com/foss-for-synopsys-dwc-arc-processors/binutils-gdb/blob/arc-2017.09/include/elf/arc.h >>>> 2. Relocation define >>>> >>>> https://github.com/foss-for-synopsys-dwc-arc-processors/binutils-gdb/blob/arc-2017.09/include/elf/arc-reloc.def >>>> 3. Relocation replace function >>>> >>>> https://github.com/foss-for-synopsys-dwc-arc-processors/binutils-gdb/blob/arc-2017.09/include/opcode/arc-func.h >>>> 4. Calculation of S, A, P, PDATA, GOT, etc. >>>> >>>> https://github.com/foss-for-synopsys-dwc-arc-processors/binutils-gdb/blob/arc-2017.09/bfd/elf32-arc.c#L1156 >>>> >>>> >>>>> 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. >>>> I implemented the MOD P for ARC: >>>> >>>> static void modifyARCAddrLoc(uint64_t &AddrLoc, const uint16_t EMachine, >>>> RelExpr Expr, uint32_t Type, uint64_t VMA, >>>> uint64_t OutSecOff, uint64_t RelOff) { >>>> if (EMachine != EM_ARC_COMPACT || EMachine != EM_ARC_COMPACT2 || >>>> Expr != R_PC || Expr != R_GOT_PC) { >>>> return; >>>> } >>>> >>>> uint64_t M = 0; >>>> if (Type == R_ARC_32_PCREL || Type == R_ARC_PC32 || Type =>>>> R_ARC_GOTPC32 >>>> || >>>> Type == R_ARC_GOTPC) { >>>> M = 4; // bitsize >= 32 ? 4 : 0 >>>> } >>>> AddrLoc = (VMA + OutSecOff + RelOff - M) & ~0x3; >>>> } >>>> >>>> modifyARCAddrLoc(AddrLoc, Config->EMachine, Expr, Type, >>>> getOutputSection()->Addr, <-- VMA is important! >>>> cast<InputSection>(this)->OutSecOff, Rel.Offset); >>>> >>>> >>>>> 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. >>>> LLD's getOutputSection()->Addr >>>> https://github.com/llvm-mirror/lld/blob/master/ELF/LinkerScript.cpp#L530 >>>> >>>> >>>> >>>>> 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/ >>>> >>>> >>>> >> -- >> Regards, >> Leslie Zhai - https://reviews.llvm.org/p/xiangzhai/ >> >> >>-- Regards, Leslie Zhai - https://reviews.llvm.org/p/xiangzhai/
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