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I get the feedback recently that enable linker relaxations may fail to build some program. Consider the following case, .text foo: addi a0, a0, %pcrel_lo(.L2) call foo .L1: auipc a1, %pcrel_hi(data_g) addi a1, a1, %pcrel_lo(.L1) lui a2, %hi(data_g) addi a2, a2, %lo(data_g) lui a3, %tprel_hi(data_t) add a3, a3, tp, %tprel_add(data_t) addi a3, a3, %tprel_lo(data_t) .L2: auipc a0, %pcrel_hi(data_g) .data .word 0x0 .global data_g data_g: .word 0x1 .section .tbss data_t: .word 0x0 The current ld reports `dangerous relocation error` when doing the pcgp relaxation, test.o: in function `foo': (.text+0x0): dangerous relocation: %pcrel_lo missing matching %pcrel_hi The .L2 auipc should not be removed since it is behind the corresponding addi, so we record the information in the pcgp_relocs table to avoid removing the auipc later. But current ld still remove it since we do not update the pcgp_relocs table while doing other relaxations. I have two solutions to fix the problem, 1. Update the pcgp_relocs table once we actually delete the code. 2. Add new relax pass to do the pcgp relaxations At first I tried to do the first solution, and we need to update at least three information - hi_sec_off of riscv_pcgp_lo_reloc, hi_sec_off and hi_addr (symbol value) of riscv_pcgp_hi_reloc. Update the hi_sec_off is simple, but it is more complicate to update the symbol value, since we almost have to do parts the same works of _bfd_riscv_relax_call again in the riscv_relax_delete_bytes to get the correct symbol value. Compared with the first solution, the second one is more intuitive and simple. We add a new relax pass to do the pcgp relaxations later, so we will get all the information correctly in the _bfd_riscv_relax_call, including the symbol value, without changing so much code. I do not see any penalty by adding a new relax pass for now, so it should be fine to delay the pcgp relaxations. Besides, I have pass all riscv-gnu-toolchain regressions for this patch. bfd/ * elfnn-riscv.c (_bfd_riscv_relax_section): Add a new relax pass to do the pcgp relaxation later, after the lui and call relaxations, but before the delete and alignment relaxations. ld/ * emultempl/riscvelf.em (riscv_elf_before_allocation): Change link_info.relax_pass from 3 to 4. * testsuite/ld-riscv-elf/pcgp-relax.d: New testcase. * testsuite/ld-riscv-elf/pcgp-relax.s: Likewise. * testsuite/ld-riscv-elf/ld-riscv-elf.exp: Updated.binutils-2_36-branch
Nelson Chu
6 years ago
7 changed files with 79 additions and 14 deletions
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#source: pcgp-relax.s |
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#ld: --relax |
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#objdump: -d -Mno-aliases |
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.*:[ ]+file format .* |
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Disassembly of section \.text: |
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0+[0-9a-f]+ <_start>: |
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.*:[ ]+[0-9a-f]+[ ]+addi[ ]+a0,a0,[0-9]+ |
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.*:[ ]+[0-9a-f]+[ ]+jal[ ]+ra,[0-9a-f]+ <_start> |
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.*:[ ]+[0-9a-f]+[ ]+addi[ ]+a1,gp,\-[0-9]+ # [0-9a-f]+ <data_g> |
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.*:[ ]+[0-9a-f]+[ ]+addi[ ]+a2,gp,\-[0-9]+ # [0-9a-f]+ <data_g> |
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.*:[ ]+[0-9a-f]+[ ]+addi[ ]+a3,tp,0 # 0 <data_t> |
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.*:[ ]+[0-9a-f]+[ ]+auipc[ ]+a0,0x[0-9a-f]+ |
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.text |
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.globl _start |
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_start: |
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addi a0, a0, %pcrel_lo(.L2) |
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call _start |
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.L1: |
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auipc a1, %pcrel_hi(data_g) |
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addi a1, a1, %pcrel_lo(.L1) |
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lui a2, %hi(data_g) |
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addi a2, a2, %lo(data_g) |
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lui a3, %tprel_hi(data_t) |
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add a3, a3, tp, %tprel_add(data_t) |
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addi a3, a3, %tprel_lo(data_t) |
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.L2: |
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auipc a0, %pcrel_hi(data_g) |
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.data |
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.word 0x0 |
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.globl data_g |
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data_g: |
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.word 0x1 |
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.section .tbss |
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data_t: |
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.word 0x0 |
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