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    • J. Neuschäfer's avatar
      py/nlrthumb: Make non-Thumb2 long-jump workaround opt-in. · 7b050b36
      J. Neuschäfer authored
      Although the original motivation given for the workaround[1] is correct,
      nlr.o and nlrthumb.o are linked with a small enough distance that the
      problem does not occur, and the workaround isn't necessary. The distance
      between the b instruction and its target (nlr_push_tail) is just 64
      bytes[2], well within the ±2046 byte range addressable by an
      unconditional branch instruction in Thumb mode.
      
      The workaround induces a relocation in the text section (textrel), which
      isn't supported everywhere, notably not on musl-libc[3], where it causes
      a crash on start-up. With the workaround removed, micropython works on an
      ARMv5T Linux system built with musl-libc.
      
      This commit changes nlrthumb.c to use a direct jump by default, but
      leaves the long jump workaround as an option for those cases where it's
      actually needed.
      
      [1]: commit dd376a23
      
      Author: Damien George <damien.p.george@gmail.com>
      Date:   Fri Sep 1 15:25:29 2017 +1000
      
          py/nlrthumb: Get working again on standard Thumb arch (ie not Thumb2).
      
          "b" on Thumb might not be long enough for the jump to nlr_push_tail so
          it must be done indirectly.
      
      [2]: Excerpt from objdump -d micropython:
      
      000095c4 <nlr_push_tail>:
          95c4:       b510            push    {r4, lr}
          95c6:       0004            movs    r4, r0
          95c8:       f02d fd42       bl      37050 <mp_thread_get_state>
          95cc:       6943            ldr     r3, [r0, #20]
          95ce:       6023            str     r3, [r4, #0]
          95d0:       6144            str     r4, [r0, #20]
          95d2:       2000            movs    r0, #0
          95d4:       bd10            pop     {r4, pc}
      
      000095d6 <nlr_pop>:
          95d6:       b510            push    {r4, lr}
          95d8:       f02d fd3a       bl      37050 <mp_thread_get_state>
          95dc:       6943            ldr     r3, [r0, #20]
          95de:       681b            ldr     r3, [r3, #0]
          95e0:       6143            str     r3, [r0, #20]
          95e2:       bd10            pop     {r4, pc}
      
      000095e4 <nlr_push>:
          95e4:       60c4            str     r4, [r0, #12]
          95e6:       6105            str     r5, [r0, #16]
          95e8:       6146            str     r6, [r0, #20]
          95ea:       6187            str     r7, [r0, #24]
          95ec:       4641            mov     r1, r8
          95ee:       61c1            str     r1, [r0, #28]
          95f0:       4649            mov     r1, r9
          95f2:       6201            str     r1, [r0, #32]
          95f4:       4651            mov     r1, sl
          95f6:       6241            str     r1, [r0, #36]   @ 0x24
          95f8:       4659            mov     r1, fp
          95fa:       6281            str     r1, [r0, #40]   @ 0x28
          95fc:       4669            mov     r1, sp
          95fe:       62c1            str     r1, [r0, #44]   @ 0x2c
          9600:       4671            mov     r1, lr
          9602:       6081            str     r1, [r0, #8]
          9604:       e7de            b.n     95c4 <nlr_push_tail>
      
      [3]: https://www.openwall.com/lists/musl/2020/09/25/4Signed-off-by: default avatarJ. Neuschäfer <j.ne@posteo.net>
      7b050b36
  12. 24 Apr, 2024 9 commits
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