Added midpoint strings #16
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kwagyeman
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Aug 22, 2023
Starting with 2757acf, the `top` variable in `nlr_jump()` in `nlraarch64.c` was assigned to register `x19` by the compiler. However, the assembly code writes over that register with ldp x19, x20, [%0, openmv#32] since `%0` is now `x19`. This causes the next line ldp lr, x9, [%0, openmv#16] to load the wrong values. To fix the issue, we move the value of the `top` variable from an unknown register to a known register at the beginning of the asm code then only use known/hard-coded registers after that. Fixes issue micropython#11754. Signed-off-by: David Lechner <david@pybricks.com>
iabdalkader
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Jul 1, 2024
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 dd376a2 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/2020/09/25/4 Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
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