py/nlrthumb: Remove non-Thumb2 workaround.

neuschaefer · neuschaefer · commit d924144dfc98 · 2023-04-27T11:46:20.000+02:00
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, where it causes a crash on start-up. With the workaround removed, micropython works on an ARMv5T Linux system built with musl-libc. [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, micropython#20] 95ce: 6023 str r3, [r4, #0] 95d0: 6144 str r4, [r0, micropython#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, micropython#20] 95de: 681b ldr r3, [r3, #0] 95e0: 6143 str r3, [r0, micropython#20] 95e2: bd10 pop {r4, pc} 000095e4 <nlr_push>: 95e4: 60c4 str r4, [r0, micropython#12] 95e6: 6105 str r5, [r0, micropython#16] 95e8: 6146 str r6, [r0, micropython#20] 95ea: 6187 str r7, [r0, micropython#24] 95ec: 4641 mov r1, r8 95ee: 61c1 str r1, [r0, micropython#28] 95f0: 4649 mov r1, r9 95f2: 6201 str r1, [r0, micropython#32] 95f4: 4651 mov r1, sl 95f6: 6241 str r1, [r0, micropython#36] @ 0x24 95f8: 4659 mov r1, fp 95fa: 6281 str r1, [r0, micropython#40] @ 0x28 95fc: 4669 mov r1, sp 95fe: 62c1 str r1, [r0, micropython#44] @ 0x2c 9600: 4671 mov r1, lr 9602: 6081 str r1, [r0, micropython#8] 9604: e7de b.n 95c4 <nlr_push_tail> Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
diff --git a/py/nlrthumb.c b/py/nlrthumb.c
@@ -71,18 +71,11 @@ __attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) {
         "str    lr, [r0, #8]        \n" // store lr into nlr_buf
         #endif
 
-        #if !defined(__thumb2__)
-        "ldr    r1, nlr_push_tail_var \n"
-        "bx     r1                  \n" // do the rest in C
-        ".align 2                   \n"
-        "nlr_push_tail_var: .word nlr_push_tail \n"
-        #else
         #if defined(__APPLE__) || defined(__MACH__)
         "b      _nlr_push_tail      \n" // do the rest in C
         #else
         "b      nlr_push_tail       \n" // do the rest in C
         #endif
-        #endif
         );
 
     #if !defined(__clang__) && defined(__GNUC__) && (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 8))
