Gary-Hobson commented on code in PR #18150:
URL: https://github.com/apache/nuttx/pull/18150#discussion_r2758166959
##########
libs/libc/machine/arm/arch_mcount.S:
##########
@@ -29,6 +29,22 @@
.type __gnu_mcount_nc, %function
__gnu_mcount_nc:
+#if defined(CONFIG_ARCH_CORTEXM0) || defined(CONFIG_ARCH_CORTEXM23)
+ /* Cortex-M0/M0+/M23 - ARMv6-M and ARMv8-M Baseline */
+ /* These cores only support limited Thumb-1 instruction set */
+ push {r0, r1, r2, r3, lr} /* Save registers */
+ mov r1, lr
+ movs r2, #1
+ bics r1, r2 /* R1 contains callee address,
with thumb bit cleared */
+ ldr r0, [sp, #20] /* R0 contains caller address */
+ movs r2, #1
+ bics r0, r2 /* Clear thumb bit */
+ bl mcount_internal /* Jump to internal _mcount()
implementation */
+ pop {r0, r1, r2, r3} /* Restore r0-r3 */
+ pop {r1} /* Pop saved lr into r1 */
Review Comment:
This is the test result from using QEMU with the micro:bit (Cortex-M0). It
can correctly collect data without modifying the caller's registers.
```
qemu-system-arm -M microbit -kernel mcount.elf -nographic -s -S
Breakpoint 1, mcount_internal (frompc=0x398 <main+28>, selfpc=0x318
<mcount_test+12>) at mcount.c:155
155 print("mcount_internal:: frompc: ");
(gdb) bt
#0 mcount_internal (frompc=0x398 <main+28>, selfpc=0x318 <mcount_test+12>)
at mcount.c:155
#1 0x000003c6 in __gnu_mcount_nc () at startup.s:91
#2 0x00000318 in mcount_test (a=17, b=0, c=1040) at mcount.c:164
#3 0x00000398 in main () at mcount.c:180
```
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