Re: Crash when cross compiling for ARM with GCC-8-2-0 and -ftree-loop-distribute-patterns

[Josef Wolf <jw@raven.inka.de>]

Tahnks for your help, Matthias!

On Thu, Oct 17, 2019 at 02:37:11PM +0200, Matthias Pfaller wrote:

> Have a look at "arm-eabi-objdump -S -d main.elf". Sometimes this is
> quite revealing.

Yeah.

> Are you using openocd or something similar for debugging?

Yes. Openocd with gdb.

> You are compiling for a cortex-m0/3/4?

Cortex-m3

> Are you single stepping through the complete startup sequence or do set a
> break point ath the top of memset (i.e. are break points working at all)?

Breakpoints are working. But there is only a limited set of hardware
breakpoints (four, AFAIR).

> Interrupts are still disabled?

There are no interrupt sources enabled yet. But I wonder why the CPU is not
starting up with disabled IRQs? I am new to the ARM architecture, but every
other architecture I know of would come out from reset with disabled
interrupts... I'd expect BASEPRI and PRIMASK to be set to sane values before
the first instruction is executed?

Anyway, explicitly calling __set_PRIMASK(1) did also not help, although
primask ist still set when the processor crashes.

> Why is the stack pointer so low at this point of execution? Using
> 0x20018000-0x20017d20 == 0x2e0 bytes of stack seems a little excessive
> for just one call.

Ah!... Looks like you've spotted the problem! Actually, the SP is decremented
on every cycle of the loop:

  (gdb) disass
  Dump of assembler code for function memset:
     0x08001008 <+0>:    push {r4, lr}
     0x0800100a <+2>:    mov  r4, r0
     0x0800100c <+4>:    cbz  r2, 0x8001014 <memset+12>
  => 0x0800100e <+6>:    uxtb r1, r1
     0x08001010 <+8>:    bl   0x8001008 <memset>
     0x08001014 <+12>:   mov  r0, r4
     0x08001016 <+14>:   pop  {r4, pc}
  End of assembler dump.

This looks REALLY suspicous to me. Every cycle of the loop in memset() is
pushing something onto the stack?!?

Without the  -ftree-loop-distribute-patterns option, the memset() function
looks entirely different:

         cbz    r2, <memset+18>
         add    r2, r0
         subs   r2, #1
         uxtb   r1, r1
         subs   r3, r0, #1
  <+10>: strb.w r1, [r3, #1]!
         cmp    r3, r2
         bne.n  <memset+10>
  <+18>: bx     lr

> I usually start toggling output lines when I'm stuck like this...

?


-- 
Josef Wolf
jw@raven.inka.de