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

[Josef Wolf]

Thanks for your help, Richard!

On Thu, Oct 17, 2019 at 03:55:31PM +0100, Richard Earnshaw (lists) wrote:
> On 17/10/2019 15:04, Josef Wolf wrote:
> >On Thu, Oct 17, 2019 at 02:37:11PM +0200, Matthias Pfaller wrote:
> >>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
> 
> The compiler has spotted that you've written something that acts like memset
> and optimized it into a function call to memset.  So now you're recursing to
> oblivion.  Try adding -fno-builtin-memset to your compile options.

This sounds reasonable, and I was actually thinking it would solve the
problem.

Unfortunately, -fno-built-memset doesn't have any effect. The same code is
generated.

-- 
Josef Wolf
jw@raven.inka.de