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@xxxxxxxxxxxxx
