From: "Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>
To: Josef Wolf <jw@raven.inka.de>, gcc-help@gcc.gnu.org
Subject: Re: Crash when cross compiling for ARM with GCC-8-2-0 and -ftree-loop-distribute-patterns
Date: Fri, 18 Oct 2019 10:26:00 -0000 [thread overview]
Message-ID: <c31ac218-801a-176c-6073-8d35b3e6a3f4@arm.com> (raw)
In-Reply-To: <20191018085314.GE11171@raven.inka.de>
On 18/10/2019 09:53, Josef Wolf wrote:
> 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.
>
Ah, yes. Looking at some libc sources it puts an explicit optimization
attribute onto the memset (and similar mem... functions) to disable
-ftree-loop-distribute-patterns for such functions. So something like
void *
__attribute__ ((__optimize__ ("-fno-tree-loop-distribute-patterns")))
memset (void *s, int c, size_t n)
{
int i;
for (i=0; i<n; i++)
((char *)s)[i] = c;
return s;
}
Though, of course, it's wrapped up in a macro to make it look a bit
prettier ;-)
This is just one of those gotchas that you have to be aware of when
implementing the standard library.
R.
next prev parent reply other threads:[~2019-10-18 10:26 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-16 13:19 Josef Wolf
2019-10-16 13:30 ` Matthias Pfaller
2019-10-17 8:10 ` Josef Wolf
2019-10-16 18:18 ` Martin Sebor
2019-10-17 11:40 ` Josef Wolf
2019-10-17 12:37 ` Matthias Pfaller
2019-10-17 14:10 ` Josef Wolf
2019-10-17 14:55 ` Richard Earnshaw (lists)
2019-10-18 9:00 ` Josef Wolf
2019-10-18 10:26 ` Richard Earnshaw (lists) [this message]
2019-10-18 12:10 ` Josef Wolf
2019-10-18 13:07 ` Segher Boessenkool
2019-10-18 13:40 ` Josef Wolf
2019-10-18 12:50 ` Josef Wolf
2019-10-18 14:04 ` Richard Earnshaw (lists)
2019-10-18 9:10 ` Propagating addresses from linker to the runtie (was: Re: Crash when cross compiling for ARM with GCC-8-2-0 and) -ftree-loop-distribute-patterns Josef Wolf
2019-10-18 9:15 ` Propagating addresses from linker to the runtie Florian Weimer
2019-10-18 9:50 ` Josef Wolf
2019-10-18 10:47 ` Florian Weimer
2019-10-18 12:51 ` Segher Boessenkool
2019-10-18 12:56 ` Florian Weimer
2019-10-18 14:14 ` Segher Boessenkool
2019-10-18 14:34 ` Florian Weimer
2019-10-18 13:30 ` Josef Wolf
2019-10-18 14:20 ` Segher Boessenkool
2019-10-18 13:10 ` Crash when cross compiling for ARM with GCC-8-2-0 and -ftree-loop-distribute-patterns Josef Wolf
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