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Date: Wed, 13 Jan 2021 19:09:35 +0300 (MSK)
From: Alexander Monakov <amonakov@ispras.ru>
To: Fredrik Noring <noring@nocrew.org>
cc: gcc-help@gcc.gnu.org
Subject: Re: m68k: Simple loop compiles into boundless recursion with -O2
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On Wed, 13 Jan 2021, Fredrik Noring wrote:

> Hi,
> 
> Compiler used is GCC m68k-elf version 10.2.0. A variant of the classic
> memset
> 
> 	void *memset2(void *s, int c, unsigned int n)
> 	{
> 		char *b = s;
> 		for (unsigned int i = 0; i < n; i++)
> 			b[i] = c;
> 		return s;
> 	}
> 
> compiles into boundless recursion with O2 optimisation and the m68k-elf
> target. This will, of course, exhaust the stack and crash badly.
> 
> The commands
> 
> 	m68k-elf-gcc -O2 -march=68000 -c -o memset2.o memset2.c
> 	m68k-elf-objdump -d memset2.o

Please invoke objdump with -dr instead to see the relocations.

> 
> produce
> 
> 	00000000 <memset2>:
> 	   0:	2f02           	movel %d2,%sp@-
> 	   2:	242f 0008      	movel %sp@(8),%d2
> 	   6:	202f 0010      	movel %sp@(16),%d0
> 	   a:	6718           	beqs 24 <memset2+0x24>
> 	   c:	2f00           	movel %d0,%sp@-
> 	   e:	102f 0013      	moveb %sp@(19),%d0
> 	  12:	4880           	extw %d0
> 	  14:	3040           	moveaw %d0,%a0
> 	  16:	2f08           	movel %a0,%sp@-
> 	  18:	2f02           	movel %d2,%sp@-
> 	  1a:	4eb9 0000 0000 	jsr 0 <memset2>   /* <<<--- recursion */

The relocation associated with this instruction should point to memset.
Most likely the compiler is optimizing your memset2 function to call
the standard function 'memset'.

When implementing memset itself you need to pass -ffreestanding to GCC,
which will disable this optimization.

Alexander