> -----Ursprüngliche Nachricht-----
> Von: Gcc-help <gcc-help-bounces@xxxxxxxxxxx> Im Auftrag von Fredrik
> Noring
> Gesendet: Mittwoch, 13. Januar 2021 17:02
> An: gcc-help@xxxxxxxxxxx
> Betreff: m68k: Simple loop compiles into boundless recursion with -O2
>
> 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
>
> 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
> */
> 20: 4fef 000c lea %sp@(12),%sp
> 24: 2002 movel %d2,%d0
> 26: 241f movel %sp@+,%d2
> 28: 4e75 rts
>
> O1 optimisation is more reasonable, as it instead produces
>
> 00000000 <memset2>:
> 0: 2f02 movel %d2,%sp@-
> 2: 202f 0008 movel %sp@(8),%d0
> 6: 242f 000c movel %sp@(12),%d2
> a: 4aaf 0010 tstl %sp@(16)
> e: 670e beqs 1e <memset2+0x1e>
> 10: 2040 moveal %d0,%a0
> 12: 222f 0010 movel %sp@(16),%d1
> 16: d280 addl %d0,%d1
> 18: 10c2 moveb %d2,%a0@+
> 1a: b288 cmpl %a0,%d1
> 1c: 66fa bnes 18 <memset2+0x18>
> 1e: 241f movel %sp@+,%d2
> 20: 4e75 rts
>
> The machine code with O2 looks like a plain compiler bug to me.
>
> What to do?
>
> Fredrik
I guess that the label here
> 1a: 4eb9 0000 0000 jsr 0 <memset2> /* <<<--- recursion
is printed incorrectly and instead a call to memset is done.
objdump is known to use the first label it finds for an offset, and also often uses the wrong section... You should look at the assembly or run objdump with `-dr` to also print relocations.
You may also use the compiler explorer here https://franke.ms/cex/ to view the results for some different m68k gcc versions. (note that the % for registers is omitted and labels have an underscore).
Here is the asm output of gcc-10.2.0-elf:
_memset2:
move.l d2,-(sp)
move.l 8(sp),d2
move.l 16(sp),d0
jeq .L4
move.l d0,-(sp)
move.b 19(sp),d0
ext.w d0
move.w d0,a0
move.l a0,-(sp)
move.l d2,-(sp)
jsr _memset
lea (12,sp),sp
.L4:
move.l d2,d0
move.l (sp)+,d2
rts
/cheers
Stefan
