m68k-linux-gnu-gcc  -mshort -m68000 -S /tmp/3.c

versus

 m68k-linux-gnu-gcc  -m68000 -S /tmp/3.c

Do you have a test program you could attach?

Created attachment 1200640 [details]
Example

Sorry thought I attached it earlier

This one isn't quite the same but generates broken output too

m68k-linux-gnu-gcc -m68000 -mshort -O2 -S /tmp/3.c
alan@localhost:~/Bulk/UZI/External/FUZIX/Kernel$ more 3.s
#NO_APP
	.file	"3.c"
	.text
	.align	2
	.globl	x
	.type	x, @function
x:
	link.w %fp,#0
	move.l 8(%fp),%d0
	ror.w #8,%d0
	and.l #65535,%d0
	unlk %fp
	rts
	.size	x, .-x
	.ident	"GCC: (GNU) 5.3.1 20160212 (Red Hat Cross 5.3.1-2)"
	.section	.note.GNU-stack,"",@progbits


versus
 m68k-linux-gnu-gcc -Os -m68000  -S /tmp/3.c
alan@localhost:~/Bulk/UZI/External/FUZIX/Kernel$ more 3.s
#NO_APP
	.file	"3.c"
	.text
	.align	2
	.globl	x
	.type	x, @function
x:
	link.w %fp,#0
	move.l 8(%fp),%d0
	ror.w #8,%d0
	swap %d0
	ror.w #8,%d0
	unlk %fp
	rts
	.size	x, .-x
	.ident	"GCC: (GNU) 5.3.1 20160212 (Red Hat Cross 5.3.1-2)"
	.section	.note.GNU-stack,"",@progbits

To quote Jeff Law on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77599 :

The problem here is you're using -mshort  on *-linux-gnu which IMHO is invalid.


Linux defines int32_t as a "int".  And -mshort changes the width of an int to 16 bits, at which point anything prototyped using int32_t has just changed its ABI in a meaningful way (similarly for uint32_t, but unsigned).  You can see this in the .optimized dump file:

x (long unsigned int y)
{
  unsigned int _1;
  unsigned int _2;
  long unsigned int _4;

;;   basic block 2, loop depth 0, count 0, freq 10000, maybe hot
;;    prev block 0, next block 1, flags: (NEW, REACHABLE, VISITED)
;;    pred:       ENTRY [100.0%]  (FALLTHRU,EXECUTABLE)
  _1 = (unsigned int) y_3(D);
  _2 = __builtin_bswap32 (_1);
  _4 = (long unsigned int) _2;
  return _4;
}

Note the type of the _1 and _2.  They are both unsigned ints, so 16 bits. 


If you try this test on a bare metal target such as m68k-elf or without the -mshort, you'll get the expected and desired result.

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