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m68k : why is bra an alias for braw, not jra

Andreas Schwab schwab@suse.de
Fri Apr 7 16:48:00 GMT 2006 
Nick Clifton <nickc@redhat.com> writes:

> Hi Philippe,
>
>> In the m68k assembler, I wonder why `bra' and friends are alias for their
>> `bxxw' counterparts.
>
>> And if one writes no size specification for a `bra' instruction the assembler
>> should be free to choose the most efficient way to generate it, i.e.
>> a `brab' if possible of even a `bral' if needed.
>
>> If no-one objects, I'll submit a patch
>
> Please feel free to submit such a patch.  I would recommend however that 
> you control the new behaviour with a command line switch as I suspect 
> that there will be assembler source files out there that rely upon the 
> current behaviour.

I've found this comment in the gas-2.x sources:

/* Define this so that jump tables with calculable offsets are possible.
   This option forces "bsr" and "bra" to always use 16-bit offsets, even
   if smaller ones work work.  Other pseudo-instructions are available for
   variable-sized offsets.  */
#define FIXED_SIZE_BRANCH

and later down:

#ifndef FIXED_SIZE_BRANCH
#define BRANCH "Bg"
#define BRANCH_MASK one(0xFF00)
#else
/* Fixed-size branches with 16-bit offsets */
#define BRANCH "BW"
#define BRANCH_MASK one(0xFFFF) /* 8-bit displacement must be 0 */
#endif

whereas gas 1.38.1 still made them variable sized.  This is the
corresponding change log:

Wed Nov  4 07:36:49 1992  Ken Raeburn  (raeburn@cygnus.com)

	* m68k.h: Define FIXED_SIZE_BRANCH, so bsr and bra instructions
	always use 16-bit offsets.  Makes calculated-size jump tables
	feasible.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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