[MIPS] avoiding certain instruction in delay slots

[Sven Anderson]

Am 03.02.2014 um 16:14 schrieb Maciej W. Rozycki <macro@linux-mips.org>:

> On Mon, 3 Feb 2014, Richard Sandiford wrote:
> 
>> If this particular errata applies specifically to delay slots though,
>> you'd need to handle it differently from -mfix-24k.  You'd need to teach
>> both GCC and GAS about the restrictions.  In the GCC case this would be
>> by setting the "can_delay" attribute to "no" for the problem cases,
>> while for GAS it would be making can_swap_branch_p return false.
> 
> You'd have to teach GAS to bail out on delay slots already scheduled 
> manually in handcoded assembly too, e.g.:
> 
> 	.set	noreorder
> 	beqz	$2, foo
> 	 lw	$2, 8($sp)
> 
> cannot be resolved automatically (the two instructions cannot be swapped 
> and NOP inserted in the delay slot instead; not that GAS already supports 
> it anyway) because of a data dependency on $2 and you do want to make the 
> user aware of this issue so that they can rewrite code instead.

Is it really like that? I assumed the dependency would trigger a hazard and the branch would wait for $2 to be ready?


BR

Sven

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