Re: Delay slots

["Maciej W. Rozycki" <macro%linux-mips.org@localhost>]

On Tue, 21 Jun 2016, Warner Losh wrote:

> For MIPS the list of instructions is simple. There’s the branch delay slot
> which always executes while the jump is happening. This is well know
> and everybody takes advantage of it to ‘hide’ the loading of args to functions
> or do other useful work. On mips1, there’s also the load delay slot, which
> means you can’t use the result of the load for 1 instruction.
> 
> That’s it.

 You missed coprocessor move delay slots, MIPS I-III.  E.g. you need to 
schedule a NOP between these pairs of instructions each if you don't have 
anything useful to put there:

	cfc1	$2, $31
	andi	$3, $2, 0x4

	mfc1	$2, $f0
	bltz	$2, foo

	dmtc1	$2, $f0
	div.d	$f2, $f1, $f0

	mfc0	$2, $15
	srl	$3, $2, 16

Needless to say GAS handles all of this automagically in the `reorder' 
mode.

 NB none of this qualifies as a hazard in MIPS-speak, these are all plain 
late data delivery cases, due to the absence of interlocking or a bypass 
(as with memory loads from MIPS II up) in the pipeline.

 FWIW,

  Maciej