Re: [PATCH v7 04/17] tracing: add static function tracer supportfor MIPS

[Maciej W. Rozycki]

On Wed, 11 Nov 2009, Ralf Baechle wrote:

> 32-bit with -mlong-call:
> 
> 	lui	$25, %hi(foo)
> 	addiu	$25, %lo(foo)
> 	jalr	$25
[...]
> It's time that we get a -G optimization that works for the kernel; it would
> allow to cut down the -mlong-calls calling sequence to just:
> 
> 	lw/ld	$25, offset($gp)
> 	jalr	$25

 Actually this may be no faster than the above.  The load produces its 
result late and the jump needs its data early, so unless a bypass has been 
implemented in the pipeline, it may well stall for the extra cycle (that's 
the reason for the load-delay slot in the original MIPS I ISA after all).

 Of course there is still the benefit of a reduced cache footprint, but 
the extra load may have to evict a cache line and flush the benefit down 
the drain.  I don't mean it's not to be considered, but it's not at all 
immediately obvious it would be a win.

  Maciej
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