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Re: DFA lookahead confusion


Excerpt of message (sent 14 May 2002) by David S. Miller:
> 
> Reading the documentation for target hook:
> 
> TARGET_SCHED_FIRST_CYCLE_MULTIPASS_DFA_LOOKAHEAD
> 
> I have a hard time coming to conclusions :-)
> 
> Alpha and Pentium are using a value that seems to be dependant upon
> the width of the processor.
> 
> However, as I read the documentation there is no direct correlation
> between width of the processor and the value to use for DFA lookahead.
> Even if processor width is 2 (as on some Alpha's) you can still get
> better schedules when using larger values for DFA lookahead.

The way I interpreted it is that the "right" value is the issue width
times the longest latency you want to deal with.  So, for example, in
a MIPS processor I was trying this stuff out on, I set it to 16 --
quad issue times 4 tick longest latency.  (I was ignoring Divide
here...)

It did seem a bit cryptic, though.

	paul


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