This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: DFA lookahead confusion
- From: Paul Koning <pkoning at equallogic dot com>
- To: davem at redhat dot com
- Cc: vmakarov at redhat dot com, gcc at gcc dot gnu dot org
- Date: Wed, 15 May 2002 11:33:00 -0400
- Subject: Re: DFA lookahead confusion
- References: <20020514.211327.04690352.davem@redhat.com>
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