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Re: RFC -- CSE compile-time stupidity
On Mon, 2005-02-21 at 14:42 -0500, Kazu Hirata wrote:
> Hi Jeff,
>
> > The second thought is to initialize all of cse_reg_info entries at the
> > beginning of cse_main. Set aside a bitmap with as many bits as
> > max_regs. Whenever we use one of these accessor macros for register
> > k, set a bit k saying "cse_reg_info_table[k] is in use." This way,
> > when we are done with a basic block, we can walk the bitmap and
> > reinitialize those that are used. Again, a good optimizer should be
> > able to eliminate most of these bit sets, but a non-pure/const
> > function call will block the cleanup opportunities. Of course, this
> > bitmap walk is far more expensive than cse_reg_info_timestamp++.
>
> I just tried this. It comes very close to the current timestamp
> approach but loses by 0.3% or so in compile time. I'm attaching a
> patch for completeness.
Thanks. I suspect this won't cut the mustard either -- the SET_BIT
twiddles memory, which, due to lameness in our aliasing code, is
likely going to cause us to miss the CSE opportunities that are
so important here.
Beyond that, I suspect we get killed zeroing the sbitmap every time we
start a new cse block. I may play with it though. Who knows, maybe
there'll be something that we can do with it.
jeff