------- Comment #96 from dberlin at gcc dot gnu dot org 2009-02-16 02:07
-------
Subject: Re: [4.3/4.4 Regression] Inordinate
compile times on large routines
Uh, it's most certainly disabled on testcases like his.
look at is_too_expensive in gcse.c
This is in fact done because LCM iteration takes too long on
flowgraphs like that, because of it's iteration order.
On Sun, Feb 15, 2009 at 6:26 AM, stevenb dot gcc at gmail dot com
<gcc-bugzi...@gcc.gnu.org> wrote:
>
>
> ------- Comment #95 from stevenb dot gcc at gmail dot com 2009-02-15 11:26
> -------
> Subject: Re: [4.3/4.4 Regression] Inordinate
> compile times on large routines
>
> Re: Comment #94
> The trouble with LCM in RTL (i.e. GCSE-PRE) is not that it is slow (or
> that it is disabled -- istr it is enabled at -O2), and also not that
> it is edge based. The problem is that it doesn't handle cascading
> expressions, because that just doesn't fit in the LCM framework. You
> have to iterate RTL GCSE-PRE to move the same invariants as what RTL
> LICM (i.e. loop-invariant.c) can achieve.
>
> (GCSE-PRE is old code from a time when GCC didn't really have a proper
> CFG. It is edge based because for block based you need critical edge
> splitting, which was was prohibitively expensive in the Old Days.
> Nowadays, gcse.c+lcm.c works in cfglayout mode and pre-splitting
> critical edges would be cheap, so it would be a good idea to
> experiment with a block based GCSE-PRE rewrite...)
>
>
> --
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26854
>
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--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26854
