Re: [PATCH] Fix PR46556 (poor address generation)

[Jeff Law <law at redhat dot com>]

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On 10/06/11 12:02, William J. Schmidt wrote:
> On Thu, 2011-10-06 at 11:35 -0600, Jeff Law wrote:
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>> 
>> On 10/06/11 04:13, Richard Guenther wrote:
>> 
>>> 
>>> People have already commented on pieces, so I'm looking only at
>>> the tree-ssa-reassoc.c pieces (did you consider piggy-backing
>>> on IVOPTs instead?  The idea is to expose additional CSE
>>> opportunities, right?  So it's sort-of a strength-reduction
>>> optimization on scalar code (classically strength reduction in
>>> loops transforms for (i) { z = i*x; } to z = 0; for (i) { z +=
>>> x }). That might be worth in general, even for non-address
>>> cases. So - if you rename that thing to
>>> tree-ssa-strength-reduce.c you can get away without 
>>> piggy-backing on anything ;)  If you structure it to detect a 
>>> strength reduction opportunity (thus, you'd need to match 
>>> two/multiple of the patterns at the same time) that would be a 
>>> bonus ... generalizing it a little bit would be another.
>> There's a variety of literature that uses PRE to detect and
>> optimize straightline code strength reduction.  I poked at it at
>> one time (RTL gcse framework) and it looked reasonably promising.
>> Never pushed it all the way through.
>> 
>> jeff
> 
> I ran across http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22586 and 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35308 that show this 
> question has come up before.  The former also suggested a
> PRE-based approach.
Yea.  We've kicked it around several times over the last 15 or so
years.

When I briefly looked at it, I was doing so more in the context of
eliminating all the optimize_related_values crap, purely as a cleanup
and utlimately couldn't justify the time.

IIRC Morgan & Muchnick both had write-ups on the basic concepts.
There's probably other literature as well.

jeff
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