Re: optimization/5738: GCSE missed optimization

[Daniel Berlin <dan@dberlin.org>]

The following reply was made to PR optimization/5738; it has been noted by GNATS.

From: Daniel Berlin <dan@dberlin.org>
To: rth@gcc.gnu.org, <dann@godzilla.ics.uci.edu>, <gcc-bugs@gcc.gnu.org>,
   <gcc-prs@gcc.gnu.org>, <nobody@gcc.gnu.org>, <gcc-gnats@gcc.gnu.org>
Cc:  
Subject: Re: optimization/5738: GCSE missed optimization
Date: Wed, 3 Apr 2002 07:58:55 -0500 (EST)

 On 3 Apr 2002 rth@gcc.gnu.org wrote:
 
 > Synopsis: GCSE missed optimization
 > 
 > State-Changed-From-To: open->closed
 > State-Changed-By: rth
 > State-Changed-When: Wed Apr  3 02:25:09 2002
 > State-Changed-Why:
 >     That's not how partial redundancy elimination (PRE) works.
 >     The object with PRE is to minimize the number of evaluations
 >     of an expression *along a path*. 
 
 No, the main object of PRE (besides performing GCSE) is to suppress 
 partial redundancies. 
 IE expressions that are available along one or more paths, but missing from some path.
 It does so by making it fully redundant, copying it to a block (or 
 blocks) such that it reaches all of the paths.  It then eliminates the 
 other copies.
 
 
 >  There is already one
 >     evaluation along each path, thus PRE considers things
 >     optimal.
 
 No it won't.
 The expression is not in the earliest place possible, and is fully 
 redundant.
 It *should* copy it to the predecessor, and eliminate the other two 
 copies.
 
 >     
 >     You want global value numbering or something, which we 
 >     don't implement.
 GVN wouldn't help here, actually.
 GVN doesn't insert new copies, it only eliminates values that are really 
 still available from some other block.
 
 
 Please don't close this PR, it's correct.
 --Dan