From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 11806 invoked by alias); 3 Apr 2002 13:06:07 -0000 Mailing-List: contact gcc-prs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Archive: List-Post: List-Help: Sender: gcc-prs-owner@gcc.gnu.org Received: (qmail 11787 invoked by uid 71); 3 Apr 2002 13:06:01 -0000 Date: Wed, 03 Apr 2002 05:06:00 -0000 Message-ID: <20020403130601.11786.qmail@sources.redhat.com> To: nobody@gcc.gnu.org Cc: gcc-prs@gcc.gnu.org, From: Daniel Berlin Subject: Re: optimization/5738: GCSE missed optimization Reply-To: Daniel Berlin X-SW-Source: 2002-04/txt/msg00224.txt.bz2 List-Id: The following reply was made to PR optimization/5738; it has been noted by GNATS. From: Daniel Berlin To: rth@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