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Re: [PATCH] Fix canonicalization of addresses

From: Eric Botcazou <ebotcazou at adacore dot com>
To: "Richard Guenther" <richard dot guenther at gmail dot com>
Cc: "Richard Earnshaw" <rearnsha at arm dot com>, gcc-patches at gcc dot gnu dot org, "Andrew Haley" <aph at redhat dot com>
Date: Tue, 6 Jan 2009 23:19:18 +0100
Subject: Re: [PATCH] Fix canonicalization of addresses
References: <200812231211.50787.ebotcazou@adacore.com> <1231254785.20016.9.camel@pc960.cambridge.arm.com> <84fc9c000901061202n1a7f9ae5s943ea9c1e45fdbdf@mail.gmail.com>

> To repeat: it's a canonicalization.  And it helps for canonicalizing a
> * (-2) - 8 to (a + 4)*(-2)
> and allowing CSE with another a + 4.  I don't think that in this case
> we want to have (-4 - a)*2.

The - - should be canonicalized elsewhere.  Changing the sign without further 
analysis is just plain wrong and appears to be an unexpected fallout of the 
change, according to the comment:

  /* No identical multiplicands; see if we can find a common
     power-of-two factor in non-power-of-two multiplies.  This
     can help in multi-dimensional array access.  */

-- 
Eric Botcazou

Follow-Ups:
- Re: [PATCH] Fix canonicalization of addresses
  - From: Richard Guenther

References:
- Re: [PATCH] Fix canonicalization of addresses
  - From: Richard Earnshaw
- Re: [PATCH] Fix canonicalization of addresses
  - From: Richard Guenther

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