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Re: Simple bitop reassoc in match.pd

From: Jeff Law <law at redhat dot com>
To: Marc Glisse <marc dot glisse at inria dot fr>, "H.J. Lu" <hjl dot tools at gmail dot com>
Cc: Richard Biener <richard dot guenther at gmail dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
Date: Wed, 11 May 2016 10:26:37 -0600
Subject: Re: Simple bitop reassoc in match.pd
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On 05/11/2016 10:17 AM, Marc Glisse wrote:

The transformation seems right to me, but we are then missing another
transformation like ~X & Y -> X ^ Y when we know that X & ~Y is 0 (the 1
bits of X are included in those of Y). That may not be easy with the
limited bit tracking we have. A version limited to constant Y of the
form 2^n-1 (i.e. 0...01...1) where X has a range included in [0, Y] may
be simpler.

While we don't have strong bit tracking, we do track [0..1] rangesreasonably well, so it may be worth doing.

We could also simplify (int)(_Bool)x to x using VRP information that x
is in [0, 1], but apparently when VRP replaces x==0 with y=x^1,(_Bool)y,
it does not compute a range for the new variable y, and by the time the
next VRP pass comes, it is too late.

Seems like a clear oversight.

jeff

Follow-Ups:
- Re: Simple bitop reassoc in match.pd
  - From: Marc Glisse
- Re: Simple bitop reassoc in match.pd
  - From: Marc Glisse

References:
- Re: Canonicalize X u< X to UNORDERED_EXPR
  - From: Richard Biener
- Re: Canonicalize X u< X to UNORDERED_EXPR
  - From: Marc Glisse
- Re: Canonicalize X u< X to UNORDERED_EXPR
  - From: Richard Biener
- Re: Canonicalize X u< X to UNORDERED_EXPR
  - From: Marc Glisse
- Re: Canonicalize X u< X to UNORDERED_EXPR
  - From: Richard Biener
- Re: Canonicalize X u< X to UNORDERED_EXPR
  - From: Marc Glisse
- Re: Canonicalize X u< X to UNORDERED_EXPR
  - From: Richard Biener
- Simple bitop reassoc in match.pd (was: Canonicalize X u< X to UNORDERED_EXPR)
  - From: Marc Glisse
- Re: Simple bitop reassoc in match.pd (was: Canonicalize X u< X to UNORDERED_EXPR)
  - From: Marc Glisse
- Re: Simple bitop reassoc in match.pd (was: Canonicalize X u< X to UNORDERED_EXPR)
  - From: H.J. Lu
- Re: Simple bitop reassoc in match.pd (was: Canonicalize X u< X to UNORDERED_EXPR)
  - From: Marc Glisse

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