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Re: Redundant limit check for switch

From: Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>
To: Piotr Fusik <pfusik at op dot pl>
Cc: GCC Development <gcc at gcc dot gnu dot org>, Diego Novillo <dnovillo at redhat dot com>,Giovanni Bajo <rasky at develer dot com>
Date: Sun, 28 Aug 2005 14:03:03 +0200
Subject: Re: Redundant limit check for switch
References: <20050822075555Z1189347-7791+1182@kps9.test.onet.pl> <014901c5a710$24b7ae60$bf03030a@trilan> <4309C40E.2060205@lu.unisi.ch> <001401c5ab99$cbb1cba0$94d24dd5@piec>

Why can't this information be used to optimize comparisons?

My patch caused GCC to infer that x & 7 > 7 is false. But I couldn't think of a quick way to use this to optimize away the check, because then GCC crashed.

Does it work only for consecutive ands?
Or is it just an early constant folding:
return (x + 1) & (0x0f & 0x0c & 0x3ff & 0xff);
?

In this case, it is.

Paolo

References:
- Redundant limit check for switch
  - From: Piotr Fusik
- Re: Redundant limit check for switch
  - From: Giovanni Bajo
- Re: Redundant limit check for switch
  - From: Paolo Bonzini
- Re: Redundant limit check for switch
  - From: Piotr Fusik

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