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Re: Conditional negation elimination in tree-ssa-phiopt.c

From: Marc Glisse <marc dot glisse at inria dot fr>
To: Kyrill Tkachov <kyrylo dot tkachov at arm dot com>
Cc: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
Date: Tue, 12 Aug 2014 13:01:42 +0200 (CEST)
Subject: Re: Conditional negation elimination in tree-ssa-phiopt.c
Authentication-results: sourceware.org; auth=none
References: <53E8C81B dot 1070303 at arm dot com>
Reply-to: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>

On Mon, 11 Aug 2014, Kyrill Tkachov wrote:

The aarch64 target has a conditional negation instruction
CSNEG Rd, Rs1, Rs2, cond

with semantics Rd = if cond then Rs1 else -Rs2.

This, however doesn't get end up getting matched for code such as:
int
foo2 (unsigned a, unsigned b)
{
 int r = 0;
 r = a & b;
 if (a & b)
   return -r;
 return r;
}

Note that in this particular case, we should just return -(a&b) like llvmdoes.


--
Marc Glisse

References:
- Conditional negation elimination in tree-ssa-phiopt.c
  - From: Kyrill Tkachov

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