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Re: [PATCH] Vectorizing abs(char/short/int) on x86.
- From: "Joseph S. Myers" <joseph at codesourcery dot com>
- To: Cong Hou <congh at google dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, Richard Biener <rguenther at suse dot de>
- Date: Wed, 23 Oct 2013 15:52:47 +0000
- Subject: Re: [PATCH] Vectorizing abs(char/short/int) on x86.
- Authentication-results: sourceware.org; auth=none
- References: <CAK=A3=3J2J_ZTT3qxO6z32kSNpA7dVFobYg=SQNi0udMY8E5VA at mail dot gmail dot com>
On Tue, 22 Oct 2013, Cong Hou wrote:
> For abs(char/short), type conversions are needed as the current abs()
> function/operation does not accept argument of char/short type.
> Therefore when we want to get the absolute value of a char_val using
> abs (char_val), it will be converted into abs ((int) char_val). It
> then can be vectorized, but the generated code is not efficient as
> lots of packings and unpackings are envolved. But if we convert
> (char) abs ((int) char_val) to abs (char_val), the vectorizer will be
> able to generate better code. Same for short.
ABS_EXPR has undefined overflow behavior. Thus, abs ((int) -128) is
defined (and we also define the subsequent conversion of +128 to signed
char, which ISO C makes implementation-defined not undefined), and
converting to an ABS_EXPR on char would wrongly make it undefined. For
such a transformation to be valid (in the absence of VRP saying that -128
isn't a possible value) you'd need a GIMPLE representation for
ABS_EXPR<overflow:wrap>, as distinct from ABS_EXPR<overflow:undefined>.
You don't have the option there is for some arithmetic operations of
converting to a corresponding operation on unsigned types.
Joseph S. Myers