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Re: [PATCH] Vectorizing abs(char/short/int) on x86.


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
joseph@codesourcery.com


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