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 jos...@codesourcery.com
