On Fri, 13 Mar 2009, Richard Guenther wrote: > Last time I sent a patch to remove the SHIFT_COUNT_TRUNCATED check > from fold-const.c the reason that this was rejected was that we want to > be consistend with target behavior...
I would disagree with such a rejection. If we want to provide any sort of consistency I think it should be optional (an option like -fwrapv) and define the semantics at the language level rather than depending on the target. > > As I understand it, SHIFT_COUNT_TRUNCATED is about the semantics of RTL, > > and has no bearing on the implemented semantics of any source language or > > of GENERIC or GIMPLE; GENERIC and GIMPLE both follow the rule that > > out-of-range shifts (or rotates?), including those by negative amounts, > > are undefined. > > "Undefined" is a bad semantic for GIMPLE. If the frontends want the > middle-end > to take advantage of undefinedness it needs to translate it to a useful > defined > state. By undefined I mean that the middle-end may assume that the value is within range [0, number of value and sign bits in type) and transform on the basis of that assumption (including value range propagation). -- Joseph S. Myers jos...@codesourcery.com
