On Mon, Aug 26, 2013 at 9:38 AM, Jan Hubicka <hubi...@ucw.cz> wrote: >> On 08/22/2013 12:45 PM, Gabriel Dos Reis wrote: >> >If the user-supplied operator new returns &a, then it must >> >also ensure that 'a' is not used anywhere else -- e.g. I you can't >> >do lvalue-to-value conversion on 'a' to see what is written there. >> >Because its storage has been reused. That is, aliasing is framed >> >in terms of object lifetime and uniqueness of ownership. >> >> Do you have a reference for this? The wording in 3.8 seems to only >> restrict how a pointer is used when there is no object in the >> storage, it doesn't say anything about using a pointer to access a >> different object at the same location. >> >> This issue seems to be core 1338: >> >> http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1338 >> >> which has priority 2, so it's not likely to get resolved any time >> soon. I'll ask to reconsider the priority at the next meeting. >> >> >>We probably can go with -fno-user-overwritten-new or something similar? >> >> I'd name it something like -fno-aliased-global-new, which would add >> the malloc attribute to the built-in declarations. > > That sounds good to me. Of course it would be really nice to go for this by > default and let users overwritting toplevel new to use an option (like we have > -freestanding).
Hmm, let's not make it a default. Replacing global operator new (e.g. for tracing purposes) is a valid C++ programming idiom. > > Can we consider the original patch (that does not change malloc attributes) > and > then I can make this as an followup? > > Also if any of these runtime parts are known to be cold in a way that is not > obvious to the backend (i.e. that it is reachable only by EH edges or leading > to noreturn), it may be good idea to add cold attribute annotations or use use > PREDICT_EXPR to mark the unlikely path. I never tought too much about this, > but especially the EH machinery seems to generate a lot of code that is > executed only for EH. I would like to try to get reorder-blocks to split > those > parts away to cold text section just as we do with the profile. > > Thank you, > Honza >> >> Jason
