On Fri, Oct 28, 2011 at 12:21 PM, David Miller <da...@davemloft.net> wrote: > From: Richard Guenther <richard.guent...@gmail.com> > Date: Fri, 28 Oct 2011 11:27:25 +0200 > >> On Fri, Oct 28, 2011 at 9:48 AM, David Miller <da...@davemloft.net> wrote: >>> >>> g++.dg/init/copy7.C makes sure that memcpy() is not emitted with >>> src and dst equal. >> >> The testcase is bogus and should be removed. See the patch I posted >> (and the PR). Unfortunately the patch didn't get any review yet. > > Thanks. Which patch and PR are you talking about, 39480?
PR50079, the patch simply deletes the testcase. > Meanwhile, if the conclusion is that memcpy(x, x, ...) is legal, I > disagree. I can think of implementations where this will break. Then we have to fix the middle-end which will happily expand block-moves to memcpy with exact overlap (a = a is valid in C). See the PR and the C testcases therein. Just trying to avoid this in the C++ frontend is bogus. > For example, if the memcpy implementation does a cache initializing > store on the first cache line of the destination before fetching the > beginning of the source, such memcpy() calls with src==dst will > produce garbage. Of course, as Linus would say, such implementation would be "broken" ;) (I expect that such implementations can (and would) easily avoid the situation with almost no cost by doing an early out for src == dst). Richard.
