Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: Invalid compilation without -fno-strict-aliasing | Date | Thu, 27 Feb 2003 05:41:06 +0100 |
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On Wednesday 26 February 2003 22:07, Horst von Brand wrote: > Jean Tourrilhes <jt@bougret.hpl.hp.com> said: > > On Wed, Feb 26, 2003 at 04:38:10PM +0100, Horst von Brand wrote: > > > Jean Tourrilhes <jt@bougret.hpl.hp.com> said: > > > > It looks like a compiler bug to me... > > > > Some users have complained that when the following code is > > > > compiled without the -fno-strict-aliasing, the order of the write and > > > > memcpy is inverted (which mean a bogus len is mem-copied into the > > > > stream). > > > > Code (from linux/include/net/iw_handler.h) : > > > > -------------------------------------------- > > > > static inline char * > > > > iwe_stream_add_event(char * stream, /* Stream of events */ > > > > char * ends, /* End of stream */ > > > > struct iw_event *iwe, /* Payload */ > > > > int event_len) /* Real size of payload */ > > > > { > > > > /* Check if it's possible */ > > > > if((stream + event_len) < ends) { > > > > iwe->len = event_len; > > > > memcpy(stream, (char *) iwe, event_len); > > > > stream += event_len; > > > > } > > > > return stream; > > > > } > > > > -------------------------------------------- > > > > IMHO, the compiler should have enough context to know that the > > > > reordering is dangerous. Any suggestion to make this simple code more > > > > bullet proof is welcomed. > > > > > > The compiler is free to assume char *stream and struct iw_event *iwe > > > point to separate areas of memory, due to strict aliasing. > > > > Which is true and which is not the problem I'm complaining about. > > ... the compiler thus goes and reorders the frobbing of the variables, as > they are (assumed) separate.
Actually, the compiler appears to view &iwe->len and (char *) iwe as separate, which I find surprising.
Regards,
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