On 03/03/15 12:57, Martin Sebor wrote:
As a data point(*) it might be interesting to note that GCC itself
relies on memcpy providing stronger guarantees than the C standard
requires it to by emitting calls to the function for large structure
self-assignments (which are strictly conforming, as discussed in bug
65029).
Right. I actually spent quite a bit of time struggling with this a
while back in a different context. The only case I could come up with
where GCC would generate an overlapping memcpy was self assignment, but
even that was bad and while we ultimately punted, I've always considered
it a wart.
[*] IMO, one in favor of tightening up the memcpy specification
to require implementations to provide the expected semantics.
That works for me :-)
The things done in glibc's memcpy are a bit on the absurd side and the
pain caused by the changes over time is almost impossible to overstate.
If the Austin group tightens memcpy to require fewer surprises I think
most developers would ultimately be happy with the result -- a few would
complain about the performance impacts for specific workloads, but I
suspect they'd be in the minority.
jeff
