-fno-strict-aliasing in CWARNFLAGS?
mcree at orcon.net.nz
Wed Feb 3 12:49:41 PST 2010
On 04/02/10 09:17, Peter Harris wrote:
> On 2010-02-03 15:02, Michael Cree wrote:
>> On 04/02/10 07:55, Soeren Sandmann wrote:
>>> I recently turned it on in pixman because completely reasonable code
>>> like this:
>>> pixman_contract (uint32_t * dst,
>>> const uint64_t *src,
>>> int width)
>>> int i;
>>> /* Start at the beginning so that we can do the contraction in
>>> * place when src == dst
>>> is actually illegal under the C aliasing rules, and GCC can and will
>>> break it unless you use -fno-strict-aliasing.
>> I'm confused. Why does this break the aliasing rules?
> If *dst and *src point to (alias) the same memory, it breaks the rules
> since they are different types.
Thanks, yes, it's now obvious.
Looking back at the code I now see that I completely missed the comment
that says it is possible that src == dest.
When I read code and see two different pointer types in the functions
arguments I naturally assume that it is _intended_ that they are two
different areas of memory. I always program like that and I guess that
programming practice comes from once knowing the C standard well many
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