Messages in this thread | ![/](/images/icornerl.gif) | | From | (Linus Torvalds) | Subject | Re: Invalid compilation without -fno-strict-aliasing | Date | Thu, 27 Feb 2003 19:30:03 +0000 (UTC) |
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In article <20030226172213.O3910@devserv.devel.redhat.com>, Jakub Jelinek <jakub@redhat.com> wrote: > >To fix that, __constant_memcpy would have to access the data through >union,
Which is impossible, since memcpy _fundamentally_ cannot know what the different types are..
> or you could as well forget about __constant_memcpy and use >__builtin_memcpy where gcc will take care about the constant copying.
Which is impossible because (a) historically __builtin_memcpy does a bad job and (b) it doesn't solve the generic case anyway, ie for other non-memcpy things.
The fact is, for type-based alias analysis gcc needs a way to tell it "this can alias", which it doesn't have. Unions are _not_ useful, _regardless_ of what silly language lawyers say, since they are not a generic method. Unions only work for trivial and largely uninteresting cases, and it doesn't _matter_ what C99 says about the issue, since that nasty thing called "real life" interferes.
Until we get some non-union way to say "this can alias", that -fno-strict-alias has to stay because gcc is too broken to allow us doing interesting stuff in-line without it.
My personal opinion is (and was several years ago when this started coming up) that a cast (any cast) should do it. But I don't are _what_ it is, as long as it is syntactically sane and isn't limited to special cases like unions.
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