On Oct 23, 2006, at 10:42 PM, Ian Lance Taylor wrote:
Chris Lattner <clattner@xxxxxxxxx> writes:
union
{
vectortype v;
float af[4];
} u;
u.v = v;
return u.af[0];
Ian
Or, alternatively:
vectortype v;
elementtype e = ((elementtype*)&v)[1]
Well, I don't recommend that in general. gcc currently has an
exception to the aliasing rules, and treats vectortype* and
elementtype* as being in the same alias set.
Right.
But at my previous
company, we got significantly better generated code in some cases by
breaking that aliasing. In particular this aliasing forces a pointer
to a vector of char to alias char* and therefore alias everything.
But if the compiler breaks that aliasing, then your suggestion of
using a cast will fail in some cases.
The compiler shouldn't break that aliasing, and using a union will
give you poor code anyway. LLVM will optimize either idiom into
element extract/insert operations, but I don't think GCC will in an
any case. If you care about performance with GCC, you have to use
ISA-specific builtins.
-Chris
