[LLVMdev] [PATCH] Add new phase to legalization to handle vector operations

[Eli Friedman]

On Wed, May 20, 2009 at 4:55 PM, Dan Gohman <gohman at apple.com> wrote:
> Can you explain why you chose the approach of using a new pass?
> I pictured removing LegalizeDAG's type legalization code would
> mostly consist of finding all the places that use TLI.getTypeAction
> and just deleting code for handling its Expand and Promote. Are you
> anticipating something more complicated?

Consider the case of an illegal operation (like division) on a <2 x
i64> on x86-32.  If we unroll it, the result has to go through type
legalization.  Therefore, if we want to remove the type legalization
code from LegalizeDAG, we have to unroll vector operations first, then
run a round of type legalization.  (There are potential alternatives,
like making LegalizeDAG call back into type legalization, but it gets
messy.)

Actually, I went ahead and tried writing the patch to remove type
legalization from LegalizeDAG; I actually ran into another issue,
which is the legalization of i32 UINT_TO_FP.  We want to legalize it
into an i64 UINT_TO_FP, but that requires type legalization.  I'm not
quite sure what to do about that; if I can't come up with a better
solution, I'll force the x86 backend to custom-lower that case.

-Eli