patch: make instcombine remove shuffles by reordering vector elements

[Nadav Rotem]

On May 5, 2013, at 7:50 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> Alternatively, we could use TTI so that only inexpensive shuffles are formed by these optimizations.

We can't estimate which shuffles are inexpensive. We have 1000 lines of c++ logic to figure this out during the x86 lowering. The second problem is that InstCombine is a canonicalization pass and not a lowering pass. Only lowering passes should use TTI. If we had a target independent way of estimating the cost of shuffles then DAGCombine would be the right place. 

Thanks,
Nadav 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20130505/fdf605a1/attachment.html>