patch: make instcombine remove shuffles by reordering vector elements

[Owen Anderson]

On May 5, 2013, at 6:53 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> On PPC, however, the situation seems slightly more friendly than on x86 because we still do have a general-purpose permutation instruction, it is just faster (and lowers register pressure) to use an alternative sequence.

ARM NEON has a general shuffle instruction as well (vtbl).  The problem is that we quite often encounter user code that was written with a specific shuffle-like instruction in mind.  If we munge that shuffle too much, it's quite possible that we'll fail to lower it back to a decent sequence, in which case we've pessimized what the user wrote.

--Owen
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20130505/233bd1dd/attachment.html>