[llvm-commits] [llvm] r153848 - in /llvm/trunk: lib/CodeGen/SelectionDAG/DAGCombiner.cpp lib/Target/X86/X86ISelLowering.cpp test/CodeGen/ARM/reg_sequence.ll test/CodeGen/CellSPU/rotate_ops.ll test/CodeGen/X86/2011-10-27-tstore.ll test/CodeGen/X86

[Eli Friedman]

2012/4/2 Rotem, Nadav <nadav.rotem at intel.com>:
> Hi Eli,
>
> Thanks for reviewing the patch.  I understand your comment and I agree that in some cases the user may generate "good" shuffles that this optimization may turn into complex shuffles, for which we generate poor code.  After reading your comment I tried to mitigate this problem by checking that the original shuffle node has a single user.  I think that users that want to generate exact shuffle instructions should use intrinsics.  Do you think that we should not optimize shuffles in the DagComb and in InstComb , or maybe we should only optimize for known patterns ?

(Sorry about the delay; I meant to send this sooner.)

Traditionally, we've assumed that a shufflevector written in IR is
likely to be a legal shuffle, and tried to avoid breaking them because
our handling of illegal shuffles is less than ideal.  So instcombine,
for example, is quite conservative in its shuffle handling.  Not sure
how much that applies to the transformation you're adding here.

-Eli