From: Bob Wilson <bob.wilson@apple.com>
Date: Thu, 13 Aug 2009 02:13:04 +0000 (+0000)
Subject: Add a comment to describe why vector shuffles are legalized to custom DAG nodes.
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=28865062c1ef60e47f0ac23a5ebc22eaf0d1b184;p=oota-llvm.git

Add a comment to describe why vector shuffles are legalized to custom DAG nodes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78884 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Target/ARM/ARMISelLowering.cpp b/lib/Target/ARM/ARMISelLowering.cpp
index 4074fc7f219..785a7ded6d3 100644
--- a/lib/Target/ARM/ARMISelLowering.cpp
+++ b/lib/Target/ARM/ARMISelLowering.cpp
@@ -2439,6 +2439,10 @@ static SDValue LowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG) {
   DebugLoc dl = Op.getDebugLoc();
   EVT VT = Op.getValueType();
 
+  // Convert shuffles that are directly supported on NEON to target-specific
+  // DAG nodes, instead of keeping them as shuffles and matching them again
+  // during code selection.  This is more efficient and avoids the possibility
+  // of inconsistencies between legalization and selection.
   if (SVN->isSplat() && SVN->getSplatIndex() == 0)
     return DAG.getNode(ARMISD::VSPLAT0, dl, VT, SVN->getOperand(0));
   if (isVREVMask(SVN, 64))