From 11330f7526ef11109ca31aa1abbb4f9a698cd95a Mon Sep 17 00:00:00 2001
From: Evan Cheng <evan.cheng@apple.com>
Date: Fri, 12 Oct 2007 09:15:53 +0000
Subject: [PATCH] Restrict EXTRACT_SUBREG coalescing to avoid negative
 performance impact.

llvm-svn: 42903
---
 llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp b/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
index 9d43750f1c68..67e829efee18 100644
--- a/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
+++ b/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
@@ -264,8 +264,13 @@ bool SimpleRegisterCoalescing::JoinCopy(MachineInstr *CopyMI,
         if (li_->hasInterval(*SR) && RHS.overlaps(li_->getInterval(*SR))) {
           DOUT << "Interfere with sub-register ";
           DEBUG(li_->getInterval(*SR).print(DOUT, mri_));
-          return true;
+          return true; // Not coalescable
         }
+    } else if (li_->getInterval(repDstReg).getSize() >
+               li_->getInterval(repSrcReg).getSize()) {
+      // Be conservative. If both sides are virtual registers, do not coalesce
+      // if the sub-register live interval is longer.
+      return false;
     }
   } else if (differingRegisterClasses(repSrcReg, repDstReg)) {
     // If they are not of the same register class, we cannot join them.