Add an MRI::tracksLiveness() flag.

Late optimization passes like branch folding and tail duplication can
transform the machine code in a way that makes it expensive to keep the
register liveness information up to date. There is a fuzzy line between
register allocation and late scheduling where the liveness information
degrades.

The MRI::tracksLiveness() flag makes the line clear: While true,
liveness information is accurate, and can be used for register
scavenging. Once the flag is false, liveness information is not
accurate, and can only be used as a hint.

Late passes generally don't need the liveness information, but they will
sometimes use the register scavenger to help update it. The scavenger
enforces strict correctness, and we have to spend a lot of code to
update register liveness that may never be used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153511 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/CodeGen/MachineRegisterInfo.cpp b/lib/CodeGen/MachineRegisterInfo.cpp
index f140decd96..7ea151713a 100644
--- a/lib/CodeGen/MachineRegisterInfo.cpp
+++ b/lib/CodeGen/MachineRegisterInfo.cpp
@@ -18,7 +18,7 @@
 using namespace llvm;
 
 MachineRegisterInfo::MachineRegisterInfo(const TargetRegisterInfo &TRI)
-  : TRI(&TRI), IsSSA(true) {
+  : TRI(&TRI), IsSSA(true), TracksLiveness(true) {
   VRegInfo.reserve(256);
   RegAllocHints.reserve(256);
   UsedPhysRegs.resize(TRI.getNumRegs());