LiveInterval: Introduce createMainRangeFromSubranges().

This function constructs the main liverange by merging all subranges if
subregister liveness tracking is available. This should be slightly
faster to compute instead of performing the liveness calculation again
for the main range. More importantly it avoids cases where the main
liverange would cover positions where no subrange was live. These cases
happened for partial definitions where the actual defined part was dead
and only the undefined parts used later.

The register coalescing requires that every part covered by the main
live range has at least one subrange live.

I also expect this function to become usefull later for places where the
subranges are modified in a way that it is hard to correctly fix the
main liverange in the machine scheduler, we can simply reconstruct it
from subranges then.

llvm-svn: 224806

1 files changed, 12 insertions, 7 deletions

diff --git a/llvm/lib/CodeGen/LiveRangeCalc.cpp b/llvm/lib/CodeGen/LiveRangeCalc.cpp
index e449b240d70..1d46161ad71 100644
--- a/llvm/lib/CodeGen/LiveRangeCalc.cpp
+++ b/llvm/lib/CodeGen/LiveRangeCalc.cpp
@@ -105,8 +105,9 @@ void LiveRangeCalc::calculate(LiveInterval &LI) {
       }
     }
 
-    // Create the def in the main liverange.
-    if (MO.isDef())
+    // Create the def in the main liverange. We do not have to do this if
+    // subranges are tracked as we recreate the main range later in this case.
+    if (MO.isDef() && !LI.hasSubRanges())
       createDeadDef(*Indexes, *Alloc, LI, MO);
   }
 
@@ -116,13 +117,17 @@ void LiveRangeCalc::calculate(LiveInterval &LI) {
 
   // Step 2: Extend live segments to all uses, constructing SSA form as
   // necessary.
-  for (LiveInterval::SubRange &S : LI.subranges()) {
+  if (LI.hasSubRanges()) {
+    for (LiveInterval::SubRange &S : LI.subranges()) {
+      resetLiveOutMap();
+      extendToUses(S, Reg, S.LaneMask);
+    }
+    LI.clear();
+    LI.constructMainRangeFromSubranges(*Indexes, *Alloc);
+  } else {
     resetLiveOutMap();
-    extendToUses(S, Reg, S.LaneMask);
+    extendToUses(LI, Reg, ~0u);
   }
-
-  resetLiveOutMap();
-  extendToUses(LI, Reg, ~0u);
 }