Fix PR2343. An *interesting* coalescer bug.

BB1:                                                                                                                                                  
  vr1025 = copy vr1024                                                                                                                                
  ..                                                                                                                                                  
BB2:                                                                                                                                                  
  vr1024 = op                                                                                                                                         
         = op vr1025                                                                                                                                     
  <loop eventually branch back to BB1>

Even though vr1025 is copied from vr1024, it's not safe to coalesced them since live range of vr1025 intersects the def of vr1024. This happens when vr1025 is assigned the value of the previous iteration of vr1024 in the loop.

llvm-svn: 51394

1 files changed, 27 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp b/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
index 5e18534c9e0..8916d9a9c5f 100644
--- a/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
+++ b/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
@@ -1322,7 +1322,20 @@ bool SimpleRegisterCoalescing::SimpleJoin(LiveInterval &LHS, LiveInterval &RHS){
         // Copy from the RHS?
         if (!RangeIsDefinedByCopyFromReg(LHS, LHSIt, RHS.reg))
           return false;    // Nope, bail out.
-        
+
+        if (LHSIt->contains(RHSIt->valno->def))
+          // Here is an interesting situation:
+          // BB1:
+          //   vr1025 = copy vr1024
+          //   ..
+          // BB2:
+          //   vr1024 = op 
+          //          = vr1025
+          // Even though vr1025 is copied from vr1024, it's not safe to
+          // coalesced them since live range of vr1025 intersects the
+          // def of vr1024. This happens because vr1025 is assigned the
+          // value of the previous iteration of vr1024.
+          return false;
         EliminatedLHSVals.push_back(LHSIt->valno);
       }
       
@@ -1348,6 +1361,19 @@ bool SimpleRegisterCoalescing::SimpleJoin(LiveInterval &LHS, LiveInterval &RHS){
           // Otherwise, if this is a copy from the RHS, mark it as being merged
           // in.
           if (RangeIsDefinedByCopyFromReg(LHS, LHSIt, RHS.reg)) {
+            if (LHSIt->contains(RHSIt->valno->def))
+              // Here is an interesting situation:
+              // BB1:
+              //   vr1025 = copy vr1024
+              //   ..
+              // BB2:
+              //   vr1024 = op 
+              //          = vr1025
+              // Even though vr1025 is copied from vr1024, it's not safe to
+              // coalesced them since live range of vr1025 intersects the
+              // def of vr1024. This happens because vr1025 is assigned the
+              // value of the previous iteration of vr1024.
+              return false;
             EliminatedLHSVals.push_back(LHSIt->valno);
 
             // We know this entire LHS live range is okay, so skip it now.