[llvm-commits] [llvm] r157001 - /llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp

[Jakob Stoklund Olesen]

Author: stoklund
Date: Thu May 17 13:32:42 2012
New Revision: 157001

URL: http://llvm.org/viewvc/llvm-project?rev=157001&view=rev
Log:
Never clear <undef> flags on already joined copies.

RegisterCoalescer set <undef> flags on all operands of copy instructions
that are scheduled to be removed. This is so they won't affect
shrinkToUses() by introducing false register reads.

Make sure those <undef> flags are never cleared, or shrinkToUses() could
cause live intervals to end at instructions about to be deleted.

This would be a lot simpler if RegisterCoalescer could just erase joined
copies immediately instead of keeping all the to-be-deleted instructions
around.

This fixes PR12862. Unfortunately, bugpoint can't create a sane test
case for this. Like many other coalescer problems, this failure depends
of a very fragile series of events.

<rdar://problem/11474428>

Modified:
    llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp

Modified: llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp?rev=157001&r1=157000&r2=157001&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp (original)
+++ llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp Thu May 17 13:32:42 2012
@@ -918,6 +918,8 @@
 
   for (MachineRegisterInfo::reg_iterator I = MRI->reg_begin(SrcReg);
        MachineInstr *UseMI = I.skipInstruction();) {
+    bool AlreadyJoined = JoinedCopies.count(UseMI);
+
     // A PhysReg copy that won't be coalesced can perhaps be rematerialized
     // instead.
     if (DstIsPhys) {
@@ -925,7 +927,7 @@
           UseMI->getOperand(1).getReg() == SrcReg &&
           UseMI->getOperand(0).getReg() != SrcReg &&
           UseMI->getOperand(0).getReg() != DstReg &&
-          !JoinedCopies.count(UseMI) &&
+          !AlreadyJoined &&
           reMaterializeTrivialDef(LIS->getInterval(SrcReg), false,
                                   UseMI->getOperand(0).getReg(), UseMI))
         continue;
@@ -937,7 +939,7 @@
 
     // If SrcReg wasn't read, it may still be the case that DstReg is live-in
     // because SrcReg is a sub-register.
-    if (!Reads && SubIdx)
+    if (!Reads && SubIdx && !AlreadyJoined)
       Reads = DstInt.liveAt(LIS->getInstructionIndex(UseMI));
 
     // Replace SrcReg with DstReg in all UseMI operands.
@@ -947,7 +949,7 @@
       // Adjust <undef> flags in case of sub-register joins. We don't want to
       // turn a full def into a read-modify-write sub-register def and vice
       // versa.
-      if (SubIdx && MO.isDef())
+      if (SubIdx && !AlreadyJoined && MO.isDef())
         MO.setIsUndef(!Reads);
 
       if (DstIsPhys)
@@ -957,7 +959,7 @@
     }
 
     // This instruction is a copy that will be removed.
-    if (JoinedCopies.count(UseMI))
+    if (AlreadyJoined)
       continue;
 
     DEBUG({