Correctly deal with identity copies in RegisterCoalescer.

Now that the coalescer keeps live intervals and machine code in sync at
all times, it needs to deal with identity copies differently.

When merging two virtual registers, all identity copies are removed
right away. This means that other identity copies must come from
somewhere else, and they are going to have a value number.

Deal with such copies by merging the value numbers before erasing the
copy instruction. Otherwise, we leave dangling value numbers in the live
interval.

This fixes PR12927.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157340 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 100 insertions, 7 deletions

diff --git a/lib/CodeGen/RegisterCoalescer.cpp b/lib/CodeGen/RegisterCoalescer.cpp
index 30997c24ba..db117ed1a0 100644
--- a/lib/CodeGen/RegisterCoalescer.cpp
+++ b/lib/CodeGen/RegisterCoalescer.cpp
@@ -988,20 +988,31 @@ bool RegisterCoalescer::joinCopy(MachineInstr *CopyMI, bool &Again) {
     return true;
   }
 
-  // If they are already joined we continue.
-  if (CP.getSrcReg() == CP.getDstReg()) {
-    DEBUG(dbgs() << "\tCopy already coalesced.\n");
+  // Eliminate undefs.
+  if (!CP.isPhys() && eliminateUndefCopy(CopyMI, CP)) {
+    DEBUG(dbgs() << "\tEliminated copy of <undef> value.\n");
     LIS->RemoveMachineInstrFromMaps(CopyMI);
     CopyMI->eraseFromParent();
     return false;  // Not coalescable.
   }
 
-  // Eliminate undefs.
-  if (!CP.isPhys() && eliminateUndefCopy(CopyMI, CP)) {
-    DEBUG(dbgs() << "\tEliminated copy of <undef> value.\n");
+  // Coalesced copies are normally removed immediately, but transformations
+  // like removeCopyByCommutingDef() can inadvertently create identity copies.
+  // When that happens, just join the values and remove the copy.
+  if (CP.getSrcReg() == CP.getDstReg()) {
+    LiveInterval &LI = LIS->getInterval(CP.getSrcReg());
+    DEBUG(dbgs() << "\tCopy already coalesced: " << LI << '\n');
+    LiveRangeQuery LRQ(LI, LIS->getInstructionIndex(CopyMI));
+    if (VNInfo *DefVNI = LRQ.valueDefined()) {
+      VNInfo *ReadVNI = LRQ.valueIn();
+      assert(ReadVNI && "No value before copy and no <undef> flag.");
+      assert(ReadVNI != DefVNI && "Cannot read and define the same value.");
+      LI.MergeValueNumberInto(DefVNI, ReadVNI);
+      DEBUG(dbgs() << "\tMerged values:          " << LI << '\n');
+    }
     LIS->RemoveMachineInstrFromMaps(CopyMI);
     CopyMI->eraseFromParent();
-    return false;  // Not coalescable.
+    return true;
   }
 
   // Enforce policies.
diff --git a/test/CodeGen/X86/coalescer-identity.ll b/test/CodeGen/X86/coalescer-identity.ll
new file mode 100644
index 0000000000..9c72ee6296
--- /dev/null
+++ b/test/CodeGen/X86/coalescer-identity.ll
@@ -0,0 +1,82 @@
+; RUN: llc < %s -verify-coalescing
+; PR12927
+target triple = "x86_64-apple-macosx10.8.0"
+
+; This is a case where removeCopyByCommutingDef() creates an identity copy that
+; joinCopy must then deal with correctly.
+
+@s = common global i16 0, align 2
+@g1 = common global i32 0, align 4
+@g2 = common global i32 0, align 4
+@g0 = common global i32 0, align 4
+
+define void @func() nounwind uwtable ssp {
+for.body.lr.ph:
+  %0 = load i32* @g2, align 4, !tbaa !0
+  %tobool6 = icmp eq i32 %0, 0
+  %s.promoted = load i16* @s, align 2
+  %.pre = load i32* @g1, align 4, !tbaa !0
+  br i1 %tobool6, label %for.body.us, label %for.body
+
+for.body.us:                                      ; preds = %for.body.lr.ph, %for.inc.us
+  %1 = phi i32 [ %3, %for.inc.us ], [ %.pre, %for.body.lr.ph ]
+  %dec13.us = phi i16 [ %dec12.us, %for.inc.us ], [ %s.promoted, %for.body.lr.ph ]
+  %i.011.us = phi i32 [ %inc.us, %for.inc.us ], [ undef, %for.body.lr.ph ]
+  %v.010.us = phi i32 [ %phitmp.us, %for.inc.us ], [ 1, %for.body.lr.ph ]
+  %tobool1.us = icmp ne i32 %v.010.us, 0
+  %2 = zext i1 %tobool1.us to i16
+  %lnot.ext.us = xor i16 %2, 1
+  %add.us = add i16 %dec13.us, %lnot.ext.us
+  %conv3.us = zext i16 %add.us to i32
+  %add4.us = sub i32 0, %1
+  %tobool5.us = icmp eq i32 %conv3.us, %add4.us
+  br i1 %tobool5.us, label %for.inc.us, label %if.then7.us
+
+for.inc.us:                                       ; preds = %cond.end.us, %for.body.us
+  %3 = phi i32 [ %1, %for.body.us ], [ %4, %cond.end.us ]
+  %dec12.us = phi i16 [ %add.us, %for.body.us ], [ %dec.us, %cond.end.us ]
+  %inc.us = add i32 %i.011.us, 1
+  %phitmp.us = udiv i32 %v.010.us, 12
+  %tobool.us = icmp eq i32 %inc.us, 0
+  br i1 %tobool.us, label %for.end, label %for.body.us
+
+cond.end.us:                                      ; preds = %if.then7.us, %cond.false.us
+  %4 = phi i32 [ 0, %cond.false.us ], [ %1, %if.then7.us ]
+  %cond.us = phi i32 [ 0, %cond.false.us ], [ %v.010.us, %if.then7.us ]
+  store i32 %cond.us, i32* @g0, align 4, !tbaa !0
+  br label %for.inc.us
+
+cond.false.us:                                    ; preds = %if.then7.us
+  store i32 0, i32* @g1, align 4, !tbaa !0
+  br label %cond.end.us
+
+if.then7.us:                                      ; preds = %for.body.us
+  %dec.us = add i16 %add.us, -1
+  br i1 %tobool1.us, label %cond.end.us, label %cond.false.us
+
+for.body:                                         ; preds = %for.body.lr.ph, %for.body
+  %dec13 = phi i16 [ %dec12, %for.body ], [ %s.promoted, %for.body.lr.ph ]
+  %i.011 = phi i32 [ %inc, %for.body ], [ undef, %for.body.lr.ph ]
+  %v.010 = phi i32 [ %phitmp, %for.body ], [ 1, %for.body.lr.ph ]
+  %tobool1 = icmp eq i32 %v.010, 0
+  %lnot.ext = zext i1 %tobool1 to i16
+  %add = add i16 %dec13, %lnot.ext
+  %conv3 = zext i16 %add to i32
+  %add4 = sub i32 0, %.pre
+  %not.tobool5 = icmp ne i32 %conv3, %add4
+  %dec = sext i1 %not.tobool5 to i16
+  %dec12 = add i16 %add, %dec
+  %inc = add i32 %i.011, 1
+  %phitmp = udiv i32 %v.010, 12
+  %tobool = icmp eq i32 %inc, 0
+  br i1 %tobool, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.inc.us, %for.body
+  %dec12.lcssa = phi i16 [ %dec12.us, %for.inc.us ], [ %dec12, %for.body ]
+  store i16 %dec12.lcssa, i16* @s, align 2
+  ret void
+}
+
+!0 = metadata !{metadata !"int", metadata !1}
+!1 = metadata !{metadata !"omnipotent char", metadata !2}
+!2 = metadata !{metadata !"Simple C/C++ TBAA"}