Correctly handle <undef> tied uses when rewriting after a split.

This fixes PR10463. A two-address instruction with an <undef> use
operand was incorrectly rewritten so the def and use no longer used the
same register, violating the tie constraint.

Fix this by always rewriting <undef> operands with the register a def
operand would use.

llvm-svn: 135885

1 files changed, 4 insertions, 8 deletions

diff --git a/llvm/lib/CodeGen/SplitKit.cpp b/llvm/lib/CodeGen/SplitKit.cpp
index d607e37331a..46a43d8bd72 100644
--- a/llvm/lib/CodeGen/SplitKit.cpp
+++ b/llvm/lib/CodeGen/SplitKit.cpp
@@ -938,15 +938,11 @@ void SplitEditor::rewriteAssigned(bool ExtendRanges) {
       continue;
     }
 
-    // <undef> operands don't really read the register, so just assign them to
-    // the complement.
-    if (MO.isUse() && MO.isUndef()) {
-      MO.setReg(Edit->get(0)->reg);
-      continue;
-    }
-
+    // <undef> operands don't really read the register, so it doesn't matter
+    // which register we choose.  When the use operand is tied to a def, we must
+    // use the same register as the def, so just do that always.
     SlotIndex Idx = LIS.getInstructionIndex(MI);
-    if (MO.isDef())
+    if (MO.isDef() || MO.isUndef())
       Idx = MO.isEarlyClobber() ? Idx.getUseIndex() : Idx.getDefIndex();
 
     // Rewrite to the mapped register at Idx.