IfConversion: Add implicit uses for redefined regs with live subregisters

Normally, if conversion would add implicit uses for redefined registers,
e.g. R0<def> = add_if ..., R0<imp-use>. However, if only subregisters of
R0 are known to be live but not R0 itself, such implicit uses will not be
added, causing prior definitions of such subregisters and R0 itself to
become dead.

llvm-svn: 282626

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/IfConversion.cpp b/llvm/lib/CodeGen/IfConversion.cpp
index f5f752ac27e..025eea229cc 100644
--- a/llvm/lib/CodeGen/IfConversion.cpp
+++ b/llvm/lib/CodeGen/IfConversion.cpp
@@ -1453,6 +1453,17 @@ static void UpdatePredRedefs(MachineInstr &MI, LivePhysRegs &Redefs) {
     }
     if (LiveBeforeMI.count(Reg))
       MIB.addReg(Reg, RegState::Implicit);
+    else {
+      bool HasLiveSubReg = false;
+      for (MCSubRegIterator S(Reg, TRI); S.isValid(); ++S) {
+        if (!LiveBeforeMI.count(*S))
+          continue;
+        HasLiveSubReg = true;
+        break;
+      }
+      if (HasLiveSubReg)
+        MIB.addReg(Reg, RegState::Implicit);
+    }
   }
 }