Index: lib/CodeGen/PeepholeOptimizer.cpp
===================================================================
--- lib/CodeGen/PeepholeOptimizer.cpp
+++ lib/CodeGen/PeepholeOptimizer.cpp
@@ -1715,7 +1715,8 @@
   // Bitcasts with more than one def are not supported.
   if (Def->getDesc().getNumDefs() != 1)
     return ValueTrackerResult();
-  if (Def->getOperand(DefIdx).getSubReg() != DefSubReg)
+  const MachineOperand DefOp = Def->getOperand(DefIdx);
+  if (DefOp.getSubReg() != DefSubReg)
     // If we look for a different subreg, it means we want a subreg of the src.
     // Bails as we do not support composing subregs yet.
     return ValueTrackerResult();
@@ -1735,6 +1736,14 @@
       return ValueTrackerResult();
     SrcIdx = OpIdx;
   }
+
+  // Stop when any user of the bitcast is a SUBREG_TO_REG, replacing with a COPY
+  // will break the assumed guarantees for the upper bits.
+  for (const MachineInstr &UseMI : MRI.use_nodbg_instructions(DefOp.getReg())) {
+    if (UseMI.isSubregToReg())
+      return ValueTrackerResult();
+  }
+
   const MachineOperand &Src = Def->getOperand(SrcIdx);
   return ValueTrackerResult(Src.getReg(), Src.getSubReg());
 }
Index: test/CodeGen/X86/peephole.mir
===================================================================
--- /dev/null
+++ test/CodeGen/X86/peephole.mir
@@ -0,0 +1,40 @@
+# RUN: llc -mtriple=x86_64-- -run-pass=peephole-opt %s -o - | FileCheck %s
+--- |
+  define void @func() { ret void }
+...
+---
+# Check that instructions with MI.isBitcast() are only replaced by COPY if there
+# are no SUBREG_TO_REG users.
+# CHECK-LABEL: name: func
+name: func
+registers:
+  - { id: 0, class: gr32 }
+  - { id: 1, class: fr32 }
+  - { id: 2, class: gr32 }
+
+  - { id: 3, class: gr32 }
+  - { id: 4, class: fr32 }
+  - { id: 5, class: gr32 }
+  - { id: 6, class: gr64 }
+
+body: |
+  bb.0:
+    ; CHECK: %1 = VMOVDI2SSrr %0
+    ; CHECK: %7 = COPY %0
+    ; CHECK: NOOP implicit %7
+    %0 = MOV32ri 42
+    %1 = VMOVDI2SSrr %0
+    %2 = MOVSS2DIrr %1
+    NOOP implicit %2
+
+    ; CHECK: %4 = VMOVDI2SSrr %3
+    ; CHECK-NOT: COPY
+    ; CHECK: %5 = MOVSS2DIrr %4
+    ; CHECK: %6 = SUBREG_TO_REG %5, 0
+    ; CHECK: NOOP implicit %6
+    %3 = MOV32ri 42
+    %4 = VMOVDI2SSrr %3
+    %5 = MOVSS2DIrr %4
+    %6 = SUBREG_TO_REG %5, 0, %subreg.sub_32bit
+    NOOP implicit %6
+...