diff --git a/llvm/lib/CodeGen/MachineSink.cpp b/llvm/lib/CodeGen/MachineSink.cpp
--- a/llvm/lib/CodeGen/MachineSink.cpp
+++ b/llvm/lib/CodeGen/MachineSink.cpp
@@ -1805,11 +1805,30 @@
     }
     auto DbgValsToSink = DbgValsToSinkMap.takeVector();
 
+    LLVM_DEBUG(dbgs() << "Sink instr " << MI << "\tinto block " << *SuccBB);
+
+    MachineBasicBlock::iterator InsertPos =
+        SuccBB->SkipPHIsAndLabels(SuccBB->begin());
+    if (InsertPos != SuccBB->begin()) {
+      bool UsesClobbered = false;
+      for (MachineBasicBlock::iterator PI = SuccBB->begin();
+           PI != InsertPos && !UsesClobbered; ++PI) {
+        for (auto U : UsedOpsInCopy) {
+          Register SrcReg = MI.getOperand(U).getReg();
+          if (PI->modifiesRegister(SrcReg, TRI)) {
+            LLVM_DEBUG(dbgs() << " *** Not sinking: prologue clobbers uses.\n");
+            UsesClobbered = true;
+            break;
+          }
+        }
+      }
+      if (UsesClobbered)
+        continue;
+    }
+
     // Clear the kill flag if SrcReg is killed between MI and the end of the
     // block.
     clearKillFlags(&MI, CurBB, UsedOpsInCopy, UsedRegUnits, TRI);
-    MachineBasicBlock::iterator InsertPos =
-        SuccBB->SkipPHIsAndLabels(SuccBB->begin());
     performSink(MI, *SuccBB, InsertPos, DbgValsToSink);
     updateLiveIn(&MI, SuccBB, UsedOpsInCopy, DefedRegsInCopy);
 
diff --git a/llvm/test/CodeGen/AMDGPU/sink-after-control-flow-postra.mir b/llvm/test/CodeGen/AMDGPU/sink-after-control-flow-postra.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/sink-after-control-flow-postra.mir
@@ -0,0 +1,72 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -mcpu=gfx1010 -verify-machineinstrs -run-pass=postra-machine-sink -mattr=-wavefrontsize32,+wavefrontsize64 -o - %s | FileCheck -check-prefixes=GFX10 %s
+
+# Ensure that PostRA Machine Sink does not sink instructions
+# past block prologues which would overwrite their uses.
+
+---
+name:            _amdgpu_ps_main
+alignment:       1
+tracksRegLiveness: true
+registers:       []
+liveins:
+  - { reg: '$sgpr4', virtual-reg: '' }
+body:             |
+  ; GFX10-LABEL: name: _amdgpu_ps_main
+  ; GFX10: bb.0:
+  ; GFX10-NEXT:   successors: %bb.1(0x80000000)
+  ; GFX10-NEXT:   liveins: $sgpr4
+  ; GFX10-NEXT: {{  $}}
+  ; GFX10-NEXT:   renamable $sgpr9 = COPY $sgpr4
+  ; GFX10-NEXT:   renamable $vgpr5 = IMPLICIT_DEF
+  ; GFX10-NEXT:   renamable $sgpr0_sgpr1 = nofpexcept V_CMP_NGT_F32_e64 0, 0, 0, $vgpr5, 0, implicit $mode, implicit $exec
+  ; GFX10-NEXT:   S_BRANCH %bb.1
+  ; GFX10-NEXT: {{  $}}
+  ; GFX10-NEXT: bb.1:
+  ; GFX10-NEXT:   successors: %bb.2(0x40000000), %bb.3(0x40000000)
+  ; GFX10-NEXT:   liveins: $sgpr6, $sgpr9, $sgpr0_sgpr1
+  ; GFX10-NEXT: {{  $}}
+  ; GFX10-NEXT:   $sgpr4_sgpr5 = S_AND_SAVEEXEC_B64 $sgpr0_sgpr1, implicit-def $exec, implicit-def $scc, implicit $exec
+  ; GFX10-NEXT:   renamable $sgpr14_sgpr15 = S_XOR_B64 $exec, killed renamable $sgpr4_sgpr5, implicit-def dead $scc
+  ; GFX10-NEXT:   S_CBRANCH_EXECZ %bb.3, implicit $exec
+  ; GFX10-NEXT:   S_BRANCH %bb.2
+  ; GFX10-NEXT: {{  $}}
+  ; GFX10-NEXT: bb.2:
+  ; GFX10-NEXT:   successors: %bb.3(0x80000000)
+  ; GFX10-NEXT:   liveins: $sgpr6
+  ; GFX10-NEXT: {{  $}}
+  ; GFX10-NEXT:   $m0 = COPY killed renamable $sgpr6
+  ; GFX10-NEXT:   S_BRANCH %bb.3
+  ; GFX10-NEXT: {{  $}}
+  ; GFX10-NEXT: bb.3:
+  ; GFX10-NEXT:   S_ENDPGM 0
+  bb.0:
+    successors: %bb.1(0x80000000)
+    liveins: $sgpr4
+
+    renamable $sgpr9 = COPY $sgpr4
+    renamable $vgpr5 = IMPLICIT_DEF
+    renamable $sgpr0_sgpr1 = nofpexcept V_CMP_NGT_F32_e64 0, 0, 0, $vgpr5, 0, implicit $mode, implicit $exec
+    S_BRANCH %bb.1
+
+  bb.1:
+    successors: %bb.2(0x40000000), %bb.8(0x40000000)
+    liveins: $sgpr6, $sgpr9, $sgpr0_sgpr1
+
+    $sgpr4_sgpr5 = S_AND_SAVEEXEC_B64 $sgpr0_sgpr1, implicit-def $exec, implicit-def $scc, implicit $exec
+    renamable $sgpr14_sgpr15 = S_XOR_B64 $exec, killed renamable $sgpr4_sgpr5, implicit-def dead $scc
+    S_CBRANCH_EXECZ %bb.8, implicit $exec
+    S_BRANCH %bb.2
+
+  bb.2:
+    successors: %bb.8(0x40000000)
+    liveins: $sgpr6
+
+    $m0 = COPY killed renamable $sgpr6
+    S_BRANCH %bb.8
+
+  bb.8:
+
+    S_ENDPGM 0
+
+...