Index: llvm/trunk/lib/Target/AMDGPU/GCNHazardRecognizer.cpp
===================================================================
--- llvm/trunk/lib/Target/AMDGPU/GCNHazardRecognizer.cpp
+++ llvm/trunk/lib/Target/AMDGPU/GCNHazardRecognizer.cpp
@@ -215,6 +215,14 @@
   if (!CurrCycleInstr)
     return;
 
+  // Do not track non-instructions which do not affect the wait states.
+  // If included, these instructions can lead to buffer overflow such that
+  // detectable hazards are missed.
+  if (CurrCycleInstr->getOpcode() == AMDGPU::IMPLICIT_DEF)
+    return;
+  else if (CurrCycleInstr->isDebugInstr())
+    return;
+
   unsigned NumWaitStates = TII.getNumWaitStates(*CurrCycleInstr);
 
   // Keep track of emitted instructions
@@ -253,8 +261,7 @@
         return WaitStates;
 
       unsigned Opcode = MI->getOpcode();
-      if (Opcode == AMDGPU::DBG_VALUE || Opcode == AMDGPU::IMPLICIT_DEF ||
-          Opcode == AMDGPU::INLINEASM)
+      if (Opcode == AMDGPU::INLINEASM)
         continue;
     }
     ++WaitStates;
Index: llvm/trunk/test/CodeGen/AMDGPU/hazard.mir
===================================================================
--- llvm/trunk/test/CodeGen/AMDGPU/hazard.mir
+++ llvm/trunk/test/CodeGen/AMDGPU/hazard.mir
@@ -1,4 +1,5 @@
 # RUN: llc -march=amdgcn -mcpu=tonga -verify-machineinstrs -run-pass post-RA-hazard-rec %s -o - | FileCheck -check-prefix=GCN -check-prefix=VI %s
+# RUN: llc -march=amdgcn -mcpu=gfx803 -verify-machineinstrs -run-pass post-RA-hazard-rec %s -o - | FileCheck -check-prefix=GCN -check-prefix=GFX8 %s
 # RUN: llc -march=amdgcn -mcpu=gfx900 -verify-machineinstrs -run-pass post-RA-hazard-rec %s -o - | FileCheck -check-prefix=GCN -check-prefix=GFX9 %s
 
 # GCN-LABEL: name: hazard_implicit_def
@@ -58,3 +59,69 @@
     $vgpr0 = V_INTERP_P1_F32 killed $vgpr4, 0, 0, implicit $m0, implicit $exec
     SI_RETURN_TO_EPILOG killed $vgpr5, killed $vgpr0
 ...
+
+# IMPLICIT_DEF/DBG_VALUE instructions should not prevent the hazard recognizer
+# from adding s_nop instructions between m0 update and s_sendmsg.
+
+# GCN-LABEL: name: hazard-lookahead-implicit-def
+# GCN: $vgpr6 = IMPLICIT_DEF
+# GFX8-NEXT: S_NOP 0
+# GFX9-NEXT: S_NOP 0
+# GCN: S_SENDMSG 3, implicit $exec, implicit $m0
+---
+name: hazard-lookahead-implicit-def
+body: |
+  bb.0:
+    $m0 = S_MOV_B32 killed $sgpr12
+    $vgpr0 = IMPLICIT_DEF
+    $vgpr1 = IMPLICIT_DEF
+    $vgpr2 = IMPLICIT_DEF
+    $vgpr3 = IMPLICIT_DEF
+    $vgpr4 = IMPLICIT_DEF
+    $vgpr5 = IMPLICIT_DEF
+    $vgpr6 = IMPLICIT_DEF
+    S_SENDMSG 3, implicit $exec, implicit $m0
+    S_ENDPGM
+...
+
+# GCN-LABEL: name: hazard-lookahead-dbg-value
+# GCN: DBG_VALUE 6
+# GFX8-NEXT: S_NOP 0
+# GFX9-NEXT: S_NOP 0
+# GCN: S_SENDMSG 3, implicit $exec, implicit $m0
+---
+name: hazard-lookahead-dbg-value
+body: |
+  bb.0:
+    $m0 = S_MOV_B32 killed $sgpr12
+    DBG_VALUE 0
+    DBG_VALUE 1
+    DBG_VALUE 2
+    DBG_VALUE 3
+    DBG_VALUE 4
+    DBG_VALUE 5
+    DBG_VALUE 6
+    S_SENDMSG 3, implicit $exec, implicit $m0
+    S_ENDPGM
+...
+
+# GCN-LABEL: name: hazard-lookahead-dbg-label
+# GCN: DBG_LABEL 6
+# GFX8-NEXT: S_NOP 0
+# GFX9-NEXT: S_NOP 0
+# GCN: S_SENDMSG 3, implicit $exec, implicit $m0
+---
+name: hazard-lookahead-dbg-label
+body: |
+  bb.0:
+    $m0 = S_MOV_B32 killed $sgpr12
+    DBG_LABEL 0
+    DBG_LABEL 1
+    DBG_LABEL 2
+    DBG_LABEL 3
+    DBG_LABEL 4
+    DBG_LABEL 5
+    DBG_LABEL 6
+    S_SENDMSG 3, implicit $exec, implicit $m0
+    S_ENDPGM
+...