diff --git a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
--- a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
@@ -751,7 +751,10 @@
       // with a conservative value of 0 for the counter.
       addWait(Wait, T, 0);
     } else {
-      addWait(Wait, T, UB - ScoreToWait);
+      // If a counter has been maxed out avoid overflow by waiting for
+      // MAX(CounterType) - 1 instead.
+      uint32_t NeededWait = std::min(UB - ScoreToWait, getWaitCountMax(T) - 1);
+      addWait(Wait, T, NeededWait);
     }
   }
 }
diff --git a/llvm/test/CodeGen/AMDGPU/waitcnt-overflow.mir b/llvm/test/CodeGen/AMDGPU/waitcnt-overflow.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/waitcnt-overflow.mir
@@ -0,0 +1,172 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -mcpu=gfx900 -verify-machineinstrs -run-pass si-insert-waitcnts -o - %s | FileCheck -check-prefixes=GFX9,GFX9_10 %s
+# RUN: llc -march=amdgcn -mcpu=gfx1010 -verify-machineinstrs -run-pass si-insert-waitcnts -o - %s | FileCheck -check-prefixes=GFX10,GFX9_10 %s
+
+# Check that we handle cases where a counter has overflowed.
+
+# Overflows lgkmcnt with gfx9 but not with gfx10.
+---
+name:            max-counter-lgkmcnt
+body:             |
+  bb.0:
+    liveins: $vgpr99
+
+    ; GFX9-LABEL: name: max-counter-lgkmcnt
+    ; GFX9: S_WAITCNT 0
+    ; GFX9: $vgpr34_vgpr35 = DS_READ2_B32_gfx9 renamable $vgpr99, 34, 35, 0, implicit $exec
+    ; GFX9-NOT: S_WAITCNT 53119
+    ; GFX9-NEXT: S_WAITCNT 52863
+    ; GFX9-NEXT: $vgpr0 = V_MAC_F32_e32 0, $vgpr1, $vgpr0, implicit $exec
+    ; GFX9-NEXT: $vgpr2 = V_MAC_F32_e32 0, $vgpr3, $vgpr2, implicit $exec
+    ; GFX9-NEXT: $vgpr4 = V_MAC_F32_e32 0, $vgpr5, $vgpr4, implicit $exec
+    ; GFX9-NEXT: $vgpr6 = V_MAC_F32_e32 0, $vgpr7, $vgpr6, implicit $exec
+    ; GFX9-NEXT: S_ENDPGM 0
+    ; GFX10-LABEL: name: max-counter-lgkmcnt
+    ; GFX10: $vgpr34_vgpr35 = DS_READ2_B32_gfx9 renamable $vgpr99, 34, 35, 0, implicit $exec
+    ; GFX10-NEXT: S_WAITCNT 53631
+    ; GFX10-NEXT: $vgpr0 = V_MAC_F32_e32 0, $vgpr1, $vgpr0, implicit $exec
+    ; GFX10-NEXT: S_WAITCNT 53375
+    ; GFX10-NEXT: $vgpr2 = V_MAC_F32_e32 0, $vgpr3, $vgpr2, implicit $exec
+    ; GFX10-NEXT: S_WAITCNT 53119
+    ; GFX10-NEXT: $vgpr4 = V_MAC_F32_e32 0, $vgpr5, $vgpr4, implicit $exec
+    ; GFX10-NEXT: S_WAITCNT 52863
+    ; GFX10-NEXT: $vgpr6 = V_MAC_F32_e32 0, $vgpr7, $vgpr6, implicit $exec
+    ; GFX10-NEXT: S_ENDPGM 0
+    $vgpr0_vgpr1 = DS_READ2_B32_gfx9 renamable $vgpr99, 0, 1, 0, implicit $exec
+    $vgpr2_vgpr3 = DS_READ2_B32_gfx9 renamable $vgpr99, 2, 3, 0, implicit $exec
+    $vgpr4_vgpr5 = DS_READ2_B32_gfx9 renamable $vgpr99, 4, 5, 0, implicit $exec
+    $vgpr6_vgpr7 = DS_READ2_B32_gfx9 renamable $vgpr99, 6, 7, 0, implicit $exec
+    $vgpr8_vgpr9 = DS_READ2_B32_gfx9 renamable $vgpr99, 8, 9, 0, implicit $exec
+    $vgpr10_vgpr11 = DS_READ2_B32_gfx9 renamable $vgpr99, 10, 11, 0, implicit $exec
+    $vgpr12_vgpr13 = DS_READ2_B32_gfx9 renamable $vgpr99, 12, 13, 0, implicit $exec
+    $vgpr14_vgpr15 = DS_READ2_B32_gfx9 renamable $vgpr99, 14, 15, 0, implicit $exec
+    $vgpr16_vgpr17 = DS_READ2_B32_gfx9 renamable $vgpr99, 16, 17, 0, implicit $exec
+    $vgpr18_vgpr19 = DS_READ2_B32_gfx9 renamable $vgpr99, 18, 19, 0, implicit $exec
+    $vgpr20_vgpr21 = DS_READ2_B32_gfx9 renamable $vgpr99, 20, 21, 0, implicit $exec
+    $vgpr22_vgpr23 = DS_READ2_B32_gfx9 renamable $vgpr99, 22, 23, 0, implicit $exec
+    $vgpr24_vgpr25 = DS_READ2_B32_gfx9 renamable $vgpr99, 24, 25, 0, implicit $exec
+    $vgpr26_vgpr27 = DS_READ2_B32_gfx9 renamable $vgpr99, 26, 27, 0, implicit $exec
+    $vgpr28_vgpr29 = DS_READ2_B32_gfx9 renamable $vgpr99, 28, 29, 0, implicit $exec
+    $vgpr30_vgpr31 = DS_READ2_B32_gfx9 renamable $vgpr99, 30, 31, 0, implicit $exec
+    $vgpr32_vgpr33 = DS_READ2_B32_gfx9 renamable $vgpr99, 32, 33, 0, implicit $exec
+    $vgpr34_vgpr35 = DS_READ2_B32_gfx9 renamable $vgpr99, 34, 35, 0, implicit $exec
+    $vgpr0 = V_MAC_F32_e32 0, $vgpr1, $vgpr0, implicit $exec
+    $vgpr2 = V_MAC_F32_e32 0, $vgpr3, $vgpr2, implicit $exec
+    $vgpr4 = V_MAC_F32_e32 0, $vgpr5, $vgpr4, implicit $exec
+    $vgpr6 = V_MAC_F32_e32 0, $vgpr7, $vgpr6, implicit $exec
+    S_ENDPGM 0
+...
+
+# Overflows vmcnt with gfx9 and gfx10.
+---
+name:            max-counter-vmcnt
+body:             |
+  bb.0:
+    liveins: $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4
+
+    ; GFX9_10-LABEL: name: max-counter-vmcnt
+    ; GFX9_10: $vgpr66 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 264, 0, 0, 0, 0, 0, implicit $exec
+    ; GFX9-NOT: S_WAITCNT 53119
+    ; GFX10-NOT: S_WAITCNT 65407
+    ; GFX9-NEXT: S_WAITCNT 53118
+    ; GFX10-NEXT: S_WAITCNT 65406
+    ; GFX9_10-NEXT: $vgpr0 = V_MAC_F32_e32 0, $vgpr1, $vgpr0, implicit $exec
+    ; GFX9_10-NEXT: $vgpr1 = V_MAC_F32_e32 0, $vgpr2, $vgpr1, implicit $exec
+    ; GFX9_10-NEXT: $vgpr2 = V_MAC_F32_e32 0, $vgpr3, $vgpr2, implicit $exec
+    ; GFX9_10-NEXT: $vgpr3 = V_MAC_F32_e32 0, $vgpr4, $vgpr3, implicit $exec
+    ; GFX9_10-NEXT: S_ENDPGM 0
+    $vgpr0 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 0, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr1 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 4, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr2 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 8, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr3 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 12, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr4 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 16, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr5 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 20, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr6 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 24, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr7 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 28, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr8 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 32, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr9 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 36, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr10 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 40, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr11 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 44, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr12 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 48, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr13 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 52, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr14 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 56, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr15 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 60, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr16 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 64, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr17 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 68, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr18 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 72, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr19 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 76, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr20 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 80, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr21 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 84, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr22 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 88, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr23 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 92, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr24 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 96, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr25 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 100, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr26 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 104, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr27 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 108, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr28 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 112, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr29 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 116, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr30 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 120, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr31 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 124, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr32 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 128, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr33 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 132, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr34 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 136, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr35 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 140, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr36 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 144, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr37 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 148, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr38 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 152, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr39 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 156, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr40 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 160, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr41 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 164, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr42 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 168, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr43 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 172, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr44 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 176, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr45 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 180, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr46 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 184, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr47 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 188, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr48 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 192, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr49 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 196, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr50 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 200, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr51 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 204, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr52 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 208, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr53 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 212, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr54 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 216, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr55 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 220, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr56 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 224, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr57 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 228, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr58 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 232, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr59 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 236, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr60 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 240, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr61 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 244, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr62 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 248, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr63 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 252, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr64 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 256, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr65 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 260, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr66 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, 264, 0, 0, 0, 0, 0, implicit $exec
+    $vgpr0 = V_MAC_F32_e32 0, $vgpr1, $vgpr0, implicit $exec
+    $vgpr1 = V_MAC_F32_e32 0, $vgpr2, $vgpr1, implicit $exec
+    $vgpr2 = V_MAC_F32_e32 0, $vgpr3, $vgpr2, implicit $exec
+    $vgpr3 = V_MAC_F32_e32 0, $vgpr4, $vgpr3, implicit $exec
+    S_ENDPGM 0
+...
+
+# Should be no waitcnt if expcnt overflows.
+---
+name:            max-counter-expcnt
+body:             |
+  bb.0:
+    liveins: $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr4, $vgpr0, $vgpr1
+
+    ; GFX9_10-LABEL: name: max-counter-expcnt
+    ; GFX9_10: EXP
+    ; GFX9_10-NOT: S_WAITCNT
+    EXP 0, $vgpr0, $vgpr0, $vgpr0, $vgpr0, -1, -1, 15, implicit $exec
+    EXP 0, $vgpr1, $vgpr1, $vgpr1, $vgpr1, -1, -1, 15, implicit $exec
+    EXP 0, $vgpr1, $vgpr1, $vgpr1, $vgpr1, -1, -1, 15, implicit $exec
+    EXP 0, $vgpr1, $vgpr1, $vgpr1, $vgpr1, -1, -1, 15, implicit $exec
+    EXP 0, $vgpr1, $vgpr1, $vgpr1, $vgpr1, -1, -1, 15, implicit $exec
+    EXP 0, $vgpr1, $vgpr1, $vgpr1, $vgpr1, -1, -1, 15, implicit $exec
+    EXP 0, $vgpr1, $vgpr1, $vgpr1, $vgpr1, -1, -1, 15, implicit $exec
+    EXP 0, $vgpr1, $vgpr1, $vgpr1, $vgpr1, -1, -1, 15, implicit $exec
+    $vgpr0 = V_MAC_F32_e32 0, $vgpr1, $vgpr0, implicit $exec
+    S_ENDPGM 0
+...