Index: lib/Target/AMDGPU/AMDGPUISelLowering.cpp
===================================================================
--- lib/Target/AMDGPU/AMDGPUISelLowering.cpp
+++ lib/Target/AMDGPU/AMDGPUISelLowering.cpp
@@ -4067,6 +4067,20 @@
       Known.Zero.setHighBits(32 - MaxValBits);
     break;
   }
+  case ISD::INTRINSIC_WO_CHAIN: {
+    unsigned IID = cast<ConstantSDNode>(Op.getOperand(0))->getZExtValue();
+    switch (IID) {
+    case Intrinsic::amdgcn_mbcnt_lo:
+    case Intrinsic::amdgcn_mbcnt_hi: {
+      // These return at most the wavefront size - 1.
+      unsigned Size = Op.getValueType().getSizeInBits();
+      Known.Zero.setHighBits(Size- Subtarget->getWavefrontSizeLog2());
+      break;
+    }
+    default:
+      break;
+    }
+  }
   }
 }
 
Index: lib/Target/AMDGPU/AMDGPUSubtarget.h
===================================================================
--- lib/Target/AMDGPU/AMDGPUSubtarget.h
+++ lib/Target/AMDGPU/AMDGPUSubtarget.h
@@ -229,6 +229,10 @@
     return WavefrontSize;
   }
 
+  unsigned getWavefrontSizeLog2() const {
+    return Log2_32(WavefrontSize);
+  }
+
   int getLocalMemorySize() const {
     return LocalMemorySize;
   }
Index: test/CodeGen/AMDGPU/llvm.amdgcn.mbcnt.ll
===================================================================
--- test/CodeGen/AMDGPU/llvm.amdgcn.mbcnt.ll
+++ test/CodeGen/AMDGPU/llvm.amdgcn.mbcnt.ll
@@ -14,6 +14,24 @@
   ret void
 }
 
+; GCN-LABEL: {{^}}mbcnt_lo_known_bits:
+; GCN: v_mbcnt_lo_u32_b32
+; GCN-NOT: and
+define i32 @mbcnt_lo_known_bits(i32 %x, i32 %y) #0 {
+  %lo = call i32 @llvm.amdgcn.mbcnt.lo(i32 %x, i32 %y)
+  %mask = and i32 %lo, 63
+  ret i32 %mask
+}
+
+; GCN-LABEL: {{^}}mbcnt_hi_known_bits:
+; GCN: v_mbcnt_hi_u32_b32
+; GCN-NOT: and
+define i32 @mbcnt_hi_known_bits(i32 %x, i32 %y) #0 {
+  %hi = call i32 @llvm.amdgcn.mbcnt.hi(i32 %x, i32 %y)
+  %mask = and i32 %hi, 63
+  ret i32 %mask
+}
+
 declare i32 @llvm.amdgcn.mbcnt.lo(i32, i32) #0
 declare i32 @llvm.amdgcn.mbcnt.hi(i32, i32) #0
 declare void @llvm.amdgcn.exp.f32(i32, i32, float, float, float, float, i1, i1) #1