2 files changed, 18 insertions, 0 deletions
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
index bb08fd2f54a..0ec338e2e12 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
@@ -4084,6 +4084,20 @@ void AMDGPUTargetLowering::computeKnownBitsForTargetNode(
       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;
+    }
+  }
   }
 }
 
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUSubtarget.h b/llvm/lib/Target/AMDGPU/AMDGPUSubtarget.h
index 2cc48a7f9f2..460ff82efc5 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUSubtarget.h
+++ b/llvm/lib/Target/AMDGPU/AMDGPUSubtarget.h
@@ -228,6 +228,10 @@ public:
     return WavefrontSize;
   }
 
+  unsigned getWavefrontSizeLog2() const {
+    return Log2_32(WavefrontSize);
+  }
+
   int getLocalMemorySize() const {
     return LocalMemorySize;
   }