diff --git a/llvm/lib/Target/AMDGPU/Utils/AMDGPUBaseInfo.cpp b/llvm/lib/Target/AMDGPU/Utils/AMDGPUBaseInfo.cpp
--- a/llvm/lib/Target/AMDGPU/Utils/AMDGPUBaseInfo.cpp
+++ b/llvm/lib/Target/AMDGPU/Utils/AMDGPUBaseInfo.cpp
@@ -268,6 +268,13 @@
 }
 
 unsigned getEUsPerCU(const MCSubtargetInfo *STI) {
+  // "Per CU" really means "per whatever functional block the waves of a
+  // workgroup must share". For gfx10 in CU mode this is the CU, which contains
+  // two SIMDs.
+  if (isGFX10(*STI) && STI->getFeatureBits().test(FeatureCuMode))
+    return 2;
+  // Pre-gfx10 a CU contains four SIMDs. For gfx10 in WGP mode the WGP contains
+  // two CUs, so a total of four SIMDs.
   return 4;
 }