diff --git a/src/coreclr/runtime/amd64/AllocFast.S b/src/coreclr/runtime/amd64/AllocFast.S
index d5b366b876dad6..c1cdf4cb73a1e5 100644
--- a/src/coreclr/runtime/amd64/AllocFast.S
+++ b/src/coreclr/runtime/amd64/AllocFast.S
@@ -233,8 +233,11 @@ LEAF_END RhpNewArrayFast, _TEXT
 LEAF_ENTRY RhpNewPtrArrayFast, _TEXT
 
         // Delegate overflow handling to the generic helper conservatively
+        // The constant 0x8000000 is (0x40000000 / sizeof(void*))
+        // Some assemblers don't like an expression here, so the
+        // constant expression is reduced to it's simple form.
 
-        cmp         rsi, (0x40000000 / 8) // sizeof(void*)
+        cmp         rsi, 0x8000000        // (0x40000000 / 8)
         jae         C_FUNC(RhpNewArrayFast)
 
         // In this case we know the element size is sizeof(void *), or 8 for x64