diff --git a/simd_x86_64.S b/simd_x86_64.S
index 38ed58b..5c56c2a 100644
--- a/simd_x86_64.S
+++ b/simd_x86_64.S
@@ -1,7 +1,8 @@
 // SIMD operations on potentially-indeterminate Node16::index[16] bytes.
-// Written in assembly so msan doesn't track the loads. The caller is
-// responsible for masking the returned bitfield to [0, numChildren) before
-// using it.
+// Written in assembly because loading and operating on indeterminate values
+// is UB in C++ but well-defined in assembly. (A side effect is that msan
+// doesn't track the loads.) The caller is responsible for masking the
+// returned bitfield to [0, numChildren) before using it.
 //
 // All functions return a 16-bit bitmask in %eax (bit i set = condition true
 // at index i). The upper 16 bits of %eax are zero.