diff --git a/s/cpnanopass.ss b/s/cpnanopass.ss
index c91d1056ad..bd38f64418 100644
--- a/s/cpnanopass.ss
+++ b/s/cpnanopass.ss
@@ -13670,7 +13670,7 @@
                ,(constant-case architecture
                   [(x86_64)
                    `(seq
-                     (set! ,%rax (inline ,(make-info-inline) ,%popcount ,%rdi))
+                     (set! ,%rax (inline ,(make-info-inline) ,%popcount ,%rcx))
                      (asm-c-return ,null-info ,%rax))]
                   [else
                    ;; Generate anything, since this should not get called
diff --git a/s/x86_64.ss b/s/x86_64.ss
index 5c187b9915..9cb5b3e869 100644
--- a/s/x86_64.ss
+++ b/s/x86_64.ss
@@ -1642,9 +1642,9 @@
                (asm-helper-call code* target dest-rax)]
               [else
                ;; Used for the body of `popcount-slow`.
-               ;; This is the sequence generated by LLVM's __builtin_popcountl()
-               ;; __builtin_popcountl() intrinsic, but with pushes and pops
-               ;; to save used registers other than the result register %rax.
+               ;; This is the sequence generated by LLVM's __builtin_popcountl(),
+               ;; but with pushes and pops to save used registers other than the
+               ;; result register %rax.
                (emit-literal-code (op dest-rax src-rcx code*)
                  51              ; pushq   %rcx
                  57              ; pushq   %rdi