diff --git a/gcc/mux-utils.h b/gcc/mux-utils.h
index 169a12e7eaa..e252b56e84b 100644
--- a/gcc/mux-utils.h
+++ b/gcc/mux-utils.h
@@ -139,10 +139,10 @@ public:
 private:
   pointer_mux (char *ptr) : m_ptr (ptr) {}
 
-  // The pointer value for A pointers, or the pointer value + 1 for B pointers.
-  // Using a pointer rather than a uintptr_t tells the compiler that second ()
-  // can never return null, and that second_or_null () is only null if
-  // is_first ().
+  // Points to the first byte of an object for A pointers or the second
+  // byte of an object for B pointers.  Using a pointer rather than a
+  // uintptr_t tells the compiler that second () can never return null,
+  // and that second_or_null () is only null if is_first ().
   char *m_ptr;
 };
 
diff --git a/gcc/rtlanal.c b/gcc/rtlanal.c
index d5b203e1372..d1240b0b7c5 100644
--- a/gcc/rtlanal.c
+++ b/gcc/rtlanal.c
@@ -1458,11 +1458,13 @@ set_of (const_rtx pat, const_rtx insn)
 /* Check whether instruction pattern PAT contains a SET with the following
    properties:
 
-   - the SET is executed unconditionally;
-   - the destination of the SET is write-only rather than read-write; and
+   - the SET is executed unconditionally; and
    - either:
      - the destination of the SET is a REG that contains REGNO; or
-     - the destination of the SET is a SUBREG of such a REG.
+     - both:
+       - the destination of the SET is a SUBREG of such a REG; and
+       - writing to the subreg clobbers all of the SUBREG_REG
+	 (in other words, read_modify_subreg_p is false).
 
    If PAT does have a SET like that, return the set, otherwise return null.