--- emacs-26.1+1.orig/src/alloc.c
+++ emacs-26.1+1/src/alloc.c
@@ -1766,14 +1766,14 @@ static char const string_overrun_cookie[
 
 /* The 'max' reserves space for the nbytes union member even when NBYTES + 1 is
    less than the size of that member.  The 'max' is not needed when
-   SDATA_DATA_OFFSET is a multiple of FLEXALIGNOF (struct sdata),
-   because then the alignment code reserves enough space.  */
+   the tail padding of "struct sdata" with a single payload byte is big enough
+   to accomodate the full union "sdata". */
 
 #define SDATA_SIZE(NBYTES)				      \
      ((SDATA_DATA_OFFSET				      \
-       + (SDATA_DATA_OFFSET % FLEXALIGNOF (struct sdata) == 0 \
-	  ? NBYTES					      \
-	  : max (NBYTES, FLEXALIGNOF (struct sdata) - 1))     \
+       + (FLEXSIZEOF (struct sdata, data, 1) < sizeof (sdata) \
+	  ? max (NBYTES, sizeof (sdata) - SDATA_DATA_OFFSET - 1) \
+	  : NBYTES)                                           \
        + 1						      \
        + FLEXALIGNOF (struct sdata) - 1)		      \
       & ~(FLEXALIGNOF (struct sdata) - 1))