commit f189e5dc1020db9b41e77488d6290d6e7f032b13 (HEAD, refs/remotes/origin/master)
Author: Mattias Engdegård <mattiase@acm.org>
Date:   Tue Mar 17 13:18:14 2020 +0100

    Don't generate useless range table entries for ASCII chars
    
    In multibyte regexps, each ASCII char or range in a character
    alternative produces a nonsense range table entry in addition to the
    correct bits in the ASCII bitmap.  Those entries do not match anything
    but waste space and time.
    
    * src/regex-emacs.c (regex_compile): Don't generate reversed intervals.

diff --git a/src/regex-emacs.c b/src/regex-emacs.c
index 694431c95e..0ae004e463 100644
--- a/src/regex-emacs.c
+++ b/src/regex-emacs.c
@@ -2113,17 +2113,20 @@ regex_compile (re_char *pattern, ptrdiff_t size,
 			if (CHAR_BYTE8_P (c1))
 			  c = BYTE8_TO_CHAR (128);
 		      }
-		    if (CHAR_BYTE8_P (c))
-		      {
-			c = CHAR_TO_BYTE8 (c);
-			c1 = CHAR_TO_BYTE8 (c1);
-			for (; c <= c1; c++)
-			  SET_LIST_BIT (c);
-		      }
-		    else if (multibyte)
-		      SETUP_MULTIBYTE_RANGE (range_table_work, c, c1);
-		    else
-		      SETUP_UNIBYTE_RANGE (range_table_work, c, c1);
+                    if (c <= c1)
+                      {
+                        if (CHAR_BYTE8_P (c))
+                          {
+                            c = CHAR_TO_BYTE8 (c);
+                            c1 = CHAR_TO_BYTE8 (c1);
+                            for (; c <= c1; c++)
+                              SET_LIST_BIT (c);
+                          }
+                        else if (multibyte)
+                          SETUP_MULTIBYTE_RANGE (range_table_work, c, c1);
+                        else
+                          SETUP_UNIBYTE_RANGE (range_table_work, c, c1);
+                      }
 		  }
 	      }