--- a/m4/alloca.m4
+++ b/m4/alloca.m4
@@ -17,19 +17,22 @@
 
   # Define an additional variable used in the Makefile substitution.
   if test $ac_cv_working_alloca_h = yes; then
-    AC_CACHE_CHECK([whether alloca must be replaced], [gl_cv_rpl_alloca], [
+    AC_CACHE_CHECK([for alloca as a compiler built-in], [gl_cv_rpl_alloca], [
       AC_EGREP_CPP([Need own alloca], [
 #if defined __GNUC__ || defined _AIX || defined _MSC_VER
-	  Need own alloca
+        Need own alloca
 #endif
-	], [gl_cv_rpl_alloca=yes], [gl_cv_rpl_alloca=no])
+        ], [gl_cv_rpl_alloca=yes], [gl_cv_rpl_alloca=no])
     ])
     if test $gl_cv_rpl_alloca = yes; then
+      dnl OK, alloca can be implemented through a compiler built-in.
       AC_DEFINE([HAVE_ALLOCA], 1,
-	[Define to 1 if you have `alloca' after including <alloca.h>,
-	 a header that may be supplied by this distribution.])
+        [Define to 1 if you have 'alloca' after including <alloca.h>,
+         a header that may be supplied by this distribution.])
       ALLOCA_H=alloca.h
     else
+      dnl alloca exists as a library function, i.e. it is slow and probably
+      dnl a memory leak. Don't define HAVE_ALLOCA in this case.
       ALLOCA_H=
     fi
   else