While working on reproducible builds for openSUSE, I found that
our smlnj package varies for each build.

+++ new /usr/lib64/smlnj/lib/xml-lib.cm/.cm/amd64-unix/xml-lib.cm (hex)
@@ -1,4 +1,4 @@
-00000000  ac 24 34 bc 54 ff 03 61  53 35 85 7e 76 45 dd c4  |.$4.T..aS5.~vE..|
+00000000  cc f9 af ec d7 3a 5c c5  24 ac e7 60 af f4 dc d1  |.....:\.$..`....|

@@ -30,7 +30,7 @@
 000001d0  b1 00 00 12 e6 6e b2 00  e4 a2 3c 50 65 72 76 61  |.....n....<Perva|
 000001e0  73 69 76 65 3e 22 ff 86  3c ff 85 13 ff 86 3c 00  |sive>"..<.....<.|
 000001f0  98 18 31 70 ff 81 30 37  47 63 6e c3 42 f0 a2 47  |..1p..07Gcn.B..G|
-00000200  5a ab fb 4e cb f5 f8 8d  78 67 e8 4f 94 24 ed 22  |Z..N....xg.O.$."|
+00000200  5a ab fb 4e cb f5 f9 84  7a 61 e8 4e 94 27 ed 22  |Z..N....za.N.'."|
 00000210  00 f3 e3 a2 58 4d 4c 5f  53 43 48 45 4d 41 22 74  |....XML_SCHEMA"t|


my tools tell me that such files are created by
"/home/abuild/rpmbuild/BUILD/smlnj-110.99/bin/.run/run.amd64-linux",
"@SMLcmdname=/home/abuild/rpmbuild/BUILD/smlnj-110.99/bin/sml",
"@SMLload=/home/abuild/rpmbuild/BUILD/smlnj-110.99/bin/.heap/sml", "-m",
"$smlnj/installer.cm"

Also it is not one of the usual sources of non-determinism.
Maybe from randomness or ASLR?

https://salsa.debian.org/reproducible-builds/reproducible-notes/-/blob/0f182efded/packages.yml#L26854
lists
    - random_order_in_static_libraries