I received a notification that at least one abidiff is sensitive to the particular implementation of std::sort.

std::sort gives no guarantees as to the relative ordering of elements that compare equal (!(a < b) && !(b < a)).

The comparison functor diff_less_than_functor only examines the first diff subjects and in general there is no guarantee that sorting distinct diffs will yield a consistent ordering.

The particular test case that was reported was this one:

--- tests/data/test-abidiff-exit/test-member-size-report0.txt	2022-01-30 18:02:19.412941913 -0800
+++ tests/output/test-abidiff-exit/test-member-size-report0.txt	2022-01-30 18:02:20.180941537 -0800
@@ -4,16 +4,15 @@
 2 functions with some indirect sub-type change:
 
   [C] 'function void reg1(S*, T*, T*)' at test-member-size-v1.cc:26:1 has some indirect sub-type changes:
-    parameter 1 of type 'S*' has sub-type changes:
-      in pointed to type 'struct S' at test-member-size-v1.cc:3:1:
-        type size changed from 128 to 192 (in bits)
-        1 data member insertion:
-          'int y', at offset 128 (in bits) at test-member-size-v1.cc:6:1
-        no data member change (1 filtered);
-    parameter 2 of type 'T*' has sub-type changes:
+    parameter 3 of type 'T*' has sub-type changes:
       in pointed to type 'struct T' at test-member-size-v1.cc:14:1:
         type size changed from 192 to 256 (in bits)
-        1 data member changes (1 filtered):
+        2 data member changes:
+          type of 'S s' changed:
+            type size changed from 128 to 192 (in bits)
+            1 data member insertion:
+              'int y', at offset 128 (in bits) at test-member-size-v1.cc:6:1
+            no data member change (1 filtered);
           'int a' offset changed from 128 to 192 (in bits) (by +64 bits)
 

In general, dependence on the implementation of std::sort can be discovered by randomising the input order before sorting.

There are two approaches to improving order stability:

1. increase the discrimination of the comparison function
2. use stable_sort

I'll try to post a patch that does both.