[llvm] update_test_checks: keep meta variables stable by default (PR #76748)

[Nicolai Hähnle via llvm-commits]

================
@@ -1187,20 +1233,317 @@ def may_clash_with_default_check_prefix_name(check_prefix, var):
     )
 
 
+def find_diff_matching(lhs: List[str], rhs: List[str]) -> List[int]:
+    """
+    Find a large ordered matching between strings in lhs and rhs.
+
+    Think of this as finding the *unchanged* lines in a diff, where the entries
+    of lhs and rhs are lines of the files being diffed.
+
+    Returns a list of matched (lhs_idx, rhs_idx) pairs.
+    """
+
+    # Collect matches in reverse order.
+    matches = []
+
+    def recurse(lhs_start, lhs_end, rhs_start, rhs_end):
+        if lhs_start == lhs_end or rhs_start == rhs_end:
+            return
+
+        # First, collect a set of candidate matching edges. We limit this to a
+        # constant multiple of the input size to avoid quadratic runtime.
+        patterns = collections.defaultdict(lambda: ([], []))
+
+        for idx in range(lhs_start, lhs_end):
+            patterns[lhs[idx]][0].append(idx)
+        for idx in range(rhs_start, rhs_end):
+            patterns[rhs[idx]][1].append(idx)
+
+        multiple_patterns = []
+
+        candidates = []
+        for pattern in patterns.values():
+            if not pattern[0] or not pattern[1]:
+                continue
+
+            if len(pattern[0]) == len(pattern[1]) == 1:
+                candidates.append((pattern[0][0], pattern[1][0]))
+            else:
+                multiple_patterns.append(pattern)
+
+        multiple_patterns.sort(key=lambda pattern: len(pattern[0]) * len(pattern[1]))
+
+        for pattern in multiple_patterns:
+            if len(candidates) + len(pattern[0]) * len(pattern[1]) > 2 * (len(lhs) + len(rhs)):
----------------
nhaehnle wrote:

Yes, I considered such options, but I also wanted to be able to write a reasonable test case that still hits the case where we drop candidates due to this check. If you feel really strongly about it (and can ideally support it with a concrete test case) I can look at it, but I'd prefer to just land something simpler for the first version.

https://github.com/llvm/llvm-project/pull/76748