[mlir] Use MLIR op names when generating FileCheck variables in generate-test-checks.py (#160820)

Motivation
----------
Improve readability and stability of autogenerated CHECK lines by using
operation-aware FileCheck variable names instead of generic VAL_N.

What changes
------------
- When possible, variable names are derived from the MLIR op name, e.g.
  `vector.transfer_read` → `TRANSFER_READ_0`.
- Unknown ops (e.g., from out-of-tree dialects) fall back to the prior
  `VAL_N` scheme.

Before
------
```mlir
  // CHECK: %[[VAL_4:.*]] = vector.transfer_read ...
  // CHECK: %[[VAL_5:.*]] = "val_use"(%[[VAL_4]]) : ...
```

After
-----
```mlir
  // CHECK: %[[TRANSFER_READ_0:.*]] = vector.transfer_read ...
  // CHECK: %[[VAL_1:.*]] = "val_use"(%[[TRANSFER_READ_0]]) : ...
```

Rationale
---------
Using op-derived names (e.g., `TRANSFER_READ_0`) makes tests easier to
read and audit, while remaining more stable across unrelated edits (e.g.
there will always be fewer `TRANSFER_READ_#N` variables than `VAL_#N`).
The fallback to `VAL_N` preserves compatibility for unknown ops.

Author: banach-space

mlir/utils/generate-test-checks.py

@@ -31,20 +31,26 @@
 import os  # Used to advertise this file's name ("autogenerated_note").
 import re
 import sys
+from collections import Counter
 
 ADVERT_BEGIN = "// NOTE: Assertions have been autogenerated by "
 ADVERT_END = """
-// The script is designed to make adding checks to
-// a test case fast, it is *not* designed to be authoritative
-// about what constitutes a good test! The CHECK should be
-// minimized and named to reflect the test intent.
+// This script is intended to make adding checks to a test case quick and easy.
+// It is *not* authoritative about what constitutes a good test. After using the
+// script, be sure to review and refine the generated checks. For example,
+// CHECK lines should be minimized and named to reflect the test’s intent.
+// For comprehensive guidelines, see:
+//   * https://mlir.llvm.org/getting_started/TestingGuide/
 """
 
 
 # Regex command to match an SSA identifier.
 SSA_RE_STR = "[0-9]+|[a-zA-Z$._-][a-zA-Z0-9$._-]*"
 SSA_RE = re.compile(SSA_RE_STR)
 
+# Regex matching `dialect.op_name` (e.g. `vector.transfer_read`).
+SSA_OP_NAME_RE = re.compile(r"\b(?:\s=\s[a-z_]+)[.]([a-z_]+)\b")
+
 # Regex matching the left-hand side of an assignment
 SSA_RESULTS_STR = r'\s*(%' + SSA_RE_STR + r')(\s*,\s*(%' + SSA_RE_STR + r'))*\s*='
 SSA_RESULTS_RE = re.compile(SSA_RESULTS_STR)
@@ -63,7 +69,12 @@
 class VariableNamer:
     def __init__(self, variable_names):
         self.scopes = []
+        # Counter for generic FileCHeck names, e.g. VAL_#N
         self.name_counter = 0
+        # Counters for FileCheck names derived from Op names, e.g.
+        # TRANSFER_READ_#N (based on `vector.transfer_read`). Note, there's a
+        # dedicated counter for every Op type present in the input.
+        self.op_name_counter = Counter()
 
         # Number of variable names to still generate in parent scope
         self.generate_in_parent_scope_left = 0
@@ -77,17 +88,29 @@ def generate_in_parent_scope(self, n):
         self.generate_in_parent_scope_left = n
 
     # Generate a substitution name for the given ssa value name.
-    def generate_name(self, source_variable_name, use_ssa_name):
+    def generate_name(self, source_variable_name, use_ssa_name, op_name=""):
 
         # Compute variable name
-        variable_name = self.variable_names.pop(0) if len(self.variable_names) > 0 else ''
-        if variable_name == '':
+        variable_name = (
+            self.variable_names.pop(0) if len(self.variable_names) > 0 else ""
+        )
+        if variable_name == "":
             # If `use_ssa_name` is set, use the MLIR SSA value name to generate
             # a FileCHeck substation string. As FileCheck requires these
             # strings to start with a character, skip MLIR variables starting
             # with a digit (e.g. `%0`).
+            #
+            # The next fallback option is to use the op name, if the
+            # corresponding match succeeds.
+            #
+            # If neither worked, use a generic name: `VAL_#N`.
             if use_ssa_name and source_variable_name[0].isalpha():
                 variable_name = source_variable_name.upper()
+            elif op_name != "":
+                variable_name = (
+                    op_name.upper() + "_" + str(self.op_name_counter[op_name])
+                )
+                self.op_name_counter[op_name] += 1
             else:
                 variable_name = "VAL_" + str(self.name_counter)
                 self.name_counter += 1
@@ -123,6 +146,7 @@ def num_scopes(self):
     def clear_names(self):
         self.name_counter = 0
         self.used_variable_names = set()
+        self.op_name_counter.clear()
 
 class AttributeNamer:
 
@@ -170,8 +194,12 @@ def process_line(line_chunks, variable_namer, use_ssa_name=False, strict_name_re
 
     # Process the rest that contained an SSA value name.
     for chunk in line_chunks:
-        m = SSA_RE.match(chunk)
-        ssa_name = m.group(0) if m is not None else ''
+        ssa = SSA_RE.match(chunk)
+        op_name_with_dialect = SSA_OP_NAME_RE.search(chunk)
+        ssa_name = ssa.group(0) if ssa is not None else ""
+        op_name = (
+            op_name_with_dialect.group(1) if op_name_with_dialect is not None else ""
+        )
 
         # Check if an existing variable exists for this name.
         variable = None
@@ -185,7 +213,7 @@ def process_line(line_chunks, variable_namer, use_ssa_name=False, strict_name_re
             output_line += "%[[" + variable + "]]"
         else:
             # Otherwise, generate a new variable.
-            variable = variable_namer.generate_name(ssa_name, use_ssa_name)
+            variable = variable_namer.generate_name(ssa_name, use_ssa_name, op_name)
             if strict_name_re:
                 # Use stricter regexp for the variable name, if requested.
                 # Greedy matching may cause issues with the generic '.*'