Index: utils/update_test_checks.py
===================================================================
--- utils/update_test_checks.py
+++ utils/update_test_checks.py
@@ -68,7 +68,9 @@
     flags=(re.M | re.S))
 CHECK_PREFIX_RE = re.compile('--check-prefix=(\S+)')
 CHECK_RE = re.compile(r'^\s*;\s*([^:]+?)(?:-NEXT|-NOT|-DAG|-LABEL)?:')
-IR_VALUE_DEF_RE = re.compile(r'\s+%(.*) =')
+# Match things that look at identifiers, but only if they are followed by
+# spaces, commas, paren, or end of the string
+IR_VALUE_RE = re.compile(r'\s+%(.+?)(?:[,\s\(\)]|\Z)')
 
 
 # Invoke the tool that is being tested.
@@ -160,29 +162,28 @@
 # Replace IR value defs and uses with FileCheck variables.
 def genericize_check_lines(lines):
   lines_with_def = []
-  vars_seen = []
+  vars_seen = set()
   for line in lines:
     # An IR variable named '%.' matches the FileCheck regex string.
     line = line.replace('%.', '%dot')
-    m = IR_VALUE_DEF_RE.match(line)
-    if m:
-      vars_seen.append(m.group(1))
-      line = line.replace('%' + m.group(1), get_value_definition(m.group(1)))
-
+    # Ignore any comments, since the check lines will too. We don't
+    # delete them however.
+    scrubbed_line = SCRUB_IR_COMMENT_RE.sub(r'', line)
+    matchiter = IR_VALUE_RE.finditer(scrubbed_line)
+    for match in matchiter:
+      # Vars we've already seen are uses
+      if match.group(1) in vars_seen:
+        line = line.replace('%' + match.group(1), get_value_use(match.group(1)), 1)
+        continue
+      # Vars we've not are defs
+      vars_seen.add(match.group(1))
+      line = line.replace('%' + match.group(1), get_value_definition(match.group(1)), 1)
     lines_with_def.append(line)
-
   # A single def isn't worth replacing?
   #if len(vars_seen) < 2:
   #  return lines
 
-  output_lines = []
-  vars_seen.sort(key=len, reverse=True)
-  for line in lines_with_def:
-    for var in vars_seen:
-      line = line.replace('%' + var, get_value_use(var))
-    output_lines.append(line)
-
-  return output_lines
+  return lines_with_def
 
 
 def add_checks(output_lines, prefix_list, func_dict, func_name, tool_basename):