diff --git a/llvm/test/tools/UpdateTestChecks/update_llc_test_checks/prefix-never-matches.test b/llvm/test/tools/UpdateTestChecks/update_llc_test_checks/prefix-never-matches.test
--- a/llvm/test/tools/UpdateTestChecks/update_llc_test_checks/prefix-never-matches.test
+++ b/llvm/test/tools/UpdateTestChecks/update_llc_test_checks/prefix-never-matches.test
@@ -4,5 +4,5 @@
 # RUN: %update_llc_test_checks %t.ll 2>&1 | FileCheck %s
 # RUN: FileCheck --input-file=%t.ll %s --check-prefix=OUTPUT
 
-# CHECK: WARNING: Prefix A had conflicting output
-# OUTPUT-NOT: A:
\ No newline at end of file
+# CHECK: WARNING: prefix A had conflicting output
+# OUTPUT-NOT: A:
diff --git a/llvm/test/tools/UpdateTestChecks/update_test_checks/Inputs/conflict.ll b/llvm/test/tools/UpdateTestChecks/update_test_checks/Inputs/conflict.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/UpdateTestChecks/update_test_checks/Inputs/conflict.ll
@@ -0,0 +1,21 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+; RUN: opt < %s -vectorize-slp -S | FileCheck %s
+
+define i32 @diff1(i32 %X, i32 %Y) {
+  %Z = sub i32 %X, %Y
+  %Q = add i32 %Z, %Y
+  ret i32 %Q
+}
+
+define <2 x i32> @diff2(<2 x i32> %X) {
+  %A = and <2 x i32> %X, <i32 4, i32 4>
+  %B = lshr <2 x i32> %A, <i32 2, i32 2>
+  %C = icmp eq <2 x i32> %B, zeroinitializer
+  %D = zext <2 x i1> %C to <2 x i32>
+  ret <2 x i32> %D
+}
+
+define <4 x float> @same(<4 x float> %X, <4 x float> %Y) {
+  %A = shufflevector <4 x float> %X, <4 x float> %Y, <4 x i32> <i32 0, i32 2, i32 4, i32 6>
+  ret <4 x float> %A
+}
diff --git a/llvm/test/tools/UpdateTestChecks/update_test_checks/prefix-never-matches.test b/llvm/test/tools/UpdateTestChecks/update_test_checks/prefix-never-matches.test
--- a/llvm/test/tools/UpdateTestChecks/update_test_checks/prefix-never-matches.test
+++ b/llvm/test/tools/UpdateTestChecks/update_test_checks/prefix-never-matches.test
@@ -2,5 +2,5 @@
 # RUN: %update_test_checks %t.ll 2>&1 | FileCheck %s
 # RUN: FileCheck --input-file=%t.ll %s --check-prefix=OUTPUT
 
-# CHECK: WARNING: Prefix A had conflicting output
-# OUTPUT-NOT: A:
\ No newline at end of file
+# CHECK: WARNING: prefix A had conflicting output
+# OUTPUT-NOT: A:
diff --git a/llvm/test/tools/UpdateTestChecks/update_test_checks/prefix_conflicts.test b/llvm/test/tools/UpdateTestChecks/update_test_checks/prefix_conflicts.test
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/UpdateTestChecks/update_test_checks/prefix_conflicts.test
@@ -0,0 +1,4 @@
+# Verify the script warns about conflicts of prefix in multiple run lines.
+# RUN: cp %S/Inputs/conflict.ll %t.ll
+# RUN: %update_test_checks %t.ll 2>&1 | FileCheck %s
+# CHECK: prefix CHECK had conflicting output from different RUN lines for functions: diff1, diff2
diff --git a/llvm/utils/UpdateTestChecks/common.py b/llvm/utils/UpdateTestChecks/common.py
--- a/llvm/utils/UpdateTestChecks/common.py
+++ b/llvm/utils/UpdateTestChecks/common.py
@@ -315,8 +315,9 @@
         self._global_var_dict.update({prefix:dict()})
 
   def finish_and_get_func_dict(self):
-    for prefix in self._get_failed_prefixes():
-      warn('Prefix %s had conflicting output from different RUN lines for all functions in test %s' % (prefix,self._path,))
+    for prefix, funcs in self._get_failed_prefixes_with_functions():
+      warn('prefix %s had conflicting output from different RUN lines for functions: %s' % (
+          prefix, ', '.join(funcs)))
     return self._func_dict
 
   def func_order(self):
@@ -407,17 +408,17 @@
             scrubbed_body, scrubbed_extra, args_and_sig, attrs)
         self._func_order[prefix].append(func)
 
-  def _get_failed_prefixes(self):
-    # This returns the list of those prefixes that failed to match any function,
-    # because there were conflicting bodies produced by different RUN lines, in
-    # all instances of the prefix. Effectively, this prefix is unused and should
-    # be removed.
+  def _get_failed_prefixes_with_functions(self):
+    # This returns the list of prefixes that failed to match some functions,
+    # along with function names, because there were conflicting bodies produced
+    # by different RUN lines, in all instrances of the prefix. This prefix needs
+    # to be split or removed.
     for prefix in self._func_dict:
-      if (self._func_dict[prefix] and
-          (not [fct for fct in self._func_dict[prefix]
-                if self._func_dict[prefix][fct] is not None])):
-        yield prefix
-
+      if self._func_dict[prefix]:
+        funcs = [fct for fct in self._func_dict[prefix]
+                 if self._func_dict[prefix][fct] is None]
+        if funcs:
+          yield (prefix, funcs)
 
 ##### Generator of LLVM IR CHECK lines