[benchmark] Warn about incorrect --memory use

Author: palimondo

benchmark/utils/DriverUtils.swift

@@ -141,6 +141,16 @@ struct TestConfig {
                                     tags: c.tags ?? [],
                                     skipTags: c.skipTags ?? [.unstable, .skip])
 
+    if logMemory && tests.count > 1 {
+      print(
+      """
+      warning: The memory usage of a test, reported as the change in MAX_RSS,
+               is based on measuring the peak memory used by the whole process.
+               These results are meaningful only when running a single test,
+               not in the batch mode!
+      """)
+    }
+
     if verbose {
       let testList = tests.map({ $0.1.name }).joined(separator: ", ")
       print("""
@@ -186,7 +196,7 @@ struct TestConfig {
     func byNamesOrIndices(b: BenchmarkInfo) -> Bool {
       return specifiedTests.contains(b.name) ||
         specifiedTests.contains(indices[b.name]!)
-    } // !! "All registeredBenchmarks have been assigned an index"
+    } // !! "`allTests` have been assigned an index"
     return allTests
       .filter(specifiedTests.isEmpty ? byTags : byNamesOrIndices)
       .map { (index: indices[$0.name]!, info: $0) }

test/benchmark/Benchmark_O.test.md

@@ -188,6 +188,14 @@ BADSKIPTAG: error: 'bogus' is not a valid 'BenchmarkCategory'
 
 ````
 
+Measuring memory use of a test with our method is valid only for single test.
+
+````
+RUN: %Benchmark_O 1 2 --memory --list \
+RUN:         2>&1 | %FileCheck %s --check-prefix WARNMEMORY
+WARNMEMORY: warning:
+````
+
 ## Usage
 
 ````