Export of internal Abseil changes

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f0456157cdc6cef6dbb5d2f99f9dc3ca0e213fab by Abseil Team <absl-team@google.com>:

Improve flag_benchmark reproducibility, test multiple threads

This adds multiple threads to the flags benchmark, and also uses some
linker magic to ensure that the defined flags are mapped to a consistent
alignment regardless of other code changes in the benchmark binary.

The benchmark performance in multiple threads is somewhat sensitive to
the alignment of the FlagImpl class, depending on whether the flag value
and the absl::Mutex are on the same cacheline or different ones. Making
all flags be cacheline-aligned would probably waste too much memory,
so this CL instead just makes the memory alignment consistent within the
tests.

PiperOrigin-RevId: 347536882

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a019646d306b9497a1eba7efa5e2c4c651d9979d by Derek Mauro <dmauro@google.com>:

Fix other comments in node_hash_set.h

PiperOrigin-RevId: 347462975

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66a4b39fe84a3e81935ad08ee76291df13d1d5e6 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 347436244

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52f4d18c2c4439d26428f64ac4de41e6183919ee by Mark Barolak <mbar@google.com>:

container: fix introduction for node_hash_set.h

Import of https://github.com/abseil/abseil-cpp/pull/861

PiperOrigin-RevId: 347432772
GitOrigin-RevId: f0456157cdc6cef6dbb5d2f99f9dc3ca0e213fab
Change-Id: I629f27e5b9584d92b0a42284c1acf708779bad3f

3 files changed, 28 insertions, 2 deletions

diff --git a/absl/flags/BUILD.bazel b/absl/flags/BUILD.bazel
index 78d6da74..6e5c4e87 100644
--- a/absl/flags/BUILD.bazel
+++ b/absl/flags/BUILD.bazel
@@ -376,9 +376,13 @@ cc_binary(
         "flag_benchmark.cc",
     ],
     copts = ABSL_TEST_COPTS,
+    linkopts = select({
+        "//conditions:default": [],
+    }) + ABSL_DEFAULT_LINKOPTS,
     tags = ["benchmark"],
     visibility = ["//visibility:private"],
     deps = [
+        "flag_benchmark.lds",
         ":flag",
         ":marshalling",
         ":parse",
diff --git a/absl/flags/flag_benchmark.cc b/absl/flags/flag_benchmark.cc
index 9982b604..57584f85 100644
--- a/absl/flags/flag_benchmark.cc
+++ b/absl/flags/flag_benchmark.cc
@@ -103,8 +103,17 @@ std::string AbslUnparseFlag(const UDT&) { return ""; }
 
 #define FLAG_DEF(T) ABSL_FLAG(T, T##_flag, {}, "");
 
+#if defined(__clang__) && defined(__linux__)
+// Force the flags used for benchmarks into a separate ELF section.
+// This ensures that, even when other parts of the code might change size,
+// the layout of the flags across cachelines is kept constant. This makes
+// benchmark results more reproducible across unrelated code changes.
+#pragma clang section data = ".benchmark_flags"
+#endif
 BENCHMARKED_TYPES(FLAG_DEF)
-
+#if defined(__clang__) && defined(__linux__)
+#pragma clang section data = ""
+#endif
 // Register thousands of flags to bloat up the size of the registry.
 // This mimics real life production binaries.
 #define DEFINE_FLAG_0(name) ABSL_FLAG(int, name, 0, "");
@@ -130,7 +139,7 @@ namespace {
       benchmark::DoNotOptimize(absl::GetFlag(FLAGS_##T##_flag)); \
     }                                                            \
   }                                                              \
-  BENCHMARK(BM_GetFlag_##T);
+  BENCHMARK(BM_GetFlag_##T)->ThreadRange(1, 16);
 
 BENCHMARKED_TYPES(BM_GetFlag)
 
diff --git a/absl/flags/flag_benchmark.lds b/absl/flags/flag_benchmark.lds
new file mode 100644
index 00000000..af115dfc
--- /dev/null
+++ b/absl/flags/flag_benchmark.lds
@@ -0,0 +1,13 @@
+/* This linker script forces the flags used by flags_benchmark
+ * into a separate page-aligned section. This isn't necessary for
+ * correctness but ensures that the benchmark results are more
+ * reproducible across unrelated code changes.
+ */
+SECTIONS {
+  .benchmark_flags : {
+    . = ALIGN(0x1000);
+    * (.benchmark_flags);
+  }
+}
+
+INSERT AFTER .data