put the compare perf script into the repo (#564)

Motivation:

For a long while I had a script which runs NIO's performance tests
against different Swift toolchains. It should be in the repository.

Modifications:

add scripts/compare_perf_of_swift_versions.sh

Result:

everybody can run the performance tests against different Swift versions

scripts/compare_perf_of_swift_versions.sh

@@ -0,0 +1,99 @@
+#!/bin/bash
+##===----------------------------------------------------------------------===##
+##
+## This source file is part of the SwiftNIO open source project
+##
+## Copyright (c) 2017-2018 Apple Inc. and the SwiftNIO project authors
+## Licensed under Apache License v2.0
+##
+## See LICENSE.txt for license information
+## See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+##
+## SPDX-License-Identifier: Apache-2.0
+##
+##===----------------------------------------------------------------------===##
+
+set -eu
+here="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+function usage() {
+    echo >&2 "$0 SWIFT_BINARY_BASE SWIFT_BINARY_COMPETITOR"
+    echo >&2
+    echo >&2 "Runs the NIOPerformanceTester tool with 'swift' binary "
+    echo >&2 "SWIFT_BINARY_BASE against SWIFT_BINARY_COMPETITOR."
+    echo >&2
+    echo >&2 "Example:"
+    echo >&2 "$0 /Library/Developer/Toolchains/swift-4.1-RELEASE.xctoolchain/usr/bin/swift /Library/Developer/Toolchains/swift-4.2-DEVELOPMENT-SNAPSHOT-2018-08-07-a.xctoolchain/usr/bin/swift"
+}
+
+function write_compare_r_script() {
+    cat >> "$1" <<"EOF"
+#!/usr/bin/env Rscript
+
+args <- commandArgs(trailingOnly=TRUE)
+perfBase <- read.csv(args[2], header=F)
+perfOther <- read.csv(args[4], header=F)
+nameBase <- args[1]
+nameOther <- args[3]
+benchmarks <- perfBase$V2
+stopifnot(benchmarks == perfOther$V2)
+
+swifts <- list(nameBase, nameOther)
+result_cols = 3:12
+
+perfBase$swift <- rep(swifts[[1]], nrow(perfBase))
+perfOther$swift <- rep(swifts[[2]], nrow(perfOther))
+
+get_perf_matrix <- function(.df) {
+    matrix(as.vector(do.call(c, .df[,result_cols])), nrow=nrow(.df))
+}
+
+all_perfs <- rbind(perfBase, perfOther)
+all_perfs$main_indicator <- apply(get_perf_matrix(all_perfs), 1, min)
+
+for (benchmark in benchmarks) {
+    cat("benchmark:", benchmark, "\n===========\n")
+    p <- all_perfs[all_perfs$V2 == benchmark,]
+
+    for (swift in swifts) {
+        results <- unlist(p[p$swift == swift,][result_cols], use.names=F)
+
+        cat(paste('- with', swift), "\n")
+        cat(results, "\n")
+        print(summary(results))
+    }
+
+    cat(paste("winner: ", p[which.min(p$main_indicator),]$swift), "\n")
+
+    pBase <- p[p$swift == nameBase,]$main_indicator
+    pOther <- p[p$swift == nameOther,]$main_indicator
+
+    cat("comparison:", nameBase, "to", nameOther, ":", round(100 * (pOther - pBase)/pBase, 2), "%\n")
+    cat("\n")
+}
+EOF
+}
+
+if ! test $# -eq 2; then
+    usage
+    exit 1
+fi
+
+compare_r_script=$(mktemp /tmp/.compare_perf_XXXXXX)
+write_compare_r_script "$compare_r_script"
+
+(
+cd "$here/.."
+compare_args=()
+for swift_binary in "$@"; do
+    echo "running with $swift_binary"
+    perf_file=$(mktemp /tmp/.nio-perf_XXXXXX)
+    "$swift_binary" run -c release NIOPerformanceTester | grep ^measuring: | tr : , > "$perf_file" 2>&1
+    compare_args+=("$swift_binary")
+    compare_args+=("$perf_file")
+done
+
+Rscript "$compare_r_script" "${compare_args[@]}"
+)
+
+rm "$compare_r_script"