Add conditional compilation for metric measuring

Ideally this would be done in Package.swift instead of having to add a flag to swift build but alas it appears that it is not possible
- https://forums.swift.org/t/swiftpm-and-conditional-compilation/36874
- https://forums.swift.org/t/compilation-conditions-and-swift-packages/34627/4

Author: rctcwyvrn

Sources/RegexBenchmark/CLI.swift

@@ -38,10 +38,22 @@ struct Runner: ParsableCommand {
   @Flag(help: "Exclude running NSRegex benchmarks")
   var excludeNs = false
 
-  @Flag(help: "Enable tracing of the engine (warning: lots of output)")
+  @Flag(help: """
+Enable tracing of the engine (warning: lots of output). Prints out processor state each cycle
+
+Note: swift-experimental-string-processing must be built with processor measurements enabled
+
+swift build -c release -Xswiftc -DPROCESSOR_MEASUREMENTS_ENABLED
+""")
   var enableTracing: Bool = false
 
-  @Flag(help: "Enable engine metrics (warning: lots of output)")
+  @Flag(help: """
+Enable engine metrics (warning: lots of output). Prints out cycle count, instruction counts, number of backtracks
+
+Note: swift-experimental-string-processing must be built with processor measurements enabled
+
+swift build -c release -Xswiftc -DPROCESSOR_MEASUREMENTS_ENABLED
+""")
   var enableMetrics: Bool = false
 
   @Flag(help: "Include firstMatch benchmarks in CrossBenchmark (off by default")

Sources/_StringProcessing/Engine/Processor.swift

@@ -400,6 +400,8 @@ extension Processor {
   mutating func cycle() {
     _checkInvariants()
     assert(state == .inProgress)
+
+#if PROCESSOR_MEASUREMENTS_ENABLED
     if cycleCount == 0 {
       trace()
       measureMetrics()
@@ -410,6 +412,8 @@ extension Processor {
       measureMetrics()
       _checkInvariants()
     }
+#endif
+
     let (opcode, payload) = fetch().destructure
     switch opcode {
     case .invalid:

Sources/_StringProcessing/Executor.swift

@@ -30,7 +30,9 @@ struct Executor {
       input: input,
       subjectBounds: subjectBounds,
       searchBounds: searchBounds)
+#if PROCESSOR_MEASUREMENTS_ENABLED
     defer { if cpu.shouldMeasureMetrics { cpu.printMetrics() } }
+#endif
     var low = searchBounds.lowerBound
     let high = searchBounds.upperBound
     while true {
@@ -57,7 +59,9 @@ struct Executor {
   ) throws -> Regex<Output>.Match? {
     var cpu = engine.makeProcessor(
       input: input, bounds: subjectBounds, matchMode: mode)
+#if PROCESSOR_MEASUREMENTS_ENABLED
     defer { if cpu.shouldMeasureMetrics { cpu.printMetrics() } }
+#endif
     return try _match(input, from: subjectBounds.lowerBound, using: &cpu)
   }