Avoid unnecessary file IO when computing positions

The exists call introduced in #8883 and #8897 lead to a slowdown
noticeable in benchmarks. Replace them by a check on the content length,
since SourceFile caches its content this should avoid any unnecessary
IO operation. This required changing the way SourceFile handles empty
files to have it return an empty string instead of crashing.

Author: smarter

compiler/src/dotty/tools/dotc/util/SourceFile.scala

@@ -52,7 +52,14 @@ class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends
 
   def maybeIncomplete: Boolean = _maybeInComplete
 
-  def this(file: AbstractFile, codec: Codec) = this(file, new String(file.toByteArray, codec.charSet).toCharArray)
+  def this(file: AbstractFile, codec: Codec) =
+    // It would be cleaner to check if the file exists instead of catching
+    // an exception, but it turns out that Files.exists is remarkably slow,
+    // at least on Java 8 (https://rules.sonarsource.com/java/tag/performance/RSPEC-3725),
+    // this is significant enough to show up in our benchmarks.
+    this(file,
+      try new String(file.toByteArray, codec.charSet).toCharArray
+      catch case _: java.nio.file.NoSuchFileException => Array[Char]())
 
   /** Tab increment; can be overridden */
   def tabInc: Int = 8

compiler/src/dotty/tools/dotc/util/SourcePosition.scala

@@ -42,16 +42,16 @@ extends interfaces.SourcePosition with Showable {
   def beforeAndAfterPoint: (List[Int], List[Int]) =
     lineOffsets.partition(_ <= point)
 
-  def column: Int = if (source.file.exists) source.column(point) else -1
+  def column: Int = if (source.content().length != 0) source.column(point) else -1
 
   def start: Int = span.start
-  def startLine: Int = if (source.file.exists) source.offsetToLine(start) else -1
-  def startColumn: Int = if (source.file.exists) source.column(start) else -1
+  def startLine: Int = if (source.content().length != 0) source.offsetToLine(start) else -1
+  def startColumn: Int = if (source.content().length != 0) source.column(start) else -1
   def startColumnPadding: String = source.startColumnPadding(start)
 
   def end: Int = span.end
-  def endLine: Int = if (source.file.exists) source.offsetToLine(end) else -1
-  def endColumn: Int = if (source.file.exists) source.column(end) else -1
+  def endLine: Int = if (source.content().length != 0) source.offsetToLine(end) else -1
+  def endColumn: Int = if (source.content().length != 0) source.column(end) else -1
 
   def withOuter(outer: SourcePosition): SourcePosition = SourcePosition(source, span, outer)
   def withSpan(range: Span) = SourcePosition(source, range, outer)

sbt-bridge/src/xsbt/DelegatingReporter.java

@@ -91,7 +91,7 @@ public Optional<java.io.File> sourceFile() {
           return Optional.ofNullable(src.file().file());
         }
         public Optional<Integer> line() {
-          if (!src.file().exists())
+          if (src.content().length == 0)
             return Optional.empty();
 
           int line = pos.line() + 1;
@@ -101,7 +101,7 @@ public Optional<Integer> line() {
           return Optional.of(line);
         }
         public String lineContent() {
-          if (!src.file().exists())
+          if (src.content().length == 0)
             return "";
 
           String line = pos.lineContent();
@@ -116,13 +116,13 @@ public Optional<Integer> offset() {
           return Optional.of(pos.point());
         }
         public Optional<Integer> pointer() {
-          if (!src.file().exists())
+          if (src.content().length == 0)
             return Optional.empty();
 
           return Optional.of(pos.point() - src.startOfLine(pos.point()));
         }
         public Optional<String> pointerSpace() {
-          if (!src.file().exists())
+          if (src.content().length == 0)
             return Optional.empty();
 
           String lineContent = this.lineContent();