[vm] Make it easier to get IL / disassembly out

Make `dart compile exe -v` forward gen_snapshot output in
verbose mode.

Fix --write_code_comments_as_synthetic_source_to=... flag. There
was a change to Dwarf writer API which broke it.

Motivated by #50059

TEST=manually

Change-Id: I0431925f724a9b24e938bdb9d7ae1019143da83a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/261360
Commit-Queue: Slava Egorov <[email protected]>
Reviewed-by: Tess Strickland <[email protected]>

Author: mraleph

pkg/dart2native/lib/generate.dart

@@ -67,22 +67,10 @@ Future<void> generateNative({
           if (soundNullSafety != null)
             '--${soundNullSafety ? '' : 'no-'}sound-null-safety',
         ]);
+    await _forwardOutput(kernelResult);
     if (kernelResult.exitCode != 0) {
-      // We pipe both stdout and stderr to stderr because the CFE doesn't print
-      // errors to stderr. This unfortunately does emit info-only output in
-      // stderr, though.
-      stderr.write(kernelResult.stdout);
-      stderr.write(kernelResult.stderr);
-      await stderr.flush();
       throw 'Generating AOT kernel dill failed!';
     }
-    // Pipe info and warnings from the CFE to stdout since the compilation
-    // succeeded. Stderr should be empty but we pipe it to stderr for
-    // completeness.
-    stdout.write(kernelResult.stdout);
-    await stdout.flush();
-    stderr.write(kernelResult.stderr);
-    await stderr.flush();
 
     if (verbose) {
       print('Generating AOT snapshot.');
@@ -93,10 +81,11 @@ Future<void> generateNative({
         : path.join(tempDir.path, 'snapshot.aot'));
     final snapshotResult = await generateAotSnapshot(genSnapshot, kernelFile,
         snapshotFile, debugPath, enableAsserts, extraOptions);
+
+    if (verbose || kernelResult.exitCode != 0) {
+      await _forwardOutput(kernelResult);
+    }
     if (snapshotResult.exitCode != 0) {
-      stderr.writeln(snapshotResult.stdout);
-      stderr.writeln(snapshotResult.stderr);
-      await stderr.flush();
       throw 'Generating AOT snapshot failed!';
     }
 
@@ -119,3 +108,22 @@ Future<void> generateNative({
     tempDir.deleteSync(recursive: true);
   }
 }
+
+Future<void> _forwardOutput(ProcessResult result) async {
+  if (result.stdout.isNotEmpty) {
+    final bool needsNewLine = !result.stdout.endsWith('\n');
+    if (result.exitCode == 0) {
+      stdout.write(result.stdout);
+      if (needsNewLine) stdout.writeln();
+      await stdout.flush();
+    } else {
+      stderr.write(result.stdout);
+      if (needsNewLine) stderr.writeln();
+    }
+  }
+  if (result.stderr.isNotEmpty) {
+    stderr.write(result.stderr);
+    if (!result.stderr.endsWith('\n')) stderr.writeln();
+    await stderr.flush();
+  }
+}

runtime/vm/dwarf.cc

@@ -551,11 +551,11 @@ class LineNumberProgramWriter {
   // Associates the given file, line, and column information for the instruction
   // at the pc_offset into the instructions payload of the Code object with the
   // symbol asm_name. Returns whether any changes were made to the stream.
-  bool AddRow(intptr_t file,
-              intptr_t line,
-              intptr_t column,
-              const char* asm_name,
-              intptr_t pc_offset) {
+  DART_WARN_UNUSED_RESULT bool AddRow(intptr_t file,
+                                      intptr_t line,
+                                      intptr_t column,
+                                      const char* asm_name,
+                                      intptr_t pc_offset) {
     ASSERT_EQUAL(end_sequence_, false);
     bool source_info_changed = false;
     // Note that files are 1-indexed.
@@ -675,8 +675,8 @@ void Dwarf::WriteSyntheticLineNumberProgram(LineNumberProgramWriter* writer) {
     auto& comments = code.comments();
     for (intptr_t i = 0, len = comments.Length(); i < len;) {
       intptr_t current_pc_offset = comments.PCOffsetAt(i);
-      writer->AddRow(comments_file_index, current_line,
-                     DwarfPosition::kNoColumn, asm_name, current_pc_offset);
+      writer->EmitRow(comments_file_index, current_line,
+                      DwarfPosition::kNoColumn, asm_name, current_pc_offset);
       while (i < len && current_pc_offset == comments.PCOffsetAt(i)) {
         comments_buffer.AddString(comments.CommentAt(i));
         comments_buffer.AddChar('\n');