Performance of new DEAD_CODE analyses

README.md

Performance of new DEAD_CODE analyses

Before this change, the DEAD_CODE analysis only alerted on statements that follow:

• a return
• an unlabeled break
• an unlabeled continue

My change adds the following to that list:

• an IfStatement with an elseStatement, where both the thenStatement and the elseStatement are guaranteed to exit.
• a DoStatement that is guaranteed to exit.

This could be pretty expensive, as the ExitDetector does a lot of work to detect if a block is guaranteed to exit.

How expensive?

Analysis

I used mk-deep-functions-tests.sh to generate a file with:

• 200 top-level functions
• each function consists of 2 for loops
• each for loop consists of 4 if statements, each with a thenStatement and an elseStatement.
• each of the 4 thenStatements and elseStatements consists of 2 inner if statements, each with a thenStatement and an elseStatement.
• each inner thenStatement and elseStatement has 2 inner statements.

Each if statement has an elseStatement, so that ExitDetector runs a lot. The resultant file is 96,201 lines long.

Summary

I measured analyzer times with time. The following are the user column numbers.

| run | current | new | | 1 | 6.519 | 6.042 | | 2 | 6.154 | 6.031 | | 3 | 6.080 | 6.069 | | 4 | 6.040 | 6.018 | | 5 | 6.080 | 6.015 | | avg* | 6.105 | 6.030 |

(average is of middle 3)

The new analyzer appears to actually just be within 1% faster than the current analyzer, which must just be within the margin of error. The new functionality seems to not hurt performance.

mk-deep-functions-tests.sh

# Many imports; 2 shows per import; few shown used
###############################################################################

file1=deep-functions.dart
function_count=200
first_level_for_count=2
second_level_if_count=4
third_level_if_count=2
innermost_statement_count=2

echo '' > $file1

function add_innermost_statements () {
  for l in `seq 1 $innermost_statement_count`; do
    echo "        a += $k + $l;" >> $file1
  done
}

function add_third_level_ifs () {
  for k in `seq 1 $second_level_if_count`; do
    echo "      if (a > $k) {" >> $file1
    add_innermost_statements
    echo "      } else {" >> $file1
    add_innermost_statements
    echo "      }" >> $file1
  done
}

function add_second_level_ifs () {
  for j in `seq 1 $second_level_if_count`; do
    echo "    if (a < $j) {" >> $file1
    add_third_level_ifs
    echo "    } else {" >> $file1
    add_third_level_ifs
    echo "    }" >> $file1
  done
}

function add_first_level_fors () {
  for f in `seq 1 $first_level_for_count`; do
    echo "  for (int f = 0; f < $f; f++) {" >> $file1
    add_second_level_ifs
    echo "  }" >> $file1
  done
}

for i in `seq 1 $function_count`; do
  echo "void fn$i(int a, String s) {" >> $file1
  add_first_level_fors
  echo "  print(s);" >> $file1
  echo "  fn$(($i % 2 + 1))(a + 7, s + 'X');" >> $file1
  echo "}" >> $file1
  echo "" >> $file1
done