Background

Conditional Threading is an optimization that searches for separate blocks of IF statements but with exactly the same conditions. When several conditionals present the same boolean condition, merging their inside actions would avoid computing the condition several times. Also, in the cases where a second IF statement has been used with the exact negation of the condition of the first IF, the second IF could then be incorporated in an ELSE statement.

Example

Consider the following example:

code <- paste(
  "n_evens <- 0",
  "n_odds <- 0",
  "odds_sum <- 0",
  "evens_sum <- 0",
  "",
  "for (i in sample(10000, replace = TRUE)) {",
  "  if (i %% 2 == 1) {",
  "    n_odds <- n_odds + 1",
  "  }",
  "  if (i %% 2 == 1) {",
  "    odds_sum <- odds_sum + i",
  "  }",
  "  if (!(i %% 2 == 1)) {",
  "    n_evens <- n_evens + 1",
  "  }",
  "  if (!(i %% 2 == 1)) {",
  "    evens_sum <- evens_sum + i",
  "  }",
  "}",
  sep = "\n"
)
cat(code)

## n_evens <- 0
## n_odds <- 0
## odds_sum <- 0
## evens_sum <- 0
## 
## for (i in sample(10000, replace = TRUE)) {
##   if (i %% 2 == 1) {
##     n_odds <- n_odds + 1
##   }
##   if (i %% 2 == 1) {
##     odds_sum <- odds_sum + i
##   }
##   if (!(i %% 2 == 1)) {
##     n_evens <- n_evens + 1
##   }
##   if (!(i %% 2 == 1)) {
##     evens_sum <- evens_sum + i
##   }
## }

Then, the automatically optimized code would be:

opt_code <- opt_cond_thread(list(code))
cat(opt_code$codes[[1]])

## n_evens <- 0
## n_odds <- 0
## odds_sum <- 0
## evens_sum <- 0
## for (i in sample(10000, replace = TRUE)) {
## if(i %% 2 == 1) {
## n_odds <- n_odds + 1
##  odds_sum <- odds_sum + i
## }else {
##  n_evens <- n_evens + 1
##  evens_sum <- evens_sum + i
## }}

And if we measure the execution time of each one, and the speed-up:

bmark_res <- microbenchmark({
  eval(parse(text = code))
}, {
  eval(parse(text = opt_code))
})
autoplot(bmark_res)

speed_up(bmark_res)

##            Min. 1st Qu.   Median     Mean  3rd Qu.     Max.
## Expr_2 403.5224 386.094 302.3329 346.0635 304.2655 1106.613

Implementation

The opt-cond-thread optimizer simply looks for IF statements in the code snippet given as the input and if it finds an IF statement, it checks the immediate next block of code. If it is another IF statement, the optimizer merges the two IF blocks if the IF conditions were same or converts the second IF statement to ELSE if the conditions were negation of each other.