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[backport] arc: improve compile time of (nested) loops (#18890)

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Clyybber 2021-09-24 14:59:48 +02:00 • committed by GitHub
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1 changed files with 8 additions and 3 deletions
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11
compiler/dfa.nim
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@ -308,6 +308,11 @@ when true:
# We unroll every loop 3 times. We emulate 0, 1, 2 iterations
# through the loop. We need to prove this is correct for our
# purposes. But Herb Sutter claims it is. (Proof by authority.)
#
# EDIT: Actually, we only need to unroll 2 times
# because Nim doesn't have a way of breaking/goto-ing into
# a loop iteration. Unrolling 2 times is much better for compile
# times of nested loops than 3 times, so we do that here.
#[
while cond:
body
@ -347,12 +352,12 @@ when true:
# 'while true' is an idiom in Nim and so we produce
# better code for it:
withBlock(nil):
for i in 0..2:
for i in 0..1:
c.gen(n[1])
else:
withBlock(nil):
var endings: array[3, TPosition]
for i in 0..2:
var endings: array[2, TPosition]
for i in 0..1:
c.gen(n[0])
endings[i] = c.forkI(n)
c.gen(n[1])