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fixes #17198, DFA failure on large case stmts (#17210)

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This alters the DFA control flow graph generation for case statments.
Gotos are now generated as a chained link, this ensures that evaluation
of variant branches collapses as early as possible, without hitting the
2k call limit.
This commit is contained in:
Saem Ghani 2021-03-02 01:32:43 -08:00 • committed by GitHub
commit ab780f66ef
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2 changed files with 40 additions and 3 deletions
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11
compiler/dfa.nim
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@ -455,6 +455,10 @@ proc genCase(c: var Con; n: PNode) =
let isExhaustive = skipTypes(n[0].typ,
abstractVarRange-{tyTypeDesc}).kind notin {tyFloat..tyFloat128, tyString}
# we generate endings as a set of chained gotos, this is a bit awkward but it
# ensures when recursively traversing the CFG for various analysis, we don't
# artificially extended the life of each branch (for the purposes of DFA)
# beyond the minimum amount.
var endings: seq[TPosition] = @[]
c.gen(n[0])
for i in 1..<n.len:
@ -462,13 +466,14 @@ proc genCase(c: var Con; n: PNode) =
if it.len == 1 or (i == n.len-1 and isExhaustive):
# treat the last branch as 'else' if this is an exhaustive case statement.
c.gen(it.lastSon)
if endings.len != 0:
c.patch(endings[^1])
else:
forkT(it.lastSon):
c.gen(it.lastSon)
if endings.len != 0:
c.patch(endings[^1])
endings.add c.gotoI(it.lastSon)
for i in countdown(endings.high, 0):
let endPos = endings[i]
c.patch(endPos)
proc genBlock(c: var Con; n: PNode) =
withBlock(n[0].sym):