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[dev.ssa] cmd/compile: detect rewrite loops of length > 1

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Use a version of Floyd's cycle finding algorithm,
but advance by 1 and 1/2 steps per cycle rather
than by 1 and 2. It is simpler and should be cheaper
in the normal, acyclic case.

This should fix the 386 and arm builds,
which are currently hung.

Change-Id: If8bd443011b28a5ecb004a549239991d3dfc862b
Reviewed-on: https://go-review.googlesource.com/13473
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Josh Bleecher Snyder 2015-08-10 14:01:04 -07:00
parent e98edc88c9
commit baf2c3ec4a
1 changed files with 11 additions and 2 deletions
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13
src/cmd/compile/internal/ssa/rewrite.go
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@ -44,9 +44,18 @@ func applyRewrite(f *Func, rb func(*Block) bool, rv func(*Value, *Config) bool)
}
// Rewriting can generate OpCopy loops.
// They are harmless (see removePredecessor),
// but take care not to loop forever.
for a.Op == OpCopy && a != a.Args[0] {
// but take care to stop if we find a cycle.
slow := a // advances every other iteration
var advance bool
for a.Op == OpCopy {
a = a.Args[0]
if slow == a {
break
}
if advance {
slow = a
}
advance = !advance
}
v.Args[i] = a
}