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cmd/compile: simplify limit logic in prove

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This replaces the open-coded intersection of limits in the prove pass
with a general limit intersection operation. This should get identical
results except in one case where it's more precise: when handling an
equality relation, if the value is *outside* the existing range, this
will reduce the range to empty rather than resetting it. This will be
important to a follow-up CL where we can take advantage of empty
ranges.

For #23354.

Change-Id: I3d3d75924f61b1da1cb604b3a9d189b26fb3a14e
Reviewed-on: https://go-review.googlesource.com/87477
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Alexandru Moșoi <alexandru@mosoi.ro>
This commit is contained in:
Austin Clements 2018-01-10 17:39:43 -05:00
parent 44e20b64ef
commit 2e9cf5f66e
1 changed files with 28 additions and 26 deletions
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54
src/cmd/compile/internal/ssa/prove.go
View file

@ -117,6 +117,22 @@ func (l limit) String() string {
return fmt.Sprintf("sm,SM,um,UM=%d,%d,%d,%d", l.min, l.max, l.umin, l.umax)
}
func (l limit) intersect(l2 limit) limit {
if l.min < l2.min {
l.min = l2.min
}
if l.umin < l2.umin {
l.umin = l2.umin
}
if l.max > l2.max {
l.max = l2.max
}
if l.umax > l2.umax {
l.umax = l2.umax
}
return l
}
var noLimit = limit{math.MinInt64, math.MaxInt64, 0, math.MaxUint64}
// a limitFact is a limit known for a particular value.
@ -273,28 +289,20 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
if !ok {
old = noLimit
}
lim := old
// Update lim with the new information we know.
lim := noLimit
switch d {
case signed:
switch r {
case lt:
if c-1 < lim.max {
lim.max = c - 1
}
lim.max = c - 1
case lt | eq:
if c < lim.max {
lim.max = c
}
lim.max = c
case gt | eq:
if c > lim.min {
lim.min = c
}
lim.min = c
case gt:
if c+1 > lim.min {
lim.min = c + 1
}
lim.min = c + 1
case lt | gt:
lim = old
if c == lim.min {
lim.min++
}
@ -319,22 +327,15 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
}
switch r {
case lt:
if uc-1 < lim.umax {
lim.umax = uc - 1
}
lim.umax = uc - 1
case lt | eq:
if uc < lim.umax {
lim.umax = uc
}
lim.umax = uc
case gt | eq:
if uc > lim.umin {
lim.umin = uc
}
lim.umin = uc
case gt:
if uc+1 > lim.umin {
lim.umin = uc + 1
}
lim.umin = uc + 1
case lt | gt:
lim = old
if uc == lim.umin {
lim.umin++
}
@ -347,6 +348,7 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
}
}
ft.limitStack = append(ft.limitStack, limitFact{v.ID, old})
lim = old.intersect(lim)
ft.limits[v.ID] = lim
if v.Block.Func.pass.debug > 2 {
v.Block.Func.Warnl(parent.Pos, "parent=%s, new limits %s %s %s", parent, v, w, lim.String())