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cmd/compile: in prove, fail fast when unsat is found

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When an unsatisfiable relation is recorded in the facts table,
there is no need to compute further relations or updates
additional data structures.

Since we're about to transitively propagate relations, make
sure to fail as fast as possible to avoid doing useless work
in dead branches.

Passes toolstash -cmp.

Change-Id: I23eed376d62776824c33088163c7ac9620abce85
Reviewed-on: https://go-review.googlesource.com/100275
Reviewed-by: Austin Clements <austin@google.com>
This commit is contained in:
Giovanni Bajo 2018-03-13 00:21:57 +01:00
parent 79112707bb
commit 385d936fb2
1 changed files with 15 additions and 3 deletions
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18
src/cmd/compile/internal/ssa/prove.go
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@ -188,6 +188,11 @@ func newFactsTable() *factsTable {
// update updates the set of relations between v and w in domain d
// restricting it to r.
func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
// No need to do anything else if we already found unsat.
if ft.unsat {
return
}
if lessByID(w, v) {
v, w = w, v
r = reverseBits[r]
@ -202,10 +207,16 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
oldR = lt | eq | gt
}
}
// No changes compared to information already in facts table.
if oldR == r {
return
}
ft.stack = append(ft.stack, fact{p, oldR})
ft.facts[p] = oldR & r
// If this relation is not satisfiable, mark it and exit right away
if oldR&r == 0 {
ft.unsat = true
return
}
// Extract bounds when comparing against constants
@ -298,12 +309,13 @@ 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 lim.min > lim.max || lim.umin > lim.umax {
ft.unsat = true
}
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())
}
if lim.min > lim.max || lim.umin > lim.umax {
ft.unsat = true
return
}
}
// Process fence-post implications.