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cmd/compile: in prove, shortcircuit self-facts

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Sometimes, we can end up calling update with a self-relation
about a variable (x REL x). In this case, there is no need
to record anything: the relation is unsatisfiable if and only
if it doesn't contain eq.

This also helps avoiding infinite loop in next CL that will
introduce transitive closure of relations.

Passes toolstash -cmp.

Change-Id: Ic408452ec1c13653f22ada35466ec98bc14aaa8e
Reviewed-on: https://go-review.googlesource.com/100276
Reviewed-by: Austin Clements <austin@google.com>
This commit is contained in:
Giovanni Bajo 2018-03-13 00:25:06 +01:00
parent 385d936fb2
commit d54902ece9
1 changed files with 9 additions and 0 deletions
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9
src/cmd/compile/internal/ssa/prove.go
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@ -193,6 +193,15 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
return
}
// Self-fact. It's wasteful to register it into the facts
// table, so just note whether it's satisfiable
if v == w {
if r&eq == 0 {
ft.unsat = true
}
return
}
if lessByID(w, v) {
v, w = w, v
r = reverseBits[r]