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cmd/compile: fix liveness for open-coded defer args for infinite loops

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Once defined, a stack slot holding an open-coded defer arg should always be marked
live, since it may be used at any time if there is a panic. These stack slots are
typically kept live naturally by the open-defer code inlined at each return/exit point.
However, we need to do extra work to make sure that they are kept live if a
function has an infinite loop or a panic exit.

For this fix, only in the case of a function that is using open-coded defers, we
compute the set of blocks (most often empty) that cannot reach a return or a
BlockExit (panic) because of an infinite loop. Then, for each block b which
cannot reach a return or BlockExit or is a BlockExit block, we mark each defer arg
slot as live, as long as the definition of the defer arg slot dominates block b.

For this change, had to export (*Func).sdom (-> Sdom) and SparseTree.isAncestorEq
(-> IsAncestorEq)

Updates #35277

Change-Id: I7b53c9bd38ba384a3794386dd0eb94e4cbde4eb1
Reviewed-on: https://go-review.googlesource.com/c/go/+/204802
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Dan Scales 2019-11-01 14:04:08 -07:00
parent 414c1d454e
commit 1b3a1db19f
12 changed files with 140 additions and 32 deletions
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6
src/cmd/compile/internal/ssa/prove.go
View file

@ -848,7 +848,7 @@ func prove(f *Func) {
})
idom := f.Idom()
sdom := f.sdom()
sdom := f.Sdom()
// DFS on the dominator tree.
//
@ -948,10 +948,10 @@ func getBranch(sdom SparseTree, p *Block, b *Block) branch {
// has one predecessor then (apart from the degenerate case),
// there is no path from entry that can reach b through p.Succs[1].
// TODO: how about p->yes->b->yes, i.e. a loop in yes.
if sdom.isAncestorEq(p.Succs[0].b, b) && len(p.Succs[0].b.Preds) == 1 {
if sdom.IsAncestorEq(p.Succs[0].b, b) && len(p.Succs[0].b.Preds) == 1 {
return positive
}
if sdom.isAncestorEq(p.Succs[1].b, b) && len(p.Succs[1].b.Preds) == 1 {
if sdom.IsAncestorEq(p.Succs[1].b, b) && len(p.Succs[1].b.Preds) == 1 {
return negative
}
return unknown