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cmd/compile: check domination of loop return in both controls

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Fixes #74473

Change-Id: I72ff6b95955ae9407271508aa80f230dcf1b6c74
Reviewed-on: https://go-review.googlesource.com/c/go/+/685816
Reviewed-by: Mark Freeman <mark@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Jorropo 2025-07-04 08:54:14 +02:00 • committed by Gopher Robot
parent cefaed0de0
commit 8cd85e602a
2 changed files with 36 additions and 10 deletions
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35
src/cmd/compile/internal/ssa/loopbce.go
View file

@ -37,19 +37,20 @@ type indVar struct {
// - the minimum bound
// - the increment value
// - the "next" value (SSA value that is Phi'd into the induction variable every loop)
// - the header's edge returning from the body
//
// Currently, we detect induction variables that match (Phi min nxt),
// with nxt being (Add inc ind).
// If it can't parse the induction variable correctly, it returns (nil, nil, nil).
func parseIndVar(ind *Value) (min, inc, nxt *Value) {
func parseIndVar(ind *Value) (min, inc, nxt *Value, loopReturn Edge) {
if ind.Op != OpPhi {
return
}
if n := ind.Args[0]; (n.Op == OpAdd64 || n.Op == OpAdd32 || n.Op == OpAdd16 || n.Op == OpAdd8) && (n.Args[0] == ind || n.Args[1] == ind) {
min, nxt = ind.Args[1], n
min, nxt, loopReturn = ind.Args[1], n, ind.Block.Preds[0]
} else if n := ind.Args[1]; (n.Op == OpAdd64 || n.Op == OpAdd32 || n.Op == OpAdd16 || n.Op == OpAdd8) && (n.Args[0] == ind || n.Args[1] == ind) {
min, nxt = ind.Args[0], n
min, nxt, loopReturn = ind.Args[0], n, ind.Block.Preds[1]
} else {
// Not a recognized induction variable.
return
@ -111,13 +112,13 @@ func findIndVar(f *Func) []indVar {
// See if this is really an induction variable
less := true
init, inc, nxt := parseIndVar(ind)
init, inc, nxt, loopReturn := parseIndVar(ind)
if init == nil {
// We failed to parse the induction variable. Before punting, we want to check
// whether the control op was written with the induction variable on the RHS
// instead of the LHS. This happens for the downwards case, like:
// for i := len(n)-1; i >= 0; i--
init, inc, nxt = parseIndVar(limit)
init, inc, nxt, loopReturn = parseIndVar(limit)
if init == nil {
// No recognized induction variable on either operand
continue
@ -145,6 +146,20 @@ func findIndVar(f *Func) []indVar {
continue
}
// startBody is the edge that eventually returns to the loop header.
var startBody Edge
switch {
case sdom.IsAncestorEq(b.Succs[0].b, loopReturn.b):
startBody = b.Succs[0]
case sdom.IsAncestorEq(b.Succs[1].b, loopReturn.b):
// if x { goto exit } else { goto entry } is identical to if !x { goto entry } else { goto exit }
startBody = b.Succs[1]
less = !less
inclusive = !inclusive
default:
continue
}
// Increment sign must match comparison direction.
// When incrementing, the termination comparison must be ind </<= limit.
// When decrementing, the termination comparison must be ind >/>= limit.
@ -172,14 +187,14 @@ func findIndVar(f *Func) []indVar {
// First condition: loop entry has a single predecessor, which
// is the header block. This implies that b.Succs[0] is
// reached iff ind < limit.
if len(b.Succs[0].b.Preds) != 1 {
// b.Succs[1] must exit the loop.
if len(startBody.b.Preds) != 1 {
// the other successor must exit the loop.
continue
}
// Second condition: b.Succs[0] dominates nxt so that
// Second condition: startBody.b dominates nxt so that
// nxt is computed when inc < limit.
if !sdom.IsAncestorEq(b.Succs[0].b, nxt.Block) {
if !sdom.IsAncestorEq(startBody.b, nxt.Block) {
// inc+ind can only be reached through the branch that enters the loop.
continue
}
@ -298,7 +313,7 @@ func findIndVar(f *Func) []indVar {
nxt: nxt,
min: min,
max: max,
entry: b.Succs[0].b,
entry: startBody.b,
flags: flags,
})
b.Logf("found induction variable %v (inc = %v, min = %v, max = %v)\n", ind, inc, min, max)

11
test/prove.go
View file

@ -2319,6 +2319,17 @@ func resliceBytes(b []byte) byte {
return 0
}
func issue74473(s []uint) {
i := 0
for {
if i >= len(s) { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
break
}
_ = s[i] // ERROR "Proved IsInBounds$"
i++
}
}
//go:noinline
func useInt(a int) {
}