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cmd/compile: make prove use poset to check non-negatives
Prove currently fails to remove bounds checks of the form: if i >= 0 { // hint that i is non-negative for i < len(data) { // i becomes Phi in the loop SSA _ = data[i] // data[Phi]; bounds check!! i++ } } addIndVarRestrictions fails to identify that the loop induction variable, (Phi), is non-negative. As a result, the restrictions, i <= Phi < len(data), are only added for the signed domain. When testing the bounds check, addBranchRestrictions is similarly unable to infer that Phi is non-negative. As a result, the restriction, Phi >= len(data), is only added/tested for the unsigned domain. This CL changes the isNonNegative method to utilise the factTable's partially ordered set (poset). It also adds field factTable.zero to allow isNonNegative to query the poset using the zero(0) constant found or created early in prove. Fixes #28956 Change-Id: I792f886c652eeaa339b0d57d5faefbf5922fe44f Reviewed-on: https://go-review.googlesource.com/c/go/+/161437 Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Giovanni Bajo <rasky@develer.com>
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@ -174,6 +174,10 @@ type factsTable struct {
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// more than one len(s) for a slice. We could keep a list if necessary.
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lens map[ID]*Value
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caps map[ID]*Value
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// zero is a reference to the zero-valued constant assigned or created
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// during the len/cap sweep that begins prove.
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zero *Value
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}
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// checkpointFact is an invalid value used for checkpointing
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@ -566,7 +570,11 @@ func (ft *factsTable) isNonNegative(v *Value) bool {
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}
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}
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return false
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// Check if the signed poset can prove that the value is >= 0
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if ft.zero == nil {
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ft.zero = v.Block.NewValue0I(v.Block.Pos, OpConst64, v.Block.Func.Config.Types.Int64, 0)
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}
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return ft.order[0].OrderedOrEqual(ft.zero, v)
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}
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// checkpoint saves the current state of known relations.
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@ -734,13 +742,12 @@ func prove(f *Func) {
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ft.checkpoint()
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// Find length and capacity ops.
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var zero *Value
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for _, b := range f.Blocks {
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for _, v := range b.Values {
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// If we found a zero constant, save it (so we don't have
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// to build one later).
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if zero == nil && v.Op == OpConst64 && v.AuxInt == 0 {
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zero = v
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if ft.zero == nil && v.Op == OpConst64 && v.AuxInt == 0 {
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ft.zero = v
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}
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if v.Uses == 0 {
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// We don't care about dead values.
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@ -749,28 +756,28 @@ func prove(f *Func) {
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}
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switch v.Op {
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case OpStringLen:
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if zero == nil {
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zero = b.NewValue0I(b.Pos, OpConst64, f.Config.Types.Int64, 0)
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if ft.zero == nil {
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ft.zero = b.NewValue0I(b.Pos, OpConst64, f.Config.Types.Int64, 0)
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}
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ft.update(b, v, zero, signed, gt|eq)
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ft.update(b, v, ft.zero, signed, gt|eq)
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case OpSliceLen:
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if ft.lens == nil {
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ft.lens = map[ID]*Value{}
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}
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ft.lens[v.Args[0].ID] = v
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if zero == nil {
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zero = b.NewValue0I(b.Pos, OpConst64, f.Config.Types.Int64, 0)
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if ft.zero == nil {
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ft.zero = b.NewValue0I(b.Pos, OpConst64, f.Config.Types.Int64, 0)
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}
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ft.update(b, v, zero, signed, gt|eq)
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ft.update(b, v, ft.zero, signed, gt|eq)
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case OpSliceCap:
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if ft.caps == nil {
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ft.caps = map[ID]*Value{}
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}
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ft.caps[v.Args[0].ID] = v
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if zero == nil {
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zero = b.NewValue0I(b.Pos, OpConst64, f.Config.Types.Int64, 0)
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if ft.zero == nil {
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ft.zero = b.NewValue0I(b.Pos, OpConst64, f.Config.Types.Int64, 0)
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}
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ft.update(b, v, zero, signed, gt|eq)
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ft.update(b, v, ft.zero, signed, gt|eq)
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}
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}
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}
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@ -904,7 +911,7 @@ func getBranch(sdom SparseTree, p *Block, b *Block) branch {
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// starting in Block b.
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func addIndVarRestrictions(ft *factsTable, b *Block, iv indVar) {
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d := signed
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if isNonNegative(iv.min) && isNonNegative(iv.max) {
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if ft.isNonNegative(iv.min) && ft.isNonNegative(iv.max) {
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d |= unsigned
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}
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@ -706,6 +706,26 @@ func range2(b [][32]int) {
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}
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}
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// signhint1-2 test whether the hint (int >= 0) is propagated into the loop.
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func signHint1(i int, data []byte) {
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if i >= 0 {
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for i < len(data) { // ERROR "Induction variable: limits \[\?,\?\), increment 1$"
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_ = data[i] // ERROR "Proved IsInBounds$"
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i++
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}
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}
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}
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func signHint2(b []byte, n int) {
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if n < 0 {
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panic("")
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}
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_ = b[25]
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for i := n; i <= 25; i++ { // ERROR "Induction variable: limits \[\?,25\], increment 1$"
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b[i] = 123 // ERROR "Proved IsInBounds$"
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}
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}
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//go:noinline
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func useInt(a int) {
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}
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