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cmd/compile: rework induction variable detector
Induction variable detection is still not quite right. I've added another failing test. Redo the overflow/underflow detector so it is more obviously correct. Update #53600 Fixes #53653 Fixes #53663 Change-Id: Id95228e282fdbf6bd80b26e1c41d62e935ba08ff Reviewed-on: https://go-review.googlesource.com/c/go/+/415874 Run-TryBot: Keith Randall <khr@golang.org> TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Russ Cox <rsc@golang.org> Reviewed-by: David Chase <drchase@google.com>
This commit is contained in:
Keith Randall
Heschi Kreinick
parent
53a4152d47
commit
2acd3646fc
@ -5,6 +5,7 @@
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package ssa
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import (
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"cmd/compile/internal/base"
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"fmt"
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"math"
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)
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@ -90,41 +91,42 @@ func findIndVar(f *Func) []indVar {
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continue
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}
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var flags indVarFlags
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var ind, max *Value // induction, and maximum
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var ind *Value // induction variable
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var init *Value // starting value
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var limit *Value // ending value
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// Check thet the control if it either ind </<= max or max >/>= ind.
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// Check thet the control if it either ind </<= limit or limit </<= ind.
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// TODO: Handle 32-bit comparisons.
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// TODO: Handle unsigned comparisons?
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c := b.Controls[0]
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inclusive := false
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switch c.Op {
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case OpLeq64:
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flags |= indVarMaxInc
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inclusive = true
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fallthrough
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case OpLess64:
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ind, max = c.Args[0], c.Args[1]
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ind, limit = c.Args[0], c.Args[1]
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default:
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continue
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}
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// See if this is really an induction variable
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less := true
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min, inc, nxt := parseIndVar(ind)
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if min == nil {
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init, inc, nxt := parseIndVar(ind)
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if init == nil {
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// We failed to parse the induction variable. Before punting, we want to check
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// whether the control op was written with arguments in non-idiomatic order,
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// so that we believe being "max" (the upper bound) is actually the induction
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// variable itself. This would happen for code like:
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// for i := 0; len(n) > i; i++
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min, inc, nxt = parseIndVar(max)
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if min == nil {
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// whether the control op was written with the induction variable on the RHS
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// instead of the LHS. This happens for the downwards case, like:
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// for i := len(n)-1; i >= 0; i--
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init, inc, nxt = parseIndVar(limit)
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if init == nil {
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// No recognied induction variable on either operand
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continue
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}
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// Ok, the arguments were reversed. Swap them, and remember that we're
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// looking at a ind >/>= loop (so the induction must be decrementing).
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ind, max = max, ind
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ind, limit = limit, ind
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less = false
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}
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@ -138,8 +140,8 @@ func findIndVar(f *Func) []indVar {
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}
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// Increment sign must match comparison direction.
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// When incrementing, the termination comparison must be ind </<= max.
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// When decrementing, the termination comparison must be ind >/>= max.
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// When incrementing, the termination comparison must be ind </<= limit.
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// When decrementing, the termination comparison must be ind >/>= limit.
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// See issue 26116.
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if step > 0 && !less {
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continue
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@ -148,177 +150,229 @@ func findIndVar(f *Func) []indVar {
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continue
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}
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// If the increment is negative, swap min/max and their flags
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if step < 0 {
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min, max = max, min
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oldf := flags
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flags = indVarMaxInc
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if oldf&indVarMaxInc == 0 {
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flags |= indVarMinExc
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}
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step = -step
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}
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if flags&indVarMaxInc != 0 && max.Op == OpConst64 && max.AuxInt+step < max.AuxInt {
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// For a <= comparison, we need to make sure that a value equal to
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// max can be incremented without overflowing.
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// (For a < comparison, the %step check below ensures no overflow.)
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continue
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}
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// Up to now we extracted the induction variable (ind),
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// the increment delta (inc), the temporary sum (nxt),
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// the minimum value (min) and the maximum value (max).
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// the initial value (init) and the limiting value (limit).
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//
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// We also know that ind has the form (Phi min nxt) where
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// We also know that ind has the form (Phi init nxt) where
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// nxt is (Add inc nxt) which means: 1) inc dominates nxt
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// and 2) there is a loop starting at inc and containing nxt.
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//
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// We need to prove that the induction variable is incremented
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// only when it's smaller than the maximum value.
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// only when it's smaller than the limiting value.
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// Two conditions must happen listed below to accept ind
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// as an induction variable.
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// First condition: loop entry has a single predecessor, which
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// is the header block. This implies that b.Succs[0] is
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// reached iff ind < max.
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// reached iff ind < limit.
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if len(b.Succs[0].b.Preds) != 1 {
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// b.Succs[1] must exit the loop.
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continue
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}
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// Second condition: b.Succs[0] dominates nxt so that
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// nxt is computed when inc < max, meaning nxt <= max.
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// nxt is computed when inc < limit.
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if !sdom.IsAncestorEq(b.Succs[0].b, nxt.Block) {
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// inc+ind can only be reached through the branch that enters the loop.
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continue
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}
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// We can only guarantee that the loop runs within limits of induction variable
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// if (one of)
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// (1) the increment is ±1
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// (2) the limits are constants
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// (3) loop is of the form k0 upto Known_not_negative-k inclusive, step <= k
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// (4) loop is of the form k0 upto Known_not_negative-k exclusive, step <= k+1
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// (5) loop is of the form Known_not_negative downto k0, minint+step < k0
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if step > 1 {
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ok := false
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if min.Op == OpConst64 && max.Op == OpConst64 {
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if max.AuxInt > min.AuxInt && max.AuxInt%step == min.AuxInt%step { // handle overflow
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ok = true
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}
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}
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// Handle induction variables of these forms.
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// KNN is known-not-negative.
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// SIGNED ARITHMETIC ONLY. (see switch on c above)
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// Possibilities for KNN are len and cap; perhaps we can infer others.
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// for i := 0; i <= KNN-k ; i += k
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// for i := 0; i < KNN-(k-1); i += k
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// Also handle decreasing.
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// "Proof" copied from https://go-review.googlesource.com/c/go/+/104041/10/src/cmd/compile/internal/ssa/loopbce.go#164
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//
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// In the case of
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// // PC is Positive Constant
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// L := len(A)-PC
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// for i := 0; i < L; i = i+PC
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//
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// we know:
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//
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// 0 + PC does not over/underflow.
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// len(A)-PC does not over/underflow
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// maximum value for L is MaxInt-PC
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// i < L <= MaxInt-PC means i + PC < MaxInt hence no overflow.
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// To match in SSA:
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// if (a) min.Op == OpConst64(k0)
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// and (b) k0 >= MININT + step
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// and (c) max.Op == OpSubtract(Op{StringLen,SliceLen,SliceCap}, k)
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// or (c) max.Op == OpAdd(Op{StringLen,SliceLen,SliceCap}, -k)
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// or (c) max.Op == Op{StringLen,SliceLen,SliceCap}
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// and (d) if upto loop, require indVarMaxInc && step <= k or !indVarMaxInc && step-1 <= k
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if min.Op == OpConst64 && min.AuxInt >= step+math.MinInt64 {
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knn := max
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k := int64(0)
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var kArg *Value
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switch max.Op {
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case OpSub64:
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knn = max.Args[0]
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kArg = max.Args[1]
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case OpAdd64:
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knn = max.Args[0]
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kArg = max.Args[1]
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if knn.Op == OpConst64 {
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knn, kArg = kArg, knn
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}
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}
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switch knn.Op {
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case OpSliceLen, OpStringLen, OpSliceCap:
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default:
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knn = nil
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}
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if kArg != nil && kArg.Op == OpConst64 {
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k = kArg.AuxInt
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if max.Op == OpAdd64 {
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k = -k
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}
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}
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if k >= 0 && knn != nil {
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if inc.AuxInt > 0 { // increasing iteration
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// The concern for the relation between step and k is to ensure that iv never exceeds knn
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// i.e., iv < knn-(K-1) ==> iv + K <= knn; iv <= knn-K ==> iv +K < knn
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if step <= k || flags&indVarMaxInc == 0 && step-1 == k {
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ok = true
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// Check for overflow/underflow. We need to make sure that inc never causes
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// the induction variable to wrap around.
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// We use a function wrapper here for easy return true / return false / keep going logic.
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// This function returns true if the increment will never overflow/underflow.
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ok := func() bool {
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if step > 0 {
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if limit.Op == OpConst64 {
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// Figure out the actual largest value.
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v := limit.AuxInt
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if !inclusive {
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if v == math.MinInt64 {
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return false // < minint is never satisfiable.
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}
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} else { // decreasing iteration
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// Will be decrementing from max towards min; max is knn-k; will only attempt decrement if
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// knn-k >[=] min; underflow is only a concern if min-step is not smaller than min.
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// This all assumes signed integer arithmetic
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// This is already assured by the test above: min.AuxInt >= step+math.MinInt64
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ok = true
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v--
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}
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if init.Op == OpConst64 {
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// Use stride to compute a better lower limit.
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if init.AuxInt > v {
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return false
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}
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v = addU(init.AuxInt, diff(v, init.AuxInt)/uint64(step)*uint64(step))
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}
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// It is ok if we can't overflow when incrementing from the largest value.
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return !addWillOverflow(v, step)
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}
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if step == 1 && !inclusive {
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// Can't overflow because maxint is never a possible value.
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return true
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}
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// If the limit is not a constant, check to see if it is a
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// negative offset from a known non-negative value.
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knn, k := findKNN(limit)
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if knn == nil || k < 0 {
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return false
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}
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// limit == (something nonnegative) - k. That subtraction can't underflow, so
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// we can trust it.
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if inclusive {
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// ind <= knn - k cannot overflow if step is at most k
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return step <= k
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}
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// ind < knn - k cannot overflow if step is at most k+1
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return step <= k+1 && k != math.MaxInt64
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} else { // step < 0
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if limit.Op == OpConst64 {
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// Figure out the actual smallest value.
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v := limit.AuxInt
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if !inclusive {
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if v == math.MaxInt64 {
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return false // > maxint is never satisfiable.
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}
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v++
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}
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if init.Op == OpConst64 {
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// Use stride to compute a better lower limit.
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if init.AuxInt < v {
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return false
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}
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v = subU(init.AuxInt, diff(init.AuxInt, v)/uint64(-step)*uint64(-step))
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}
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// It is ok if we can't underflow when decrementing from the smallest value.
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return !subWillUnderflow(v, -step)
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}
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if step == -1 && !inclusive {
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// Can't underflow because minint is never a possible value.
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return true
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}
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}
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return false
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// TODO: other unrolling idioms
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// for i := 0; i < KNN - KNN % k ; i += k
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// for i := 0; i < KNN&^(k-1) ; i += k // k a power of 2
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// for i := 0; i < KNN&(-k) ; i += k // k a power of 2
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}
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if !ok {
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continue
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if ok() {
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flags := indVarFlags(0)
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var min, max *Value
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if step > 0 {
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min = init
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max = limit
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if inclusive {
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flags |= indVarMaxInc
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}
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} else {
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min = limit
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max = init
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flags |= indVarMaxInc
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if !inclusive {
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flags |= indVarMinExc
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}
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step = -step
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}
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if f.pass.debug >= 1 {
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printIndVar(b, ind, min, max, step, flags)
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}
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iv = append(iv, indVar{
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ind: ind,
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min: min,
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max: max,
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entry: b.Succs[0].b,
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flags: flags,
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})
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b.Logf("found induction variable %v (inc = %v, min = %v, max = %v)\n", ind, inc, min, max)
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}
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if f.pass.debug >= 1 {
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printIndVar(b, ind, min, max, step, flags)
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}
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iv = append(iv, indVar{
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ind: ind,
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min: min,
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max: max,
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entry: b.Succs[0].b,
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flags: flags,
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})
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b.Logf("found induction variable %v (inc = %v, min = %v, max = %v)\n", ind, inc, min, max)
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// TODO: other unrolling idioms
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// for i := 0; i < KNN - KNN % k ; i += k
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// for i := 0; i < KNN&^(k-1) ; i += k // k a power of 2
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// for i := 0; i < KNN&(-k) ; i += k // k a power of 2
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}
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return iv
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}
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func dropAdd64(v *Value) (*Value, int64) {
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if v.Op == OpAdd64 && v.Args[0].Op == OpConst64 {
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return v.Args[1], v.Args[0].AuxInt
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// addWillOverflow reports whether x+y would result in a value more than maxint.
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func addWillOverflow(x, y int64) bool {
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return x+y < x
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}
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// subWillUnderflow reports whether x-y would result in a value less than minint.
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func subWillUnderflow(x, y int64) bool {
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return x-y > x
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}
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// diff returns x-y as a uint64. Requires x>=y.
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func diff(x, y int64) uint64 {
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if x < y {
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base.Fatalf("diff %d - %d underflowed", x, y)
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}
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if v.Op == OpAdd64 && v.Args[1].Op == OpConst64 {
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return v.Args[0], v.Args[1].AuxInt
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return uint64(x - y)
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}
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// addU returns x+y. Requires that x+y does not overflow an int64.
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func addU(x int64, y uint64) int64 {
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if y >= 1<<63 {
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if x >= 0 {
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base.Fatalf("addU overflowed %d + %d", x, y)
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}
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x += 1<<63 - 1
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x += 1
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y -= 1 << 63
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}
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return v, 0
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if addWillOverflow(x, int64(y)) {
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base.Fatalf("addU overflowed %d + %d", x, y)
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}
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return x + int64(y)
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}
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// subU returns x-y. Requires that x-y does not underflow an int64.
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func subU(x int64, y uint64) int64 {
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if y >= 1<<63 {
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if x < 0 {
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base.Fatalf("subU underflowed %d - %d", x, y)
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}
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x -= 1<<63 - 1
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x -= 1
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y -= 1 << 63
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}
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if subWillUnderflow(x, int64(y)) {
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base.Fatalf("subU underflowed %d - %d", x, y)
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}
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return x - int64(y)
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}
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// if v is known to be x - c, where x is known to be nonnegative and c is a
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// constant, return x, c. Otherwise return nil, 0.
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func findKNN(v *Value) (*Value, int64) {
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var x, y *Value
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x = v
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switch v.Op {
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case OpSub64:
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x = v.Args[0]
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y = v.Args[1]
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case OpAdd64:
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x = v.Args[0]
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y = v.Args[1]
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if x.Op == OpConst64 {
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x, y = y, x
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}
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}
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switch x.Op {
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case OpSliceLen, OpStringLen, OpSliceCap:
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default:
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return nil, 0
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}
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if y == nil {
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return x, 0
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}
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if y.Op != OpConst64 {
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return nil, 0
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}
|
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if v.Op == OpAdd64 {
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return x, -y.AuxInt
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}
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return x, y.AuxInt
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}
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func printIndVar(b *Block, i, min, max *Value, inc int64, flags indVarFlags) {
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@ -12,6 +12,7 @@ func main() {
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f()
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g()
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h()
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j(math.MinInt64)
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}
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func f() {
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for i := int64(math.MaxInt64); i <= math.MaxInt64; i++ {
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@ -40,3 +41,13 @@ func h() {
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println(i, i < 0)
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}
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}
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//go:noinline
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func j(i int64) {
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for j := int64(math.MaxInt64); j <= i-1; j++ {
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if j < 0 {
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break
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}
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println(j)
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}
|
||||
}
|
||||
|
||||
@ -6,3 +6,4 @@ done
|
||||
9223372036854775805 false
|
||||
9223372036854775807 false
|
||||
done
|
||||
9223372036854775807
|
||||
|
||||
42
test/fixedbugs/issue53653.go
Normal file
42
test/fixedbugs/issue53653.go
Normal file
@ -0,0 +1,42 @@
|
||||
// run
|
||||
|
||||
// Copyright 2022 The Go Authors. All rights reserved.
|
||||
// Use of this source code is governed by a BSD-style
|
||||
// license that can be found in the LICENSE file.
|
||||
|
||||
package main
|
||||
|
||||
import "math"
|
||||
|
||||
func main() {
|
||||
f()
|
||||
g()
|
||||
h()
|
||||
}
|
||||
func f() {
|
||||
for i := int64(math.MinInt64); i >= math.MinInt64; i-- {
|
||||
if i > 0 {
|
||||
println("done")
|
||||
return
|
||||
}
|
||||
println(i, i > 0)
|
||||
}
|
||||
}
|
||||
func g() {
|
||||
for i := int64(math.MinInt64) + 1; i >= math.MinInt64; i-- {
|
||||
if i > 0 {
|
||||
println("done")
|
||||
return
|
||||
}
|
||||
println(i, i > 0)
|
||||
}
|
||||
}
|
||||
func h() {
|
||||
for i := int64(math.MinInt64) + 2; i >= math.MinInt64; i -= 2 {
|
||||
if i > 0 {
|
||||
println("done")
|
||||
return
|
||||
}
|
||||
println(i, i > 0)
|
||||
}
|
||||
}
|
||||
8
test/fixedbugs/issue53653.out
Normal file
8
test/fixedbugs/issue53653.out
Normal file
@ -0,0 +1,8 @@
|
||||
-9223372036854775808 false
|
||||
done
|
||||
-9223372036854775807 false
|
||||
-9223372036854775808 false
|
||||
done
|
||||
-9223372036854775806 false
|
||||
-9223372036854775808 false
|
||||
done
|
||||
@ -3,6 +3,8 @@
|
||||
|
||||
package main
|
||||
|
||||
import "math"
|
||||
|
||||
func f0a(a []int) int {
|
||||
x := 0
|
||||
for i := range a { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
|
||||
@ -281,8 +283,8 @@ func d2(a [100]int) [100]int {
|
||||
|
||||
func d3(a [100]int) [100]int {
|
||||
for i := 0; i <= 99; i++ { // ERROR "Induction variable: limits \[0,99\], increment 1$"
|
||||
for j := 0; j <= i-1; j++ { // ERROR "Induction variable: limits \[0,\?\], increment 1$"
|
||||
a[j] = 0 // ERROR "Proved IsInBounds$"
|
||||
for j := 0; j <= i-1; j++ {
|
||||
a[j] = 0
|
||||
a[j+1] = 0 // ERROR "Proved IsInBounds$"
|
||||
a[j+2] = 0
|
||||
}
|
||||
@ -290,7 +292,61 @@ func d3(a [100]int) [100]int {
|
||||
return a
|
||||
}
|
||||
|
||||
func nobce1() {
|
||||
func d4() {
|
||||
for i := int64(math.MaxInt64 - 9); i < math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775798,9223372036854775805\), increment 4$"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MaxInt64 - 8); i < math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775799,9223372036854775805\), increment 4$"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MaxInt64 - 7); i < math.MaxInt64-2; i += 4 {
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MaxInt64 - 6); i < math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775801,9223372036854775805\), increment 4$"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MaxInt64 - 9); i <= math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775798,9223372036854775805\], increment 4$"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MaxInt64 - 8); i <= math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775799,9223372036854775805\], increment 4$"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MaxInt64 - 7); i <= math.MaxInt64-2; i += 4 {
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MaxInt64 - 6); i <= math.MaxInt64-2; i += 4 {
|
||||
useString("foo")
|
||||
}
|
||||
}
|
||||
|
||||
func d5() {
|
||||
for i := int64(math.MinInt64 + 9); i > math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \(-9223372036854775806,-9223372036854775799\], increment 4"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MinInt64 + 8); i > math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \(-9223372036854775806,-9223372036854775800\], increment 4"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MinInt64 + 7); i > math.MinInt64+2; i -= 4 {
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MinInt64 + 6); i > math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \(-9223372036854775806,-9223372036854775802\], increment 4"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MinInt64 + 9); i >= math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \[-9223372036854775806,-9223372036854775799\], increment 4"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MinInt64 + 8); i >= math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \[-9223372036854775806,-9223372036854775800\], increment 4"
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MinInt64 + 7); i >= math.MinInt64+2; i -= 4 {
|
||||
useString("foo")
|
||||
}
|
||||
for i := int64(math.MinInt64 + 6); i >= math.MinInt64+2; i -= 4 {
|
||||
useString("foo")
|
||||
}
|
||||
}
|
||||
|
||||
func bce1() {
|
||||
// tests overflow of max-min
|
||||
a := int64(9223372036854774057)
|
||||
b := int64(-1547)
|
||||
@ -300,8 +356,7 @@ func nobce1() {
|
||||
panic("invalid test: modulos should differ")
|
||||
}
|
||||
|
||||
for i := b; i < a; i += z {
|
||||
// No induction variable is possible because i will overflow a first iteration.
|
||||
for i := b; i < a; i += z { // ERROR "Induction variable: limits \[-1547,9223372036854774057\), increment 1337"
|
||||
useString("foobar")
|
||||
}
|
||||
}
|
||||
|
||||
Loading…
x
Reference in New Issue
Block a user