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cmd/compile: compute bits.TrailingZeros*'s limits from argument's limits
y := bits.TrailingZeros(x) if y > bits.Len(x.umax)-1 { then must always be true 1 << y > x.umax which is impossible } Change-Id: Iab4fce1c2ef828bee3a8a4a977cbadb5f9333136 Reviewed-on: https://go-review.googlesource.com/c/go/+/603996 LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@golang.org> Auto-Submit: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: David Chase <drchase@google.com>
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@ -1648,22 +1648,22 @@ func (ft *factsTable) flowLimit(v *Value) bool {
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case OpCtz64:
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a := ft.limits[v.Args[0].ID]
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if a.nonzero() {
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return ft.unsignedMax(v, 63)
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return ft.unsignedMax(v, uint64(bits.Len64(a.umax)-1))
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}
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case OpCtz32:
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a := ft.limits[v.Args[0].ID]
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if a.nonzero() {
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return ft.unsignedMax(v, 31)
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return ft.unsignedMax(v, uint64(bits.Len32(uint32(a.umax))-1))
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}
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case OpCtz16:
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a := ft.limits[v.Args[0].ID]
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if a.nonzero() {
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return ft.unsignedMax(v, 15)
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return ft.unsignedMax(v, uint64(bits.Len16(uint16(a.umax))-1))
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}
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case OpCtz8:
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a := ft.limits[v.Args[0].ID]
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if a.nonzero() {
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return ft.unsignedMax(v, 7)
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return ft.unsignedMax(v, uint64(bits.Len8(uint8(a.umax))-1))
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}
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case OpBitLen64:
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@ -1196,6 +1196,103 @@ func f22(b bool, x, y int) int {
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return 0
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}
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func ctz64(x uint64, ensureBothBranchesCouldHappen bool) int {
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const max = math.MaxUint64
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sz := bits.Len64(max)
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log2half := uint64(max) >> (sz / 2)
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if x >= log2half || x == 0 {
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return 42
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}
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y := bits.TrailingZeros64(x) // ERROR "Proved Ctz64 non-zero$""
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z := sz / 2
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if ensureBothBranchesCouldHappen {
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if y < z { // ERROR "Proved Less64$"
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return -42
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}
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} else {
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if y >= z { // ERROR "Disproved Leq64$"
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return 1337
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}
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}
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return y
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}
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func ctz32(x uint32, ensureBothBranchesCouldHappen bool) int {
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const max = math.MaxUint32
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sz := bits.Len32(max)
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log2half := uint32(max) >> (sz / 2)
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if x >= log2half || x == 0 {
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return 42
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}
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y := bits.TrailingZeros32(x) // ERROR "Proved Ctz32 non-zero$""
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z := sz / 2
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if ensureBothBranchesCouldHappen {
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if y < z { // ERROR "Proved Less64$"
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return -42
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}
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} else {
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if y >= z { // ERROR "Disproved Leq64$"
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return 1337
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}
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}
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return y
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}
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func ctz16(x uint16, ensureBothBranchesCouldHappen bool) int {
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const max = math.MaxUint16
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sz := bits.Len16(max)
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log2half := uint16(max) >> (sz / 2)
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if x >= log2half || x == 0 {
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return 42
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}
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y := bits.TrailingZeros16(x) // ERROR "Proved Ctz16 non-zero$""
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z := sz / 2
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if ensureBothBranchesCouldHappen {
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if y < z { // ERROR "Proved Less64$"
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return -42
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}
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} else {
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if y >= z { // ERROR "Disproved Leq64$"
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return 1337
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}
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}
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return y
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}
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func ctz8(x uint8, ensureBothBranchesCouldHappen bool) int {
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const max = math.MaxUint8
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sz := bits.Len8(max)
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log2half := uint8(max) >> (sz / 2)
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if x >= log2half || x == 0 {
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return 42
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}
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y := bits.TrailingZeros8(x) // ERROR "Proved Ctz8 non-zero$""
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z := sz / 2
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if ensureBothBranchesCouldHappen {
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if y < z { // ERROR "Proved Less64$"
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return -42
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}
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} else {
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if y >= z { // ERROR "Disproved Leq64$"
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return 1337
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}
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}
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return y
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}
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func bitLen64(x uint64, ensureBothBranchesCouldHappen bool) int {
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const max = math.MaxUint64
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sz := bits.Len64(max)
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