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cmd/compile: compute bits.OnesCount's limits from argument's limits

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Change-Id: Ia90d48ea0fab363c8592221fad88958b522edefe
Reviewed-on: https://go-review.googlesource.com/c/go/+/656159
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
Jorropo 2025-03-09 14:39:49 +01:00
parent 8d767ff38d
commit 99411d7847
2 changed files with 31 additions and 0 deletions
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8
src/cmd/compile/internal/ssa/prove.go
View File

@ -1732,6 +1732,14 @@ func (ft *factsTable) flowLimit(v *Value) bool {
return ft.unsignedMax(v, uint64(bits.Len8(uint8(a.umax))-1))
}
case OpPopCount64, OpPopCount32, OpPopCount16, OpPopCount8:
a := ft.limits[v.Args[0].ID]
changingBitsCount := uint64(bits.Len64(a.umax ^ a.umin))
sharedLeadingMask := ^(uint64(1)<<changingBitsCount - 1)
fixedBits := a.umax & sharedLeadingMask
min := uint64(bits.OnesCount64(fixedBits))
return ft.unsignedMinMax(v, min, min+changingBitsCount)
case OpBitLen64:
a := ft.limits[v.Args[0].ID]
return ft.unsignedMinMax(v,

23
test/prove_popcount.go
View File

@ -33,5 +33,28 @@ func onesCountsBounds(x uint64, ensureAllBranchesCouldHappen func() bool) int {
return z
}
func onesCountsTight(x uint64, ensureAllBranchesCouldHappen func() bool) int {
const maxv = 0xff0f
const minv = 0xff00
x = max(x, minv)
x = min(x, maxv)
z := bits.OnesCount64(x)
if ensureAllBranchesCouldHappen() && z > bits.OnesCount64(maxv) { // ERROR "Disproved Less64$"
return 42
}
if ensureAllBranchesCouldHappen() && z <= bits.OnesCount64(maxv) { // ERROR "Proved Leq64$"
return 4242
}
if ensureAllBranchesCouldHappen() && z < bits.OnesCount64(minv) { // ERROR "Disproved Less64$"
return 424242
}
if ensureAllBranchesCouldHappen() && z >= bits.OnesCount64(minv) { // ERROR "Proved Leq64$"
return 42424242
}
return z
}
func main() {
}