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cmd/compile: compute XOR's limits from argument's limits

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This help to optimize code like this:

  func f(buckets *[512]bucket, v value) {
    a, b := v.computeSomething()
    // assume a and b are proved < 512
    b := &buckets[a ^ b] // pick a random bucket
    b.store(v)
  }

Change-Id: I1acf702f5a8137f9ded49081b4703922879b0288
Reviewed-on: https://go-review.googlesource.com/c/go/+/604455
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@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 2024-08-09 15:45:39 +02:00 • committed by Keith Randall
parent 9a4fe7e14a
commit 49621cc311
2 changed files with 26 additions and 2 deletions
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10
src/cmd/compile/internal/ssa/prove.go
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@ -1688,6 +1688,10 @@ func (ft *factsTable) flowLimit(v *Value) bool {
uint64(bits.Len8(uint8(a.umax))))
// Masks.
// TODO: if y.umax and y.umin share a leading bit pattern, y also has that leading bit pattern.
// we could compare the patterns of always set bits in a and b and learn more about minimum and maximum.
// But I doubt this help any real world code.
case OpAnd64, OpAnd32, OpAnd16, OpAnd8:
// AND can only make the value smaller.
a := ft.limits[v.Args[0].ID]
@ -1699,8 +1703,10 @@ func (ft *factsTable) flowLimit(v *Value) bool {
b := ft.limits[v.Args[1].ID]
return ft.unsignedMin(v, maxU(a.umin, b.umin))
case OpXor64, OpXor32, OpXor16, OpXor8:
// TODO: use leading/trailing zeroes?
// Not sure if it is worth it.
// XOR can't flip bits that are proved to be zero in both inputs.
a := ft.limits[v.Args[0].ID]
b := ft.limits[v.Args[1].ID]
return ft.unsignedMax(v, 1<<bits.Len64(a.umax|b.umax)-1)
// Arithmetic.
case OpAdd64: