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| author | Jorropo <jorropo.pgm@gmail.com> | 2024-08-09 15:45:39 +0200 |
|---|---|---|
| committer | Keith Randall <khr@golang.org> | 2024-09-03 17:12:49 +0000 |
| commit | 49621cc311a41b71f60f03202f3872c0633cac59 (patch) | |
| tree | 56612663952081bbad6db8ebd4e1ebbecbb88d93 /src/cmd | |
| parent | 9a4fe7e14a4f71267f929c5545916f9830a89187 (diff) | |
| download | go-49621cc311a41b71f60f03202f3872c0633cac59.tar.xz | |
cmd/compile: compute XOR's limits from argument's limits
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>
Diffstat (limited to 'src/cmd')
| -rw-r--r-- | src/cmd/compile/internal/ssa/prove.go | 10 |
1 files changed, 8 insertions, 2 deletions
diff --git a/src/cmd/compile/internal/ssa/prove.go b/src/cmd/compile/internal/ssa/prove.go index 1daf8d85c4..5195a48608 100644 --- a/src/cmd/compile/internal/ssa/prove.go +++ b/src/cmd/compile/internal/ssa/prove.go @@ -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: |