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cmd/compile: learn transitive proofs for safe unsigned adds

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I've split this into it's own CL to make git bisect more effective.

Change-Id: Iaab5f0bd2ad51e86ced8c6b8fbd371eb75eeef14
Reviewed-on: https://go-review.googlesource.com/c/go/+/685815
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Mark Freeman <mark@golang.org>
This commit is contained in:
Jorropo 2025-07-04 09:21:03 +02:00 • committed by Gopher Robot
parent bd80f74bc1
commit e5f202bb60
2 changed files with 94 additions and 0 deletions
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31
src/cmd/compile/internal/ssa/prove.go
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@ -5,6 +5,7 @@
package ssa
import (
"cmd/compile/internal/types"
"cmd/internal/src"
"fmt"
"math"
@ -2132,6 +2133,21 @@ func addRestrictions(parent *Block, ft *factsTable, t domain, v, w *Value, r rel
}
}
func unsignedAddOverflows(a, b uint64, t *types.Type) bool {
switch t.Size() {
case 8:
return a+b < a
case 4:
return a+b > math.MaxUint32
case 2:
return a+b > math.MaxUint16
case 1:
return a+b > math.MaxUint8
default:
panic("unreachable")
}
}
func addLocalFacts(ft *factsTable, b *Block) {
// Propagate constant ranges among values in this block.
// We do this before the second loop so that we have the
@ -2151,6 +2167,21 @@ func addLocalFacts(ft *factsTable, b *Block) {
// FIXME(go.dev/issue/68857): this loop only set up limits properly when b.Values is in topological order.
// flowLimit can also depend on limits given by this loop which right now is not handled.
switch v.Op {
case OpAdd64, OpAdd32, OpAdd16, OpAdd8:
x := ft.limits[v.Args[0].ID]
y := ft.limits[v.Args[1].ID]
if !unsignedAddOverflows(x.umax, y.umax, v.Type) {
r := gt
if !x.nonzero() {
r |= eq
}
ft.update(b, v, v.Args[1], unsigned, r)
r = gt
if !y.nonzero() {
r |= eq
}
ft.update(b, v, v.Args[0], unsigned, r)
}
case OpAnd64, OpAnd32, OpAnd16, OpAnd8:
ft.update(b, v, v.Args[0], unsigned, lt|eq)
ft.update(b, v, v.Args[1], unsigned, lt|eq)