cmd/compile: learn transitive proofs for safe unsigned adds

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>

2 files changed, 94 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/prove.go b/src/cmd/compile/internal/ssa/prove.go
index 93bd525c38..8fe1bb7050 100644
--- a/src/cmd/compile/internal/ssa/prove.go
+++ b/src/cmd/compile/internal/ssa/prove.go
@@ -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)
diff --git a/test/prove.go b/test/prove.go
index faf0b79237..e843edcbf0 100644
--- a/test/prove.go
+++ b/test/prove.go
@@ -2041,6 +2041,69 @@ func cvtBoolToUint8BCE(b bool, a [2]int64) int64 {
 	return a[c] // ERROR "Proved IsInBounds$"
 }
 
+func transitiveProofsThroughNonOverflowingUnsignedAdd(x, y, z uint64) {
+	x &= 1<<63 - 1
+	y &= 1<<63 - 1
+
+	a := x + y
+	if a > z {
+		return
+	}
+
+	if x > z { // ERROR "Disproved Less64U$"
+		return
+	}
+	if y > z { // ERROR "Disproved Less64U$"
+		return
+	}
+	if a == x {
+		return
+	}
+	if a == y {
+		return
+	}
+
+	x |= 1
+	y |= 1
+	a = x + y
+	if a == x { // ERROR "Disproved Eq64$"
+		return
+	}
+	if a == y { // ERROR "Disproved Eq64$"
+		return
+	}
+}
+
+func transitiveProofsThroughOverflowingUnsignedAdd(x, y, z uint64) {
+	a := x + y
+	if a > z {
+		return
+	}
+
+	if x > z {
+		return
+	}
+	if y > z {
+		return
+	}
+	if a == x {
+		return
+	}
+	if a == y {
+		return
+	}
+
+	x |= 1
+	y |= 1
+	a = x + y
+	if a == x {
+		return
+	}
+	if a == y {
+		return
+	}
+}
+
 //go:noinline
 func useInt(a int) {
 }