cmd/compile: use len(s)<=cap(s) to remove more bounds checks

When we discover a relation x <= len(s), also discover the relation
x <= cap(s).  That way, in situations like:

a := s[x:]  // tests 0 <= x <= len(s)
b := s[:x]  // tests 0 <= x <= cap(s)

the second check can be eliminated.

Fixes #16813

Change-Id: Ifc037920b6955e43bac1a1eaf6bac63a89cfbd44
Reviewed-on: https://go-review.googlesource.com/33633
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexandru Moșoi <alexandru@mosoi.ro>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/prove.go

@@ -97,6 +97,12 @@ type factsTable struct {
 	// known lower and upper bounds on individual values.
 	limits     map[ID]limit
 	limitStack []limitFact // previous entries
+
+	// For each slice s, a map from s to a len(s)/cap(s) value (if any)
+	// TODO: check if there are cases that matter where we have
+	// more than one len(s) for a slice. We could keep a list if necessary.
+	lens map[ID]*Value
+	caps map[ID]*Value
 }
 
 // checkpointFact is an invalid value used for checkpointing
@@ -432,7 +438,8 @@ var (
 	}
 )
 
-// prove removes redundant BlockIf controls that can be inferred in a straight line.
+// prove removes redundant BlockIf branches that can be inferred
+// from previous dominating comparisons.
 //
 // By far, the most common redundant pair are generated by bounds checking.
 // For example for the code:
@@ -455,6 +462,31 @@ var (
 // else branch of the first comparison is executed, we already know that i < len(a).
 // The code for the second panic can be removed.
 func prove(f *Func) {
+	ft := newFactsTable()
+
+	// Find length and capacity ops.
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			if v.Uses == 0 {
+				// We don't care about dead values.
+				// (There can be some that are CSEd but not removed yet.)
+				continue
+			}
+			switch v.Op {
+			case OpSliceLen:
+				if ft.lens == nil {
+					ft.lens = map[ID]*Value{}
+				}
+				ft.lens[v.Args[0].ID] = v
+			case OpSliceCap:
+				if ft.caps == nil {
+					ft.caps = map[ID]*Value{}
+				}
+				ft.caps[v.Args[0].ID] = v
+			}
+		}
+	}
+
 	// current node state
 	type walkState int
 	const (
@@ -472,7 +504,6 @@ func prove(f *Func) {
 		state: descend,
 	})
 
-	ft := newFactsTable()
 	idom := f.Idom()
 	sdom := f.sdom()
 
@@ -559,8 +590,34 @@ func updateRestrictions(parent *Block, ft *factsTable, t domain, v, w *Value, r
 		r = (lt | eq | gt) ^ r
 	}
 	for i := domain(1); i <= t; i <<= 1 {
-		if t&i != 0 {
+		if t&i == 0 {
-			ft.update(parent, v, w, i, r)
+			continue
+		}
+		ft.update(parent, v, w, i, r)
+
+		// Additional facts we know given the relationship between len and cap.
+		if i != signed && i != unsigned {
+			continue
+		}
+		if v.Op == OpSliceLen && r&lt == 0 && ft.caps[v.Args[0].ID] != nil {
+			// len(s) > w implies cap(s) > w
+			// len(s) >= w implies cap(s) >= w
+			// len(s) == w implies cap(s) >= w
+			ft.update(parent, ft.caps[v.Args[0].ID], w, i, r|gt)
+		}
+		if w.Op == OpSliceLen && r&gt == 0 && ft.caps[w.Args[0].ID] != nil {
+			// same, length on the RHS.
+			ft.update(parent, v, ft.caps[w.Args[0].ID], i, r|lt)
+		}
+		if v.Op == OpSliceCap && r&gt == 0 && ft.lens[v.Args[0].ID] != nil {
+			// cap(s) < w implies len(s) < w
+			// cap(s) <= w implies len(s) <= w
+			// cap(s) == w implies len(s) <= w
+			ft.update(parent, ft.lens[v.Args[0].ID], w, i, r|lt)
+		}
+		if w.Op == OpSliceCap && r&lt == 0 && ft.lens[w.Args[0].ID] != nil {
+			// same, capacity on the RHS.
+			ft.update(parent, v, ft.lens[w.Args[0].ID], i, r|gt)
 		}
 	}
 }

test/checkbce.go

@@ -50,7 +50,7 @@ func f5(a []int) {
 	if len(a) > 5 {
 		useInt(a[5])
 		useSlice(a[6:])
-		useSlice(a[:6]) // ERROR "Found IsSliceInBounds$"
+		useSlice(a[:6])
 	}
 }
 

test/prove.go

@@ -451,6 +451,18 @@ func f14(p, q *int, a []int) {
 	useInt(a[i2+j]) // ERROR "Proved boolean IsInBounds$"
 }
 
+func f15(s []int, x int) {
+	useSlice(s[x:])
+	useSlice(s[:x]) // ERROR "Proved IsSliceInBounds$"
+}
+
+func f16(s []int) []int {
+	if len(s) >= 10 {
+		return s[:10] // ERROR "Proved non-negative bounds IsSliceInBounds$"
+	}
+	return nil
+}
+
 //go:noinline
 func useInt(a int) {
 }