cmd/compile: init limit for newly created value in prove pass

Fixes: #70156

Change-Id: I2e5dc2a39a8e54ec5f18c5f9d1644208cffb2e9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/624695
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Mauri de Souza Meneguzzo <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

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

@@ -462,6 +462,25 @@ func newFactsTable(f *Func) *factsTable {
 	return ft
 }
 
+// initLimitForNewValue initializes the limits for newly created values,
+// possibly needing to expand the limits slice. Currently used by
+// simplifyBlock when certain provably constant results are folded.
+func (ft *factsTable) initLimitForNewValue(v *Value) {
+	if int(v.ID) >= len(ft.limits) {
+		f := v.Block.Func
+		n := f.NumValues()
+		if cap(ft.limits) >= n {
+			ft.limits = ft.limits[:n]
+		} else {
+			old := ft.limits
+			ft.limits = f.Cache.allocLimitSlice(n)
+			copy(ft.limits, old)
+			f.Cache.freeLimitSlice(old)
+		}
+	}
+	ft.limits[v.ID] = initLimit(v)
+}
+
 // signedMin records the fact that we know v is at least
 // min in the signed domain.
 func (ft *factsTable) signedMin(v *Value, min int64) bool {
@@ -2269,6 +2288,7 @@ func simplifyBlock(sdom SparseTree, ft *factsTable, b *Block) {
 				continue
 			}
 			v.SetArg(i, c)
+			ft.initLimitForNewValue(c)
 			if b.Func.pass.debug > 1 {
 				b.Func.Warnl(v.Pos, "Proved %v's arg %d (%v) is constant %d", v, i, arg, constValue)
 			}

test/fixedbugs/issue70156.go

@@ -0,0 +1,23 @@
+// run
+
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"reflect"
+)
+
+func main() {
+	pi := new(interface{})
+	v := reflect.ValueOf(pi).Elem()
+	if v.Kind() != reflect.Interface {
+		panic(0)
+	}
+	if (v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface) && v.IsNil() {
+		return
+	}
+	panic(1)
+}