cmd/compile: don't evaluate side effects of range over array

If the thing we're ranging over is an array or ptr to array, and
it doesn't have a function call or channel receive in it, then we
shouldn't evaluate it.

Typecheck the ranged-over value as a constant in that case.
That makes the unified exporter replace the range expression
with a constant int.

Change-Id: I0d4ea081de70d20cf6d1fa8d25ef6cb021975554
Reviewed-on: https://go-review.googlesource.com/c/go/+/659317
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

Author: randall77

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

@@ -217,12 +217,12 @@ func nilcheckelim2(f *Func) {
 					f.Warnl(v.Pos, "removed nil check")
 				}
 				// For bug 33724, policy is that we might choose to bump an existing position
-				// off the faulting load/store in favor of the one from the nil check.
+				// off the faulting load in favor of the one from the nil check.
 
 				// Iteration order means that first nilcheck in the chain wins, others
 				// are bumped into the ordinary statement preservation algorithm.
 				u := b.Values[unnecessary.get(v.Args[0].ID)]
-				if !u.Pos.SameFileAndLine(v.Pos) {
+				if !u.Type.IsMemory() && !u.Pos.SameFileAndLine(v.Pos) {
 					if u.Pos.IsStmt() == src.PosIsStmt {
 						pendingLines.add(u.Pos)
 					}

src/cmd/compile/internal/types2/range.go

@@ -8,6 +8,7 @@ package types2
 
 import (
 	"cmd/compile/internal/syntax"
+	"go/constant"
 	"internal/buildcfg"
 	. "internal/types/errors"
 )
@@ -23,8 +24,32 @@ import (
 func (check *Checker) rangeStmt(inner stmtContext, rangeStmt *syntax.ForStmt, noNewVarPos poser, sKey, sValue, sExtra, rangeVar syntax.Expr, isDef bool) {
 	// check expression to iterate over
 	var x operand
+
+	// From the spec:
+	//   The range expression x is evaluated before beginning the loop,
+	//   with one exception: if at most one iteration variable is present
+	//   and x or len(x) is constant, the range expression is not evaluated.
+	// So we have to be careful not to evaluate the arg in the
+	// described situation.
+
+	check.hasCallOrRecv = false
 	check.expr(nil, &x, rangeVar)
 
+	if isTypes2 && x.mode != invalid && sValue == nil && !check.hasCallOrRecv {
+		if t, ok := arrayPtrDeref(under(x.typ)).(*Array); ok {
+			// Override type of rangeVar to be a constant
+			// (and thus side-effects will not be computed
+			// by the backend).
+			check.record(&operand{
+				mode: constant_,
+				expr: rangeVar,
+				typ:  Typ[Int],
+				val:  constant.MakeInt64(t.len),
+				id:   x.id,
+			})
+		}
+	}
+
 	// determine key/value types
 	var key, val Type
 	if x.mode != invalid {

src/cmd/compile/internal/walk/range.go

@@ -5,6 +5,7 @@
 package walk
 
 import (
+	"go/constant"
 	"internal/buildcfg"
 	"unicode/utf8"
 
@@ -80,6 +81,10 @@ func walkRange(nrange *ir.RangeStmt) ir.Node {
 		base.Fatalf("walkRange")
 
 	case types.IsInt[k]:
+		if nn := arrayRangeClear(nrange, v1, v2, a); nn != nil {
+			base.Pos = lno
+			return nn
+		}
 		hv1 := typecheck.TempAt(base.Pos, ir.CurFunc, t)
 		hn := typecheck.TempAt(base.Pos, ir.CurFunc, t)
 
@@ -519,21 +524,41 @@ func arrayRangeClear(loop *ir.RangeStmt, v1, v2, a ir.Node) ir.Node {
 	}
 	lhs := stmt.X.(*ir.IndexExpr)
 	x := lhs.X
-	if a.Type().IsPtr() && a.Type().Elem().IsArray() {
-		if s, ok := x.(*ir.StarExpr); ok && s.Op() == ir.ODEREF {
-			x = s.X
+
+	// Get constant number of iterations for int and array cases.
+	n := int64(-1)
+	if ir.IsConst(a, constant.Int) {
+		n = ir.Int64Val(a)
+	} else if a.Type().IsArray() {
+		n = a.Type().NumElem()
+	} else if a.Type().IsPtr() && a.Type().Elem().IsArray() {
+		n = a.Type().Elem().NumElem()
+	}
+
+	if n >= 0 {
+		// Int/Array case.
+		if !x.Type().IsArray() {
+			return nil
+		}
+		if x.Type().NumElem() != n {
+			return nil
+		}
+	} else {
+		// Slice case.
+		if !ir.SameSafeExpr(x, a) {
+			return nil
 		}
 	}
 
-	if !ir.SameSafeExpr(x, a) || !ir.SameSafeExpr(lhs.Index, v1) {
+	if !ir.SameSafeExpr(lhs.Index, v1) {
 		return nil
 	}
 
 	if !ir.IsZero(stmt.Y) {
 		return nil
 	}
 
-	return arrayClear(stmt.Pos(), a, loop)
+	return arrayClear(stmt.Pos(), x, loop)
 }
 
 // arrayClear constructs a call to runtime.memclr for fast zeroing of slices and arrays.

src/go/types/range.go

@@ -12,6 +12,7 @@ package codegen
 type T struct {
 	a *[10]int
 	b [10]int
+	s []int
 }
 
 func (t *T) f() {
@@ -38,4 +39,15 @@ func (t *T) f() {
 	for i := range *t.a {
 		(*t.a)[i] = 0
 	}
+
+	// amd64:-".*runtime.memclrNoHeapPointers"
+	// amd64:"DUFFZERO"
+	for i := range t.b {
+		t.b[i] = 0
+	}
+
+	// amd64:".*runtime.memclrNoHeapPointers"
+	for i := range t.s {
+		t.s[i] = 0
+	}
 }