cmd/compile: make isSmallMakeSlice checks slice cap only

If slice cap is not set, it will be equal to slice len. So
isSmallMakeSlice only needs to check whether slice cap is constant.

While at it, also add test to make sure panicmakeslicecap is called
when make slice contains invalid non-constant len.

For this benchmark:

func BenchmarkMakeSliceNonConstantLen(b *testing.B) {
	len := 1
	for i := 0; i < b.N; i++ {
		s := make([]int, len, 2)
		_ = s

	}
}

Result compare with parent:

name                        old time/op  new time/op  delta
MakeSliceNonConstantLen-12  18.4ns ± 1%   0.2ns ± 2%  -98.66%  (p=0.008 n=5+5)

Fixes #37975

Change-Id: I4bc926361bc2ffeab4cfaa888ef0a30cbc3b80e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/226278
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/builtin.go

@@ -13,6 +13,7 @@ var runtimeDecls = [...]struct {
 	{"panicdivide", funcTag, 5},
 	{"panicshift", funcTag, 5},
 	{"panicmakeslicelen", funcTag, 5},
+	{"panicmakeslicecap", funcTag, 5},
 	{"throwinit", funcTag, 5},
 	{"panicwrap", funcTag, 5},
 	{"gopanic", funcTag, 7},

src/cmd/compile/internal/gc/builtin/runtime.go

@@ -18,6 +18,7 @@ func newobject(typ *byte) *any
 func panicdivide()
 func panicshift()
 func panicmakeslicelen()
+func panicmakeslicecap()
 func throwinit()
 func panicwrap()
 

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

@@ -354,14 +354,13 @@ func isSmallMakeSlice(n *Node) bool {
 	if n.Op != OMAKESLICE {
 		return false
 	}
-	l := n.Left
 	r := n.Right
 	if r == nil {
-		r = l
+		r = n.Left
 	}
 	t := n.Type
 
-	return smallintconst(l) && smallintconst(r) && (t.Elem().Width == 0 || r.Int64() < maxImplicitStackVarSize/t.Elem().Width)
+	return smallintconst(r) && (t.Elem().Width == 0 || r.Int64() < maxImplicitStackVarSize/t.Elem().Width)
 }
 
 // walk the whole tree of the body of an
@@ -1338,6 +1337,19 @@ opswitch:
 			if i < 0 {
 				Fatalf("walkexpr: invalid index %v", r)
 			}
+
+			// if len < 0 { panicmakeslicelen }
+			nif := nod(OIF, nod(OLT, l, nodintconst(0)), nil)
+			nif.Nbody.Set1(mkcall("panicmakeslicelen", nil, init))
+			nif = typecheck(nif, ctxStmt)
+			init.Append(nif)
+
+			// if len > cap { panicmakeslicecap }
+			nif = nod(OIF, nod(OGT, conv(l, types.Types[TUINT64]), nodintconst(i)), nil)
+			nif.Nbody.Set1(mkcall("panicmakeslicecap", nil, init))
+			nif = typecheck(nif, ctxStmt)
+			init.Append(nif)
+
 			t = types.NewArray(t.Elem(), i) // [r]T
 			var_ := temp(t)
 			a := nod(OAS, var_, nil) // zero temp

test/escape_slice.go

@@ -93,6 +93,14 @@ func slice10() []*int {
 	return s
 }
 
+func slice11() {
+	i := 2
+	s := make([]int, 2, 3) // ERROR "make\(\[\]int, 2, 3\) does not escape"
+	s = make([]int, i, 3)  // ERROR "make\(\[\]int, i, 3\) does not escape"
+	s = make([]int, i, 1)  // ERROR "make\(\[\]int, i, 1\) does not escape"
+	_ = s
+}
+
 func envForDir(dir string) []string { // ERROR "dir does not escape"
 	env := os.Environ()
 	return mergeEnvLists([]string{"PWD=" + dir}, env) // ERROR ".PWD=. \+ dir escapes to heap" "\[\]string literal does not escape"

test/fixedbugs/issue37975.go

@@ -0,0 +1,54 @@
+// run
+
+// Copyright 2020 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.
+
+// Make sure runtime.panicmakeslice* are called.
+
+package main
+
+import "strings"
+
+func main() {
+	// Test typechecking passes if len is valid
+	// but cap is out of range for len's type.
+	var x byte
+	_ = make([]int, x, 300)
+
+	capOutOfRange := func() {
+		i := 2
+		s := make([]int, i, 1)
+		s[0] = 1
+	}
+	lenOutOfRange := func() {
+		i := -1
+		s := make([]int, i, 3)
+		s[0] = 1
+	}
+
+	tests := []struct {
+		f        func()
+		panicStr string
+	}{
+		{capOutOfRange, "cap out of range"},
+		{lenOutOfRange, "len out of range"},
+	}
+
+	for _, tc := range tests {
+		shouldPanic(tc.panicStr, tc.f)
+	}
+
+}
+
+func shouldPanic(str string, f func()) {
+	defer func() {
+		err := recover()
+		runtimeErr := err.(error).Error()
+		if !strings.Contains(runtimeErr, str) {
+			panic("got panic " + runtimeErr + ", want " + str)
+		}
+	}()
+
+	f()
+}