cmd/compile: change mustHeapAlloc to return a reason why

This change renames mustHeapAlloc to heapAllocReason, and changes it
to return the reason why the argument must escape, so we don't have to
re-deduce it in its callers just to print the escape reason. It also
embeds isSmallMakeSlice body in heapAllocReason, since the former was
only used by the latter, and deletes isSmallMakeSlice.

An outdated TODO to remove smallintconst, which the TODO claimed was
only used in one place, was also removed, since grepping shows we
currently call smallintconst in 11 different places.

Change-Id: I0bd11bf29b92c4126f5bb455877ff73217d5a155
Reviewed-on: https://go-review.googlesource.com/c/go/+/258678
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -1134,7 +1134,6 @@ func strlit(n *Node) string {
 	return n.Val().U.(string)
 }
 
-// TODO(gri) smallintconst is only used in one place - can we used indexconst?
 func smallintconst(n *Node) bool {
 	if n.Op == OLITERAL && Isconst(n, CTINT) && n.Type != nil {
 		switch simtype[n.Type.Etype] {

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

@@ -169,36 +169,47 @@ func mayAffectMemory(n *Node) bool {
 	}
 }
 
-func mustHeapAlloc(n *Node) bool {
+// heapAllocReason returns the reason the given Node must be heap
+// allocated, or the empty string if it doesn't.
+func heapAllocReason(n *Node) string {
 	if n.Type == nil {
-		return false
+		return ""
 	}
 
 	// Parameters are always passed via the stack.
 	if n.Op == ONAME && (n.Class() == PPARAM || n.Class() == PPARAMOUT) {
-		return false
+		return ""
 	}
 
 	if n.Type.Width > maxStackVarSize {
-		return true
+		return "too large for stack"
 	}
 
 	if (n.Op == ONEW || n.Op == OPTRLIT) && n.Type.Elem().Width >= maxImplicitStackVarSize {
-		return true
+		return "too large for stack"
 	}
 
 	if n.Op == OCLOSURE && closureType(n).Size() >= maxImplicitStackVarSize {
-		return true
+		return "too large for stack"
 	}
 	if n.Op == OCALLPART && partialCallType(n).Size() >= maxImplicitStackVarSize {
-		return true
+		return "too large for stack"
 	}
 
-	if n.Op == OMAKESLICE && !isSmallMakeSlice(n) {
-		return true
+	if n.Op == OMAKESLICE {
+		r := n.Right
+		if r == nil {
+			r = n.Left
+		}
+		if !smallintconst(r) {
+			return "non-constant size"
+		}
+		if t := n.Type; t.Elem().Width != 0 && r.Int64() >= maxImplicitStackVarSize/t.Elem().Width {
+			return "too large for stack"
+		}
 	}
 
-	return false
+	return ""
 }
 
 // addrescapes tags node n as having had its address taken

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

@@ -1051,11 +1051,7 @@ func (e *Escape) newLoc(n *Node, transient bool) *EscLocation {
 		}
 		n.SetOpt(loc)
 
-		if mustHeapAlloc(n) {
-			why := "too large for stack"
-			if n.Op == OMAKESLICE && (!Isconst(n.Left, CTINT) || (n.Right != nil && !Isconst(n.Right, CTINT))) {
-				why = "non-constant size"
-			}
+		if why := heapAllocReason(n); why != "" {
 			e.flow(e.heapHole().addr(n, why), loc)
 		}
 	}

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

@@ -336,19 +336,6 @@ func walkstmt(n *Node) *Node {
 	return n
 }
 
-func isSmallMakeSlice(n *Node) bool {
-	if n.Op != OMAKESLICE {
-		return false
-	}
-	r := n.Right
-	if r == nil {
-		r = n.Left
-	}
-	t := n.Type
-
-	return smallintconst(r) && (t.Elem().Width == 0 || r.Int64() < maxImplicitStackVarSize/t.Elem().Width)
-}
-
 // walk the whole tree of the body of an
 // expression or simple statement.
 // the types expressions are calculated.
@@ -1339,8 +1326,8 @@ opswitch:
 			yyerror("%v can't be allocated in Go; it is incomplete (or unallocatable)", t.Elem())
 		}
 		if n.Esc == EscNone {
-			if !isSmallMakeSlice(n) {
-				Fatalf("non-small OMAKESLICE with EscNone: %v", n)
+			if why := heapAllocReason(n); why != "" {
+				Fatalf("%v has EscNone, but %v", n, why)
 			}
 			// var arr [r]T
 			// n = arr[:l]

test/fixedbugs/issue41635.go

@@ -7,12 +7,11 @@
 package p
 
 func f() { // ERROR ""
-	b1 := make([]byte, 1<<17)      // ERROR "too large for stack" ""
-	b2 := make([]byte, 100, 1<<17) // ERROR "too large for stack" ""
-
 	n, m := 100, 200
-	b1 = make([]byte, n)      // ERROR "non-constant size" ""
-	b2 = make([]byte, 100, m) // ERROR "non-constant size" ""
+	_ = make([]byte, 1<<17)      // ERROR "too large for stack" ""
+	_ = make([]byte, 100, 1<<17) // ERROR "too large for stack" ""
+	_ = make([]byte, n, 1<<17)   // ERROR "too large for stack" ""
 
-	_, _ = b1, b2
+	_ = make([]byte, n)      // ERROR "non-constant size" ""
+	_ = make([]byte, 100, m) // ERROR "non-constant size" ""
 }