cmd/compile: declare inlined result params early for empty returns

The code for delayed declaration of inlined result parameters only
handles non-empty return statements. This is generally okay, because
we already early declare if there are any (non-blank) named result
parameters.

But if a user writes a function with only blank result parameters and
with exactly one return statement, which is empty, then they could end
up hitting the dreaded "Value live at entry" ICE.

This CL fixes the issue by ensuring we always early declare inlined
result parameters if there are any empty return statements.

Fixes #44355.

Change-Id: I315f3853be436452883b1ce31da1bdffdf24d506
Reviewed-on: https://go-review.googlesource.com/c/go/+/293293
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/inline/inl.go

@@ -852,17 +852,25 @@ func mkinlcall(n *ir.CallExpr, fn *ir.Func, maxCost int32, inlMap map[*ir.Func]b
 		}
 	}
 
+	// We can delay declaring+initializing result parameters if:
+	// (1) there's exactly one "return" statement in the inlined function;
+	// (2) it's not an empty return statement (#44355); and
+	// (3) the result parameters aren't named.
+	delayretvars := true
+
 	nreturns := 0
 	ir.VisitList(ir.Nodes(fn.Inl.Body), func(n ir.Node) {
-		if n != nil && n.Op() == ir.ORETURN {
+		if n, ok := n.(*ir.ReturnStmt); ok {
 			nreturns++
+			if len(n.Results) == 0 {
+				delayretvars = false // empty return statement (case 2)
+			}
 		}
 	})
 
-	// We can delay declaring+initializing result parameters if:
-	// (1) there's only one "return" statement in the inlined
-	// function, and (2) the result parameters aren't named.
-	delayretvars := nreturns == 1
+	if nreturns != 1 {
+		delayretvars = false // not exactly one return statement (case 1)
+	}
 
 	// temporaries for return values.
 	var retvars []ir.Node
@@ -873,7 +881,7 @@ func mkinlcall(n *ir.CallExpr, fn *ir.Func, maxCost int32, inlMap map[*ir.Func]b
 			m = inlvar(n)
 			m = typecheck.Expr(m).(*ir.Name)
 			inlvars[n] = m
-			delayretvars = false // found a named result parameter
+			delayretvars = false // found a named result parameter (case 3)
 		} else {
 			// anonymous return values, synthesize names for use in assignment that replaces return
 			m = retvar(t, i)

test/fixedbugs/issue44355.dir/a.go

@@ -0,0 +1,7 @@
+// Copyright 2021 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 a
+
+func F() (_ *int) { return nil }

test/fixedbugs/issue44355.dir/b.go

@@ -0,0 +1,9 @@
+// Copyright 2021 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 b
+
+import "./a"
+
+var _ = a.F()

test/fixedbugs/issue44355.go

@@ -0,0 +1,7 @@
+// compiledir
+
+// Copyright 2021 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 ignored