cmd/compile: on Wasm and AIX, let deferred nil function panic at invocation

The Go spec requires

	If a deferred function value evaluates to nil, execution
	panics when the function is invoked, not when the "defer"
	statement is executed.

On Wasm and AIX, currently we actually emit a nil check at the
point of defer statement, which will make it panic too early.
This CL fixes this.

Also, on Wasm, now the nil function will be passed through
deferreturn to jmpdefer, which does an explicit nil check and
calls sigpanic if it is nil. This sigpanic, being called from
assembly, is ABI0. So change the assembler backend to also
handle sigpanic in ABI0.

Fixes #34926.
Updates #8047.

Change-Id: I28489a571cee36d2aef041f917b8cfdc31d557d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/201297
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -3779,7 +3779,8 @@ func (s *state) call(n *Node, k callKind) *ssa.Value {
 			break
 		}
 		closure = s.expr(fn)
-		if thearch.LinkArch.Family == sys.Wasm || objabi.GOOS == "aix" && k != callGo {
+		if k != callDefer && k != callDeferStack && (thearch.LinkArch.Family == sys.Wasm || objabi.GOOS == "aix" && k != callGo) {
+			// Deferred nil function needs to panic when the function is invoked, not the point of defer statement.
 			// On AIX, the closure needs to be verified as fn can be nil, except if it's a call go. This needs to be handled by the runtime to have the "go of nil func value" error.
 			// TODO(neelance): On other architectures this should be eliminated by the optimization steps
 			s.nilCheck(closure)

src/cmd/internal/obj/wasm/wasmobj.go

@@ -129,6 +129,7 @@ var (
 	morestackNoCtxt *obj.LSym
 	gcWriteBarrier  *obj.LSym
 	sigpanic        *obj.LSym
+	sigpanic0       *obj.LSym
 	deferreturn     *obj.LSym
 	jmpdefer        *obj.LSym
 )
@@ -143,6 +144,7 @@ func instinit(ctxt *obj.Link) {
 	morestackNoCtxt = ctxt.Lookup("runtime.morestack_noctxt")
 	gcWriteBarrier = ctxt.Lookup("runtime.gcWriteBarrier")
 	sigpanic = ctxt.LookupABI("runtime.sigpanic", obj.ABIInternal)
+	sigpanic0 = ctxt.LookupABI("runtime.sigpanic", 0) // sigpanic called from assembly, which has ABI0
 	deferreturn = ctxt.LookupABI("runtime.deferreturn", obj.ABIInternal)
 	// jmpdefer is defined in assembly as ABI0, but what we're
 	// looking for is the *call* to jmpdefer from the Go function
@@ -491,7 +493,7 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			}
 
 			// return value of call is on the top of the stack, indicating whether to unwind the WebAssembly stack
-			if call.As == ACALLNORESUME && call.To.Sym != sigpanic { // sigpanic unwinds the stack, but it never resumes
+			if call.As == ACALLNORESUME && call.To.Sym != sigpanic && call.To.Sym != sigpanic0 { // sigpanic unwinds the stack, but it never resumes
 				// trying to unwind WebAssembly stack but call has no resume point, terminate with error
 				p = appendp(p, AIf)
 				p = appendp(p, obj.AUNDEF)

test/defernil.go

@@ -0,0 +1,33 @@
+// run
+
+// Copyright 2019 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.
+
+// Check that deferring a nil function causes a proper
+// panic when the deferred function is invoked (not
+// when the function is deferred).
+// See Issue #8047 and #34926.
+
+package main
+
+var x = 0
+
+func main() {
+	defer func() {
+		err := recover()
+		if err == nil {
+			panic("did not panic")
+		}
+		if x != 1 {
+			panic("FAIL")
+		}
+	}()
+	f()
+}
+
+func f() {
+	var nilf func()
+	defer nilf()
+	x = 1
+}