runtime, sycall/js: add support for callbacks from JavaScript

This commit adds support for JavaScript callbacks back into
WebAssembly. This is experimental API, just like the rest of the
syscall/js package. The time package now also uses this mechanism
to properly support timers without resorting to a busy loop.

JavaScript code can call into the same entry point multiple times.
The new RUN register is used to keep track of the program's
run state. Possible values are: starting, running, paused and exited.
If no goroutine is ready any more, the scheduler can put the
program into the "paused" state and the WebAssembly code will
stop running. When a callback occurs, the JavaScript code puts
the callback data into a queue and then calls into WebAssembly
to allow the Go code to continue running.

Updates #18892
Updates #25506

Change-Id: Ib8701cfa0536d10d69bd541c85b0e2a754eb54fb
Reviewed-on: https://go-review.googlesource.com/114197
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

7 files changed, 29 insertions, 4 deletions

diff --git a/src/cmd/internal/obj/wasm/a.out.go b/src/cmd/internal/obj/wasm/a.out.go
index 9c04be2609..6f882215ff 100644
--- a/src/cmd/internal/obj/wasm/a.out.go
+++ b/src/cmd/internal/obj/wasm/a.out.go
@@ -219,6 +219,8 @@ const (
 	// However, it is not allowed to switch goroutines while inside of an ACALLNORESUME call.
 	ACALLNORESUME
 
+	ARETUNWIND
+
 	AMOVB
 	AMOVH
 	AMOVW
@@ -244,6 +246,7 @@ const (
 	REG_RET1
 	REG_RET2
 	REG_RET3
+	REG_RUN
 
 	// locals
 	REG_R0
diff --git a/src/cmd/internal/obj/wasm/anames.go b/src/cmd/internal/obj/wasm/anames.go
index 20d04446d0..745f0d773a 100644
--- a/src/cmd/internal/obj/wasm/anames.go
+++ b/src/cmd/internal/obj/wasm/anames.go
@@ -180,6 +180,7 @@ var Anames = []string{
 	"F64ReinterpretI64",
 	"RESUMEPOINT",
 	"CALLNORESUME",
+	"RETUNWIND",
 	"MOVB",
 	"MOVH",
 	"MOVW",
diff --git a/src/cmd/internal/obj/wasm/wasmobj.go b/src/cmd/internal/obj/wasm/wasmobj.go
index ca09b3fa0b..8498b40724 100644
--- a/src/cmd/internal/obj/wasm/wasmobj.go
+++ b/src/cmd/internal/obj/wasm/wasmobj.go
@@ -25,6 +25,7 @@ var Register = map[string]int16{
 	"RET1": REG_RET1,
 	"RET2": REG_RET2,
 	"RET3": REG_RET3,
+	"RUN":  REG_RUN,
 
 	"R0":  REG_R0,
 	"R1":  REG_R1,
@@ -487,7 +488,7 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 				p = appendp(p, AEnd) // end of Loop
 			}
 
-		case obj.ARET:
+		case obj.ARET, ARETUNWIND:
 			ret := *p
 			p.As = obj.ANOP
 
@@ -528,7 +529,14 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			p = appendp(p, AI32Add)
 			p = appendp(p, ASet, regAddr(REG_SP))
 
-			// not switching goroutine, return 0
+			if ret.As == ARETUNWIND {
+				// function needs to unwind the WebAssembly stack, return 1
+				p = appendp(p, AI32Const, constAddr(1))
+				p = appendp(p, AReturn)
+				break
+			}
+
+			// not unwinding the WebAssembly stack, return 0
 			p = appendp(p, AI32Const, constAddr(0))
 			p = appendp(p, AReturn)
 		}
@@ -726,7 +734,7 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			}
 			reg := p.From.Reg
 			switch {
-			case reg >= REG_PC_F && reg <= REG_RET3:
+			case reg >= REG_PC_F && reg <= REG_RUN:
 				w.WriteByte(0x23) // get_global
 				writeUleb128(w, uint64(reg-REG_PC_F))
 			case reg >= REG_R0 && reg <= REG_F15:
@@ -743,7 +751,7 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			}
 			reg := p.To.Reg
 			switch {
-			case reg >= REG_PC_F && reg <= REG_RET3:
+			case reg >= REG_PC_F && reg <= REG_RUN:
 				w.WriteByte(0x24) // set_global
 				writeUleb128(w, uint64(reg-REG_PC_F))
 			case reg >= REG_R0 && reg <= REG_F15:
diff --git a/src/cmd/link/internal/wasm/asm.go b/src/cmd/link/internal/wasm/asm.go
index aadb0c3b6e..b7beaa5d2f 100644
--- a/src/cmd/link/internal/wasm/asm.go
+++ b/src/cmd/link/internal/wasm/asm.go
@@ -304,6 +304,7 @@ func writeGlobalSec(ctxt *ld.Link) {
 		I64, // 6: RET1
 		I64, // 7: RET2
 		I64, // 8: RET3
+		I32, // 9: RUN
 	}
 
 	writeUleb128(ctxt.Out, uint64(len(globalRegs))) // number of globals
diff --git a/src/cmd/trace/annotations.go b/src/cmd/trace/annotations.go
index c91f18ef6f..96c109e0f2 100644
--- a/src/cmd/trace/annotations.go
+++ b/src/cmd/trace/annotations.go
@@ -1,3 +1,7 @@
+// Copyright 2018 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 main
 
 import (
diff --git a/src/cmd/trace/annotations_test.go b/src/cmd/trace/annotations_test.go
index 5d2b226b35..a9068d53c1 100644
--- a/src/cmd/trace/annotations_test.go
+++ b/src/cmd/trace/annotations_test.go
@@ -1,3 +1,9 @@
+// Copyright 2018 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.
+
+// +build !js
+
 package main
 
 import (
diff --git a/src/cmd/trace/trace_test.go b/src/cmd/trace/trace_test.go
index 852d745b29..9e90f50d4b 100644
--- a/src/cmd/trace/trace_test.go
+++ b/src/cmd/trace/trace_test.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !js
+
 package main
 
 import (