cmd/compile: wrap/desugar defer calls for register abi

Adds code to the compiler's "order" phase to rewrite go and defer
statements to always be argument-less. E.g.

 defer f(x,y)       =>     x1, y1 := x, y
			   defer func() { f(x1, y1) }

This transformation is not beneficial on its own, but it helps
simplify runtime defer handling for the new register ABI (when
invoking deferred functions on the panic path, the runtime doesn't
need to manage the complexity of determining which args to pass in
register vs memory).

This feature is currently enabled by default if GOEXPERIMENT=regabi or
GOEXPERIMENT=regabidefer is in effect.

Included in this CL are some workarounds in the runtime to insure that
"go" statement targets in the runtime are argument-less already (since
wrapping them can potentially introduce heap-allocated closures, which
are currently not allowed). The expectation is that these workarounds
will be temporary, and can go away once we either A) change the rules
about heap-allocated closures, or B) implement some other scheme for
handling go statements.

Change-Id: I01060d79a6b140c6f0838d6e6813f807ccdca319
Reviewed-on: https://go-review.googlesource.com/c/go/+/298669
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 10 insertions, 5 deletions

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 6927e90daa..4b99d755c4 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -207,17 +207,22 @@ func readgogc() int32 {
 	return 100
 }
 
+// Temporary in order to enable register ABI work.
+// TODO(register args): convert back to local chan in gcenabled, passed to "go" stmts.
+var gcenable_setup chan int
+
 // gcenable is called after the bulk of the runtime initialization,
 // just before we're about to start letting user code run.
 // It kicks off the background sweeper goroutine, the background
 // scavenger goroutine, and enables GC.
 func gcenable() {
 	// Kick off sweeping and scavenging.
-	c := make(chan int, 2)
-	go bgsweep(c)
-	go bgscavenge(c)
-	<-c
-	<-c
+	gcenable_setup = make(chan int, 2)
+	go bgsweep()
+	go bgscavenge()
+	<-gcenable_setup
+	<-gcenable_setup
+	gcenable_setup = nil
 	memstats.enablegc = true // now that runtime is initialized, GC is okay
 }