compiler, runtime: call gcWriteBarrier instead of writebarrierptr

In 1.11 writebarrierptr is going away, so change the compiler to call
    gcWriteBarrier instead.  We weren't using gcWriteBarrier before;
    adjust the implementation to use the putFast method.
    
    This revealed a problem in the kickoff function.  When using cgo,
    kickoff can be called on the g0 of an m allocated by newExtraM.  In
    that case the m will generally have a p, but systemstack may be called
    by wbBufFlush as part of flushing the write barrier buffer.  At that
    point the buffer is full, so we can not do a write barrier.  So adjust
    the existing code in kickoff so that in the case where we are g0,
    don't do any write barrier at all.
    
    Reviewed-on: https://go-review.googlesource.com/131395

From-SVN: r264295

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-82d7205ba9e5c1fe38fd24f89a45caf2e974975b
+218c9159635e06e39ae43d0efe1ac1e694fead2e
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/runtime.def

@@ -302,7 +302,7 @@ DEF_GO_RUNTIME(IFACEEFACEEQ, "runtime.ifaceefaceeq", P2(IFACE, EFACE),
 
 
 // Set *dst = src where dst is a pointer to a pointer and src is a pointer.
-DEF_GO_RUNTIME(WRITEBARRIERPTR, "runtime.writebarrierptr",
+DEF_GO_RUNTIME(GCWRITEBARRIER, "runtime.gcWriteBarrier",
 	       P2(POINTER, POINTER), R0())
 
 // Set *dst = *src for an arbitrary type.

gcc/go/gofrontend/wb.cc

@@ -380,7 +380,7 @@ Gogo::propagate_writebarrierrec()
 // This is compatible with the definition in the runtime package.
 //
 // For types that are pointer shared (pointers, maps, chans, funcs),
-// we replaced the call to typedmemmove with writebarrierptr(&A, B).
+// we replaced the call to typedmemmove with gcWriteBarrier(&A, B).
 // As far as the GC is concerned, all pointers are the same, so it
 // doesn't need the type descriptor.
 //
@@ -391,7 +391,7 @@ Gogo::propagate_writebarrierrec()
 // runtime package, so we could optimize by only testing it once
 // between function calls.
 //
-// A slice could be handled with a call to writebarrierptr plus two
+// A slice could be handled with a call to gcWriteBarrier plus two
 // integer moves.
 
 // Traverse the IR adding write barriers.
@@ -824,7 +824,7 @@ Gogo::assign_with_write_barrier(Function* function, Block* enclosing,
     case Type::TYPE_MAP:
     case Type::TYPE_CHANNEL:
       // These types are all represented by a single pointer.
-      call = Runtime::make_call(Runtime::WRITEBARRIERPTR, loc, 2, lhs, rhs);
+      call = Runtime::make_call(Runtime::GCWRITEBARRIER, loc, 2, lhs, rhs);
       break;
 
     case Type::TYPE_STRING:

libgo/go/runtime/mgc_gccgo.go

@@ -11,6 +11,11 @@ import (
 	"unsafe"
 )
 
+// For gccgo, use go:linkname to rename compiler-called functions to
+// themselves, so that the compiler will export them.
+//
+//go:linkname gcWriteBarrier runtime.gcWriteBarrier
+
 // gcRoot is a single GC root: a variable plus a ptrmask.
 //go:notinheap
 type gcRoot struct {
@@ -188,12 +193,7 @@ func checkPreempt() {
 //go:nowritebarrier
 func gcWriteBarrier(dst *uintptr, src uintptr) {
 	buf := &getg().m.p.ptr().wbBuf
-	next := buf.next
-	np := next + 2*sys.PtrSize
-	buf.next = np
-	*(*uintptr)(unsafe.Pointer(next)) = src
-	*(*uintptr)(unsafe.Pointer(next + sys.PtrSize)) = *dst
-	if np >= buf.end {
+	if !buf.putFast(src, *dst) {
 		wbBufFlush(dst, src)
 	}
 	*dst = src

libgo/go/runtime/proc.go

@@ -1146,24 +1146,29 @@ func kickoff() {
 
 	fv := gp.entry
 	param := gp.param
-	gp.entry = nil
 
 	// When running on the g0 stack we can wind up here without a p,
-	// for example from mcall(exitsyscall0) in exitsyscall.
-	// Setting gp.param = nil will call a write barrier, and if
-	// there is no p that write barrier will crash. When called from
-	// mcall the gp.param value will be a *g, which we don't need to
-	// shade since we know it will be kept alive elsewhere. In that
-	// case clear the field using uintptr so that the write barrier
-	// does nothing.
-	if gp.m.p == 0 {
-		if gp == gp.m.g0 && gp.param == unsafe.Pointer(gp.m.curg) {
-			*(*uintptr)(unsafe.Pointer(&gp.param)) = 0
-		} else {
-			throw("no p in kickoff")
-		}
+	// for example from mcall(exitsyscall0) in exitsyscall, in
+	// which case we can not run a write barrier.
+	// It is also possible for us to get here from the systemstack
+	// call in wbBufFlush, at which point the write barrier buffer
+	// is full and we can not run a write barrier.
+	// Setting gp.entry = nil or gp.param = nil will try to run a
+	// write barrier, so if we are on the g0 stack due to mcall
+	// (systemstack calls mcall) then clear the field using uintptr.
+	// This is OK when gp.param is gp.m.curg, as curg will be kept
+	// alive elsewhere, and gp.entry always points into g, or
+	// to a statically allocated value, or (in the case of mcall)
+	// to the stack.
+	if gp == gp.m.g0 && gp.param == unsafe.Pointer(gp.m.curg) {
+		*(*uintptr)(unsafe.Pointer(&gp.entry)) = 0
+		*(*uintptr)(unsafe.Pointer(&gp.param)) = 0
+	} else if gp.m.p == 0 {
+		throw("no p in kickoff")
+	} else {
+		gp.entry = nil
+		gp.param = nil
 	}
-	gp.param = nil
 
 	fv(param)
 	goexit1()