cmd/compile: introduce //go:systemstack annotation

//go:systemstack means that the function must run on the system stack.

Add one use in runtime as a demonstration.

Fixes #9174.

Change-Id: I8d4a509cb313541426157da703f1c022e964ace4
Reviewed-on: https://go-review.googlesource.com/10840Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>

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

@@ -654,11 +654,13 @@ var flag_race int
 
 var flag_largemodel int
 
-var noescape bool
-
-var nosplit bool
-
-var nowritebarrier bool
+// Pending annotations for next func declaration.
+var (
+	noescape       bool
+	nosplit        bool
+	nowritebarrier bool
+	systemstack    bool
+)
 
 var debuglive int
 

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

@@ -1392,6 +1392,7 @@ xfndcl:
 		$$.Noescape = noescape;
 		$$.Func.Nosplit = nosplit;
 		$$.Func.Nowritebarrier = nowritebarrier;
+		$$.Func.Systemstack = systemstack;
 		funcbody($$);
 	}
 
@@ -1580,6 +1581,7 @@ xdcl_list:
 		noescape = false
 		nosplit = false
 		nowritebarrier = false
+		systemstack = false
 	}
 
 vardcl_list:

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

@@ -1597,6 +1597,11 @@ func getlinepragma() int {
 			return c
 		}
 
+		if verb == "go:systemstack" {
+			systemstack = true
+			return c
+		}
+
 		if verb == "go:nowritebarrier" {
 			if compiling_runtime == 0 {
 				Yyerror("//go:nowritebarrier only allowed in runtime")

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

@@ -418,6 +418,7 @@ func compile(fn *Node) {
 		nam = nil
 	}
 	ptxt = Thearch.Gins(obj.ATEXT, nam, &nod1)
+	Afunclit(&ptxt.From, Curfn.Func.Nname)
 	ptxt.From3 = new(obj.Addr)
 	if fn.Func.Dupok {
 		ptxt.From3.Offset |= obj.DUPOK
@@ -431,6 +432,9 @@ func compile(fn *Node) {
 	if fn.Func.Nosplit {
 		ptxt.From3.Offset |= obj.NOSPLIT
 	}
+	if fn.Func.Systemstack {
+		ptxt.From.Sym.Cfunc = 1
+	}
 
 	// Clumsy but important.
 	// See test/recover.go for test cases and src/reflect/value.go
@@ -441,8 +445,6 @@ func compile(fn *Node) {
 		}
 	}
 
-	Afunclit(&ptxt.From, Curfn.Func.Nname)
-
 	ginit()
 
 	gcargs = makefuncdatasym("gcargs·%d", obj.FUNCDATA_ArgsPointerMaps)

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

@@ -174,6 +174,7 @@ type Func struct {
 	Dupok          bool // duplicate definitions ok
 	Wrapper        bool // is method wrapper
 	Needctxt       bool // function uses context register (has closure variables)
+	Systemstack    bool // must run on system stack
 }
 
 // Node ops.

src/runtime/malloc.go

@@ -797,6 +797,17 @@ var globalAlloc struct {
 // Intended for things like function/type/debug-related persistent data.
 // If align is 0, uses default align (currently 8).
 func persistentalloc(size, align uintptr, sysStat *uint64) unsafe.Pointer {
+	var p unsafe.Pointer
+	systemstack(func() {
+		p = persistentalloc1(size, align, sysStat)
+	})
+	return p
+}
+
+// Must run on system stack because stack growth can (re)invoke it.
+// See issue 9174.
+//go:systemstack
+func persistentalloc1(size, align uintptr, sysStat *uint64) unsafe.Pointer {
 	const (
 		chunk    = 256 << 10
 		maxBlock = 64 << 10 // VM reservation granularity is 64K on windows