cmd, runtime: eliminate runtime.no_pointers_stackmap

runtime.no_pointers_stackmap is an odd beast.
It is defined in a Go file, populated by assembly,
used by the GC, and its address is magic used
by async pre-emption to ascertain whether a
routine was implemented in assembly.

A subsequent change will force all GC data into the go.func.* linker symbol.
runtime.no_pointers_stackmap is GC data, so it must go there.
Yet it also needs to go into rodata, for the runtime address trick.

This change eliminates it entirely.

Replace the runtime address check with the newly introduced asm funcflag.

Handle the assembly macro as magic, similarly to our handling of go_args_stackmap.
This allows the no_pointers_stackmap to be identical in all ways
to other gclocals stackmaps, including content-addressability.

Change-Id: Id2f20a262cfab0719beb88e6342984ec4b196268
Reviewed-on: https://go-review.googlesource.com/c/go/+/353672
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

4 files changed, 22 insertions, 19 deletions

diff --git a/src/cmd/internal/obj/plist.go b/src/cmd/internal/obj/plist.go
index 348a16356e..e5bbdd51a7 100644
--- a/src/cmd/internal/obj/plist.go
+++ b/src/cmd/internal/obj/plist.go
@@ -54,11 +54,28 @@ func Flushplist(ctxt *Link, plist *Plist, newprog ProgAlloc, myimportpath string
 			if curtext == nil { // func _() {}
 				continue
 			}
-			if p.To.Sym.Name == "go_args_stackmap" {
+			switch p.To.Sym.Name {
+			case "go_args_stackmap":
 				if p.From.Type != TYPE_CONST || p.From.Offset != objabi.FUNCDATA_ArgsPointerMaps {
 					ctxt.Diag("FUNCDATA use of go_args_stackmap(SB) without FUNCDATA_ArgsPointerMaps")
 				}
 				p.To.Sym = ctxt.LookupDerived(curtext, curtext.Name+".args_stackmap")
+			case "no_pointers_stackmap":
+				if p.From.Type != TYPE_CONST || p.From.Offset != objabi.FUNCDATA_LocalsPointerMaps {
+					ctxt.Diag("FUNCDATA use of no_pointers_stackmap(SB) without FUNCDATA_LocalsPointerMaps")
+				}
+				// funcdata for functions with no local variables in frame.
+				// Define two zero-length bitmaps, because the same index is used
+				// for the local variables as for the argument frame, and assembly
+				// frames have two argument bitmaps, one without results and one with results.
+				// Write []uint32{2, 0}.
+				b := make([]byte, 8)
+				ctxt.Arch.ByteOrder.PutUint32(b, 2)
+				s := ctxt.GCLocalsSym(b)
+				if !s.OnList() {
+					ctxt.Globl(s, int64(len(s.P)), int(RODATA|DUPOK))
+				}
+				p.To.Sym = s
 			}
 
 		}
diff --git a/src/runtime/asm.s b/src/runtime/asm.s
index 0e14fcd3e6..84d56de7dd 100644
--- a/src/runtime/asm.s
+++ b/src/runtime/asm.s
@@ -4,14 +4,6 @@
 
 #include "textflag.h"
 
-// funcdata for functions with no local variables in frame.
-// Define two zero-length bitmaps, because the same index is used
-// for the local variables as for the argument frame, and assembly
-// frames have two argument bitmaps, one without results and one with results.
-DATA runtime·no_pointers_stackmap+0x00(SB)/4, $2
-DATA runtime·no_pointers_stackmap+0x04(SB)/4, $0
-GLOBL runtime·no_pointers_stackmap(SB),RODATA, $8
-
 #ifndef GOARCH_amd64
 TEXT ·sigpanic0(SB),NOSPLIT,$0-0
 	JMP	·sigpanic<ABIInternal>(SB)
diff --git a/src/runtime/funcdata.h b/src/runtime/funcdata.h
index 1002b181e4..15f1b5c9a1 100644
--- a/src/runtime/funcdata.h
+++ b/src/runtime/funcdata.h
@@ -44,7 +44,7 @@
 
 // NO_LOCAL_POINTERS indicates that the assembly function stores
 // no pointers to heap objects in its local stack variables.
-#define NO_LOCAL_POINTERS	FUNCDATA $FUNCDATA_LocalsPointerMaps, runtime·no_pointers_stackmap(SB)
+#define NO_LOCAL_POINTERS	FUNCDATA $FUNCDATA_LocalsPointerMaps, no_pointers_stackmap(SB)
 
 // ArgsSizeUnknown is set in Func.argsize to mark all functions
 // whose argument size is unknown (C vararg functions, and
diff --git a/src/runtime/preempt.go b/src/runtime/preempt.go
index 18566a7459..da24f5042c 100644
--- a/src/runtime/preempt.go
+++ b/src/runtime/preempt.go
@@ -56,7 +56,6 @@ import (
 	"internal/abi"
 	"internal/goarch"
 	"runtime/internal/atomic"
-	"unsafe"
 )
 
 type suspendGState struct {
@@ -405,12 +404,9 @@ func isAsyncSafePoint(gp *g, pc, sp, lr uintptr) (bool, uintptr) {
 		// functions (except at calls).
 		return false, 0
 	}
-	if fd := funcdata(f, _FUNCDATA_LocalsPointerMaps); fd == nil || fd == unsafe.Pointer(&no_pointers_stackmap) {
-		// This is assembly code. Don't assume it's
-		// well-formed. We identify assembly code by
-		// checking that it has either no stack map, or
-		// no_pointers_stackmap, which is the stack map
-		// for ones marked as NO_LOCAL_POINTERS.
+	if fd := funcdata(f, _FUNCDATA_LocalsPointerMaps); fd == nil || f.flag&funcFlag_ASM != 0 {
+		// This is assembly code. Don't assume it's well-formed.
+		// TODO: Empirically we still need the fd == nil check. Why?
 		//
 		// TODO: Are there cases that are safe but don't have a
 		// locals pointer map, like empty frame functions?
@@ -455,5 +451,3 @@ func isAsyncSafePoint(gp *g, pc, sp, lr uintptr) (bool, uintptr) {
 	}
 	return true, pc
 }
-
-var no_pointers_stackmap uint64 // defined in assembly, for NO_LOCAL_POINTERS macro