cmd/asm, cmd/link, runtime: introduce FuncInfo flag bits

The runtime traceback code has its own definition of which functions
mark the top frame of a stack, separate from the TOPFRAME bits that
exist in the assembly and are passed along in DWARF information.
It's error-prone and redundant to have two different sources of truth.
This CL provides the actual TOPFRAME bits to the runtime, so that
the runtime can use those bits instead of reinventing its own category.

This CL also adds a new bit, SPWRITE, which marks functions that
write directly to SP (anything but adding and subtracting constants).
Such functions must stop a traceback, because the traceback has no
way to rederive the SP on entry. Again, the runtime has its own definition
which is mostly correct, but also missing some functions. During ordinary
goroutine context switches, such functions do not appear on the stack,
so the incompleteness in the runtime usually doesn't matter.
But profiling signals can arrive at any moment, and the runtime may
crash during traceback if it attempts to unwind an SP-writing frame
and gets out-of-sync with the actual stack. The runtime contains code
to try to detect likely candidates but again it is incomplete.
Deriving the SPWRITE bit automatically from the actual assembly code
provides the complete truth, and passing it to the runtime lets the
runtime use it.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I227f53b23ac5b3dabfcc5e8ee3f00df4e113cf58
Reviewed-on: https://go-review.googlesource.com/c/go/+/288800
Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>

14 files changed, 165 insertions, 66 deletions

diff --git a/src/cmd/internal/goobj/funcinfo.go b/src/cmd/internal/goobj/funcinfo.go
index 2cca8f6c4e..6d33a10a51 100644
--- a/src/cmd/internal/goobj/funcinfo.go
+++ b/src/cmd/internal/goobj/funcinfo.go
@@ -19,9 +19,10 @@ type CUFileIndex uint32
 //
 // TODO: make each pcdata a separate symbol?
 type FuncInfo struct {
-	Args   uint32
-	Locals uint32
-	FuncID objabi.FuncID
+	Args     uint32
+	Locals   uint32
+	FuncID   objabi.FuncID
+	FuncFlag objabi.FuncFlag
 
 	Pcsp        SymRef
 	Pcfile      SymRef
@@ -35,6 +36,9 @@ type FuncInfo struct {
 }
 
 func (a *FuncInfo) Write(w *bytes.Buffer) {
+	writeUint8 := func(x uint8) {
+		w.WriteByte(x)
+	}
 	var b [4]byte
 	writeUint32 := func(x uint32) {
 		binary.LittleEndian.PutUint32(b[:], x)
@@ -47,8 +51,10 @@ func (a *FuncInfo) Write(w *bytes.Buffer) {
 
 	writeUint32(a.Args)
 	writeUint32(a.Locals)
-	writeUint32(uint32(a.FuncID))
-
+	writeUint8(uint8(a.FuncID))
+	writeUint8(uint8(a.FuncFlag))
+	writeUint8(0) // pad to uint32 boundary
+	writeUint8(0)
 	writeSymRef(a.Pcsp)
 	writeSymRef(a.Pcfile)
 	writeSymRef(a.Pcline)
@@ -72,46 +78,6 @@ func (a *FuncInfo) Write(w *bytes.Buffer) {
 	}
 }
 
-func (a *FuncInfo) Read(b []byte) {
-	readUint32 := func() uint32 {
-		x := binary.LittleEndian.Uint32(b)
-		b = b[4:]
-		return x
-	}
-	readSymIdx := func() SymRef {
-		return SymRef{readUint32(), readUint32()}
-	}
-
-	a.Args = readUint32()
-	a.Locals = readUint32()
-	a.FuncID = objabi.FuncID(readUint32())
-
-	a.Pcsp = readSymIdx()
-	a.Pcfile = readSymIdx()
-	a.Pcline = readSymIdx()
-	a.Pcinline = readSymIdx()
-	a.Pcdata = make([]SymRef, readUint32())
-	for i := range a.Pcdata {
-		a.Pcdata[i] = readSymIdx()
-	}
-
-	funcdataofflen := readUint32()
-	a.Funcdataoff = make([]uint32, funcdataofflen)
-	for i := range a.Funcdataoff {
-		a.Funcdataoff[i] = readUint32()
-	}
-	filelen := readUint32()
-	a.File = make([]CUFileIndex, filelen)
-	for i := range a.File {
-		a.File[i] = CUFileIndex(readUint32())
-	}
-	inltreelen := readUint32()
-	a.InlTree = make([]InlTreeNode, inltreelen)
-	for i := range a.InlTree {
-		b = a.InlTree[i].Read(b)
-	}
-}
-
 // FuncInfoLengths is a cache containing a roadmap of offsets and
 // lengths for things within a serialized FuncInfo. Each length field
 // stores the number of items (e.g. files, inltree nodes, etc), and the
@@ -159,7 +125,9 @@ func (*FuncInfo) ReadArgs(b []byte) uint32 { return binary.LittleEndian.Uint32(b
 
 func (*FuncInfo) ReadLocals(b []byte) uint32 { return binary.LittleEndian.Uint32(b[4:]) }
 
-func (*FuncInfo) ReadFuncID(b []byte) uint32 { return binary.LittleEndian.Uint32(b[8:]) }
+func (*FuncInfo) ReadFuncID(b []byte) objabi.FuncID { return objabi.FuncID(b[8]) }
+
+func (*FuncInfo) ReadFuncFlag(b []byte) objabi.FuncFlag { return objabi.FuncFlag(b[9]) }
 
 func (*FuncInfo) ReadPcsp(b []byte) SymRef {
 	return SymRef{binary.LittleEndian.Uint32(b[12:]), binary.LittleEndian.Uint32(b[16:])}
diff --git a/src/cmd/internal/goobj/objfile.go b/src/cmd/internal/goobj/objfile.go
index e6447e455d..d1b838f676 100644
--- a/src/cmd/internal/goobj/objfile.go
+++ b/src/cmd/internal/goobj/objfile.go
@@ -298,7 +298,6 @@ const (
 	SymFlagNoSplit
 	SymFlagReflectMethod
 	SymFlagGoType
-	SymFlagTopFrame
 )
 
 // Sym.Flag2
@@ -332,7 +331,6 @@ func (s *Sym) Leaf() bool          { return s.Flag()&SymFlagLeaf != 0 }
 func (s *Sym) NoSplit() bool       { return s.Flag()&SymFlagNoSplit != 0 }
 func (s *Sym) ReflectMethod() bool { return s.Flag()&SymFlagReflectMethod != 0 }
 func (s *Sym) IsGoType() bool      { return s.Flag()&SymFlagGoType != 0 }
-func (s *Sym) TopFrame() bool      { return s.Flag()&SymFlagTopFrame != 0 }
 func (s *Sym) UsedInIface() bool   { return s.Flag2()&SymFlagUsedInIface != 0 }
 func (s *Sym) IsItab() bool        { return s.Flag2()&SymFlagItab != 0 }
 
diff --git a/src/cmd/internal/obj/arm/obj5.go b/src/cmd/internal/obj/arm/obj5.go
index 29d3a5867d..7de04302d9 100644
--- a/src/cmd/internal/obj/arm/obj5.go
+++ b/src/cmd/internal/obj/arm/obj5.go
@@ -34,6 +34,7 @@ import (
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
+	"log"
 )
 
 var progedit_tlsfallback *obj.LSym
@@ -613,6 +614,21 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				p.From.Reg = REGSP
 			}
 		}
+
+		if p.To.Type == obj.TYPE_REG && p.To.Reg == REGSP && p.Spadj == 0 {
+			f := c.cursym.Func()
+			if f.FuncFlag&objabi.FuncFlag_SPWRITE == 0 {
+				c.cursym.Func().FuncFlag |= objabi.FuncFlag_SPWRITE
+				if ctxt.Debugvlog || !ctxt.IsAsm {
+					ctxt.Logf("auto-SPWRITE: %s %v\n", c.cursym.Name, p)
+					if !ctxt.IsAsm {
+						ctxt.Diag("invalid auto-SPWRITE in non-assembly")
+						ctxt.DiagFlush()
+						log.Fatalf("bad SPWRITE")
+					}
+				}
+			}
+		}
 	}
 }
 
diff --git a/src/cmd/internal/obj/arm64/obj7.go b/src/cmd/internal/obj/arm64/obj7.go
index 0baf51973a..3b88543852 100644
--- a/src/cmd/internal/obj/arm64/obj7.go
+++ b/src/cmd/internal/obj/arm64/obj7.go
@@ -35,6 +35,7 @@ import (
 	"cmd/internal/objabi"
 	"cmd/internal/src"
 	"cmd/internal/sys"
+	"log"
 	"math"
 )
 
@@ -970,6 +971,21 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				p = q5
 			}
 		}
+
+		if p.To.Type == obj.TYPE_REG && p.To.Reg == REGSP && p.Spadj == 0 {
+			f := c.cursym.Func()
+			if f.FuncFlag&objabi.FuncFlag_SPWRITE == 0 {
+				c.cursym.Func().FuncFlag |= objabi.FuncFlag_SPWRITE
+				if ctxt.Debugvlog || !ctxt.IsAsm {
+					ctxt.Logf("auto-SPWRITE: %s %v\n", c.cursym.Name, p)
+					if !ctxt.IsAsm {
+						ctxt.Diag("invalid auto-SPWRITE in non-assembly")
+						ctxt.DiagFlush()
+						log.Fatalf("bad SPWRITE")
+					}
+				}
+			}
+		}
 	}
 }
 
diff --git a/src/cmd/internal/obj/link.go b/src/cmd/internal/obj/link.go
index 8206902328..a48db3bdc8 100644
--- a/src/cmd/internal/obj/link.go
+++ b/src/cmd/internal/obj/link.go
@@ -454,6 +454,7 @@ type FuncInfo struct {
 	Locals   int32
 	Align    int32
 	FuncID   objabi.FuncID
+	FuncFlag objabi.FuncFlag
 	Text     *Prog
 	Autot    map[*LSym]struct{}
 	Pcln     Pcln
@@ -619,10 +620,6 @@ const (
 	// target of an inline during compilation
 	AttrWasInlined
 
-	// TopFrame means that this function is an entry point and unwinders should not
-	// keep unwinding beyond this frame.
-	AttrTopFrame
-
 	// Indexed indicates this symbol has been assigned with an index (when using the
 	// new object file format).
 	AttrIndexed
@@ -663,7 +660,6 @@ func (a *Attribute) NeedCtxt() bool           { return a.load()&AttrNeedCtxt !=
 func (a *Attribute) NoFrame() bool            { return a.load()&AttrNoFrame != 0 }
 func (a *Attribute) Static() bool             { return a.load()&AttrStatic != 0 }
 func (a *Attribute) WasInlined() bool         { return a.load()&AttrWasInlined != 0 }
-func (a *Attribute) TopFrame() bool           { return a.load()&AttrTopFrame != 0 }
 func (a *Attribute) Indexed() bool            { return a.load()&AttrIndexed != 0 }
 func (a *Attribute) UsedInIface() bool        { return a.load()&AttrUsedInIface != 0 }
 func (a *Attribute) ContentAddressable() bool { return a.load()&AttrContentAddressable != 0 }
@@ -713,14 +709,13 @@ var textAttrStrings = [...]struct {
 	{bit: AttrNoFrame, s: "NOFRAME"},
 	{bit: AttrStatic, s: "STATIC"},
 	{bit: AttrWasInlined, s: ""},
-	{bit: AttrTopFrame, s: "TOPFRAME"},
 	{bit: AttrIndexed, s: ""},
 	{bit: AttrContentAddressable, s: ""},
 	{bit: AttrABIWrapper, s: "ABIWRAPPER"},
 }
 
-// TextAttrString formats a for printing in as part of a TEXT prog.
-func (a Attribute) TextAttrString() string {
+// String formats a for printing in as part of a TEXT prog.
+func (a Attribute) String() string {
 	var s string
 	for _, x := range textAttrStrings {
 		if a&x.bit != 0 {
@@ -746,6 +741,18 @@ func (a Attribute) TextAttrString() string {
 	return s
 }
 
+// TextAttrString formats the symbol attributes for printing in as part of a TEXT prog.
+func (s *LSym) TextAttrString() string {
+	attr := s.Attribute.String()
+	if s.Func().FuncFlag&objabi.FuncFlag_TOPFRAME != 0 {
+		if attr != "" {
+			attr += "|"
+		}
+		attr += "TOPFRAME"
+	}
+	return attr
+}
+
 func (s *LSym) String() string {
 	return s.Name
 }
diff --git a/src/cmd/internal/obj/mips/obj0.go b/src/cmd/internal/obj/mips/obj0.go
index 135a8df3aa..91bba90d41 100644
--- a/src/cmd/internal/obj/mips/obj0.go
+++ b/src/cmd/internal/obj/mips/obj0.go
@@ -35,6 +35,7 @@ import (
 	"cmd/internal/sys"
 	"encoding/binary"
 	"fmt"
+	"log"
 	"math"
 )
 
@@ -536,6 +537,21 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				p.From.Reg = REGSP
 			}
 		}
+
+		if p.To.Type == obj.TYPE_REG && p.To.Reg == REGSP && p.Spadj == 0 {
+			f := c.cursym.Func()
+			if f.FuncFlag&objabi.FuncFlag_SPWRITE == 0 {
+				c.cursym.Func().FuncFlag |= objabi.FuncFlag_SPWRITE
+				if ctxt.Debugvlog || !ctxt.IsAsm {
+					ctxt.Logf("auto-SPWRITE: %s %v\n", c.cursym.Name, p)
+					if !ctxt.IsAsm {
+						ctxt.Diag("invalid auto-SPWRITE in non-assembly")
+						ctxt.DiagFlush()
+						log.Fatalf("bad SPWRITE")
+					}
+				}
+			}
+		}
 	}
 
 	if c.ctxt.Arch.Family == sys.MIPS {
diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index bb58b4f0c2..85f0570e5d 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -330,9 +330,6 @@ func (w *writer) Sym(s *LSym) {
 	if s.ReflectMethod() {
 		flag |= goobj.SymFlagReflectMethod
 	}
-	if s.TopFrame() {
-		flag |= goobj.SymFlagTopFrame
-	}
 	if strings.HasPrefix(s.Name, "type.") && s.Name[5] != '.' && s.Type == objabi.SRODATA {
 		flag |= goobj.SymFlagGoType
 	}
@@ -673,9 +670,10 @@ func genFuncInfoSyms(ctxt *Link) {
 			continue
 		}
 		o := goobj.FuncInfo{
-			Args:   uint32(fn.Args),
-			Locals: uint32(fn.Locals),
-			FuncID: objabi.FuncID(fn.FuncID),
+			Args:     uint32(fn.Args),
+			Locals:   uint32(fn.Locals),
+			FuncID:   fn.FuncID,
+			FuncFlag: fn.FuncFlag,
 		}
 		pc := &fn.Pcln
 		o.Pcsp = makeSymRef(preparePcSym(pc.Pcsp))
@@ -788,7 +786,7 @@ func (ctxt *Link) writeSymDebugNamed(s *LSym, name string) {
 	if s.NoSplit() {
 		fmt.Fprintf(ctxt.Bso, "nosplit ")
 	}
-	if s.TopFrame() {
+	if s.Func() != nil && s.Func().FuncFlag&objabi.FuncFlag_TOPFRAME != 0 {
 		fmt.Fprintf(ctxt.Bso, "topframe ")
 	}
 	fmt.Fprintf(ctxt.Bso, "size=%d", s.Size)
diff --git a/src/cmd/internal/obj/plist.go b/src/cmd/internal/obj/plist.go
index 679ce7eb8f..177083261c 100644
--- a/src/cmd/internal/obj/plist.go
+++ b/src/cmd/internal/obj/plist.go
@@ -134,6 +134,7 @@ func (ctxt *Link) InitTextSym(s *LSym, flag int) {
 	}
 	name := strings.Replace(s.Name, "\"\"", ctxt.Pkgpath, -1)
 	s.Func().FuncID = objabi.GetFuncID(name, flag&WRAPPER != 0)
+	s.Func().FuncFlag = toFuncFlag(flag)
 	s.Set(AttrOnList, true)
 	s.Set(AttrDuplicateOK, flag&DUPOK != 0)
 	s.Set(AttrNoSplit, flag&NOSPLIT != 0)
@@ -142,7 +143,6 @@ func (ctxt *Link) InitTextSym(s *LSym, flag int) {
 	s.Set(AttrABIWrapper, flag&ABIWRAPPER != 0)
 	s.Set(AttrNeedCtxt, flag&NEEDCTXT != 0)
 	s.Set(AttrNoFrame, flag&NOFRAME != 0)
-	s.Set(AttrTopFrame, flag&TOPFRAME != 0)
 	s.Type = objabi.STEXT
 	ctxt.Text = append(ctxt.Text, s)
 
@@ -150,6 +150,14 @@ func (ctxt *Link) InitTextSym(s *LSym, flag int) {
 	ctxt.dwarfSym(s)
 }
 
+func toFuncFlag(flag int) objabi.FuncFlag {
+	var out objabi.FuncFlag
+	if flag&TOPFRAME != 0 {
+		out |= objabi.FuncFlag_TOPFRAME
+	}
+	return out
+}
+
 func (ctxt *Link) Globl(s *LSym, size int64, flag int) {
 	if s.OnList() {
 		ctxt.Diag("symbol %s listed multiple times", s.Name)
diff --git a/src/cmd/internal/obj/ppc64/obj9.go b/src/cmd/internal/obj/ppc64/obj9.go
index fddf552156..a77be29cf0 100644
--- a/src/cmd/internal/obj/ppc64/obj9.go
+++ b/src/cmd/internal/obj/ppc64/obj9.go
@@ -34,6 +34,7 @@ import (
 	"cmd/internal/objabi"
 	"cmd/internal/src"
 	"cmd/internal/sys"
+	"log"
 )
 
 func progedit(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
@@ -984,6 +985,21 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				p.From.Reg = REGSP
 			}
 		}
+
+		if p.To.Type == obj.TYPE_REG && p.To.Reg == REGSP && p.Spadj == 0 && p.As != ACMPU {
+			f := c.cursym.Func()
+			if f.FuncFlag&objabi.FuncFlag_SPWRITE == 0 {
+				c.cursym.Func().FuncFlag |= objabi.FuncFlag_SPWRITE
+				if ctxt.Debugvlog || !ctxt.IsAsm {
+					ctxt.Logf("auto-SPWRITE: %s %v\n", c.cursym.Name, p)
+					if !ctxt.IsAsm {
+						ctxt.Diag("invalid auto-SPWRITE in non-assembly")
+						ctxt.DiagFlush()
+						log.Fatalf("bad SPWRITE")
+					}
+				}
+			}
+		}
 	}
 }
 
diff --git a/src/cmd/internal/obj/riscv/obj.go b/src/cmd/internal/obj/riscv/obj.go
index 5ef334dd6a..d104f1cfa5 100644
--- a/src/cmd/internal/obj/riscv/obj.go
+++ b/src/cmd/internal/obj/riscv/obj.go
@@ -25,6 +25,7 @@ import (
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
 	"fmt"
+	"log"
 )
 
 func buildop(ctxt *obj.Link) {}
@@ -716,6 +717,21 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				p.Spadj = int32(-p.From.Offset)
 			}
 		}
+
+		if p.To.Type == obj.TYPE_REG && p.To.Reg == REGSP && p.Spadj == 0 {
+			f := cursym.Func()
+			if f.FuncFlag&objabi.FuncFlag_SPWRITE == 0 {
+				f.FuncFlag |= objabi.FuncFlag_SPWRITE
+				if ctxt.Debugvlog || !ctxt.IsAsm {
+					ctxt.Logf("auto-SPWRITE: %s %v\n", cursym.Name, p)
+					if !ctxt.IsAsm {
+						ctxt.Diag("invalid auto-SPWRITE in non-assembly")
+						ctxt.DiagFlush()
+						log.Fatalf("bad SPWRITE")
+					}
+				}
+			}
+		}
 	}
 
 	// Rewrite MOV pseudo-instructions. This cannot be done in
diff --git a/src/cmd/internal/obj/s390x/objz.go b/src/cmd/internal/obj/s390x/objz.go
index 970cf827d6..a02c4fc17f 100644
--- a/src/cmd/internal/obj/s390x/objz.go
+++ b/src/cmd/internal/obj/s390x/objz.go
@@ -33,6 +33,7 @@ import (
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
+	"log"
 	"math"
 )
 
@@ -545,6 +546,21 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				p.From.Reg = REGSP
 			}
 		}
+
+		if p.To.Type == obj.TYPE_REG && p.To.Reg == REGSP && p.Spadj == 0 {
+			f := c.cursym.Func()
+			if f.FuncFlag&objabi.FuncFlag_SPWRITE == 0 {
+				c.cursym.Func().FuncFlag |= objabi.FuncFlag_SPWRITE
+				if ctxt.Debugvlog || !ctxt.IsAsm {
+					ctxt.Logf("auto-SPWRITE: %s\n", c.cursym.Name)
+					if !ctxt.IsAsm {
+						ctxt.Diag("invalid auto-SPWRITE in non-assembly")
+						ctxt.DiagFlush()
+						log.Fatalf("bad SPWRITE")
+					}
+				}
+			}
+		}
 	}
 	if wasSplit {
 		c.stacksplitPost(pLast, pPre, pPreempt, autosize) // emit post part of split check
diff --git a/src/cmd/internal/obj/util.go b/src/cmd/internal/obj/util.go
index b9bacb7a22..1c34b4e833 100644
--- a/src/cmd/internal/obj/util.go
+++ b/src/cmd/internal/obj/util.go
@@ -187,7 +187,7 @@ func (p *Prog) WriteInstructionString(w io.Writer) {
 		// In short, print one of these two:
 		// TEXT	foo(SB), DUPOK|NOSPLIT, $0
 		// TEXT	foo(SB), $0
-		s := p.From.Sym.Attribute.TextAttrString()
+		s := p.From.Sym.TextAttrString()
 		if s != "" {
 			fmt.Fprintf(w, "%s%s", sep, s)
 			sep = ", "
diff --git a/src/cmd/internal/obj/x86/obj6.go b/src/cmd/internal/obj/x86/obj6.go
index 84de58a4c4..bc3a3b4bbe 100644
--- a/src/cmd/internal/obj/x86/obj6.go
+++ b/src/cmd/internal/obj/x86/obj6.go
@@ -35,6 +35,7 @@ import (
 	"cmd/internal/objabi"
 	"cmd/internal/src"
 	"cmd/internal/sys"
+	"log"
 	"math"
 	"strings"
 )
@@ -839,6 +840,20 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 
 		switch p.As {
 		default:
+			if p.To.Type == obj.TYPE_REG && p.To.Reg == REG_SP && p.As != ACMPL && p.As != ACMPQ {
+				f := cursym.Func()
+				if f.FuncFlag&objabi.FuncFlag_SPWRITE == 0 {
+					f.FuncFlag |= objabi.FuncFlag_SPWRITE
+					if ctxt.Debugvlog || !ctxt.IsAsm {
+						ctxt.Logf("auto-SPWRITE: %s %v\n", cursym.Name, p)
+						if !ctxt.IsAsm {
+							ctxt.Diag("invalid auto-SPWRITE in non-assembly")
+							ctxt.DiagFlush()
+							log.Fatalf("bad SPWRITE")
+						}
+					}
+				}
+			}
 			continue
 
 		case APUSHL, APUSHFL:
diff --git a/src/cmd/internal/objabi/funcid.go b/src/cmd/internal/objabi/funcid.go
index e921a82c0c..6e188e31bb 100644
--- a/src/cmd/internal/objabi/funcid.go
+++ b/src/cmd/internal/objabi/funcid.go
@@ -6,13 +6,22 @@ package objabi
 
 import "strings"
 
+// A FuncFlag records bits about a function, passed to the runtime.
+type FuncFlag uint8
+
+// Note: This list must match the list in runtime/symtab.go.
+const (
+	FuncFlag_TOPFRAME = 1 << iota
+	FuncFlag_SPWRITE
+)
+
 // A FuncID identifies particular functions that need to be treated
 // specially by the runtime.
 // Note that in some situations involving plugins, there may be multiple
 // copies of a particular special runtime function.
-// Note: this list must match the list in runtime/symtab.go.
 type FuncID uint8
 
+// Note: this list must match the list in runtime/symtab.go.
 const (
 	FuncID_normal FuncID = iota // not a special function
 	FuncID_asmcgocall