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>

2 files changed, 14 insertions, 48 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 }