cmd/compile: generate frame pointers for otherwise frameless functions

func f() {
    g()
}

We mistakenly don't add a frame pointer for f.  This means f
isn't seen when walking the frame pointer linked list.  That
matters for kernel-gathered profiles, and is an impediment for
issues like #16638.

To fix, allocate a stack frame even for otherwise frameless functions
like f.  It is a bit tricky because we need to avoid some runtime
internals that really, really don't want one.

No test at the moment, as only kernel CPU profiles would catch it.
Tests will come with the implementation of #16638.

Fixes #18103

Change-Id: I411206cc9de4c8fdd265bee2e4fa61d161ad1847
Reviewed-on: https://go-review.googlesource.com/33754
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

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

@@ -380,6 +380,9 @@ func compile(fn *Node) {
 	if fn.Func.Wrapper {
 		ptxt.From3.Offset |= obj.WRAPPER
 	}
+	if fn.Func.NoFramePointer {
+		ptxt.From3.Offset |= obj.NOFRAME
+	}
 	if fn.Func.Needctxt {
 		ptxt.From3.Offset |= obj.NEEDCTXT
 	}

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

@@ -1810,6 +1810,8 @@ func genwrapper(rcvr *Type, method *Field, newnam *Sym, iface int) {
 		n := nod(ORETJMP, nil, nil)
 		n.Left = newname(methodsym(method.Sym, methodrcvr, 0))
 		fn.Nbody.Append(n)
+		// When tail-calling, we can't use a frame pointer.
+		fn.Func.NoFramePointer = true
 	} else {
 		fn.Func.Wrapper = true // ignore frame for panic+recover matching
 		call := nod(OCALL, dot, nil)

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

@@ -317,6 +317,7 @@ type Func struct {
 	Needctxt        bool   // function uses context register (has closure variables)
 	ReflectMethod   bool   // function calls reflect.Type.Method or MethodByName
 	IsHiddenClosure bool
+	NoFramePointer  bool // Must not use a frame pointer for this function
 }
 
 type Op uint8

src/cmd/internal/obj/x86/obj6.go

@@ -632,11 +632,27 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
 		autoffset = 0
 	}
 
+	hasCall := false
+	for q := p; q != nil; q = q.Link {
+		if q.As == obj.ACALL || q.As == obj.ADUFFCOPY || q.As == obj.ADUFFZERO {
+			hasCall = true
+			break
+		}
+	}
+
 	var bpsize int
-	if p.Mode == 64 && ctxt.Framepointer_enabled && autoffset > 0 && p.From3.Offset&obj.NOFRAME == 0 {
-		// Make room for to save a base pointer. If autoffset == 0,
-		// this might do something special like a tail jump to
-		// another function, so in that case we omit this.
+	if p.Mode == 64 && ctxt.Framepointer_enabled &&
+		p.From3.Offset&obj.NOFRAME == 0 && // (1) below
+		!(autoffset == 0 && p.From3.Offset&obj.NOSPLIT != 0) && // (2) below
+		!(autoffset == 0 && !hasCall) { // (3) below
+		// Make room to save a base pointer.
+		// There are 2 cases we must avoid:
+		// 1) If noframe is set (which we do for functions which tail call).
+		// 2) Scary runtime internals which would be all messed up by frame pointers.
+		//    We detect these using a heuristic: frameless nosplit functions.
+		//    TODO: Maybe someday we label them all with NOFRAME and get rid of this heuristic.
+		// For performance, we also want to avoid:
+		// 3) Frameless leaf functions
 		bpsize = ctxt.Arch.PtrSize
 		autoffset += int32(bpsize)
 		p.To.Offset += int64(bpsize)