cmd/compile: use getcallersp for gorecover "fp" arg

Currently, the compiler synthesize a special ".fp" node, which
points to the FP of the current frame, be to used to call
gorecover. Later that node turns to an Arg in SSA that is not
really an arg, causing problems for the new ABI work which changes
the handling of Args, so we have to special-case that node.

This CL changes the compiler to get the FP by using getcallersp,
which is an intrinsic in SSA and works on all platforms. As we
need the FP, not the caller SP, one drawback is that we have to
add FixedFrameSize for LR machines. But it does allow us to remove
that special node.

Change-Id: Ie721d51efca8116c9d23cc4f79738fffcf847df8
Reviewed-on: https://go-review.googlesource.com/c/go/+/297930
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ir/name.go

@@ -509,5 +509,3 @@ func NewPkgName(pos src.XPos, sym *types.Sym, pkg *types.Pkg) *PkgName {
 	p.pos = pos
 	return p
 }
-
-var RegFP *Name

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

@@ -93,12 +93,7 @@ func (s *ssafn) AllocFrame(f *ssa.Func) {
 		for _, v := range b.Values {
 			if n, ok := v.Aux.(*ir.Name); ok {
 				switch n.Class {
-				case ir.PPARAM, ir.PPARAMOUT:
-					// Don't modify RegFP; it is a global.
-					if n != ir.RegFP {
-						n.SetUsed(true)
-					}
-				case ir.PAUTO:
+				case ir.PPARAM, ir.PPARAMOUT, ir.PAUTO:
 					n.SetUsed(true)
 				}
 			}

src/cmd/compile/internal/ssagen/ssa.go

@@ -5176,10 +5176,6 @@ func (s *state) addr(n ir.Node) *ssa.Value {
 			if v != nil {
 				return v
 			}
-			if n == ir.RegFP {
-				// Special arg that points to the frame pointer (Used by ORECOVER).
-				return s.entryNewValue2A(ssa.OpLocalAddr, t, n, s.sp, s.startmem)
-			}
 			s.Fatalf("addr of undeclared ONAME %v. declared: %v", n, s.decladdrs)
 			return nil
 		case ir.PAUTO:

src/cmd/compile/internal/typecheck/builtin.go

@@ -178,6 +178,7 @@ var runtimeDecls = [...]struct {
 	{"uint32tofloat64", funcTag, 117},
 	{"complex128div", funcTag, 118},
 	{"getcallerpc", funcTag, 119},
+	{"getcallersp", funcTag, 119},
 	{"racefuncenter", funcTag, 31},
 	{"racefuncexit", funcTag, 9},
 	{"raceread", funcTag, 31},

src/cmd/compile/internal/typecheck/builtin/runtime.go

@@ -226,6 +226,7 @@ func uint32tofloat64(uint32) float64
 func complex128div(num complex128, den complex128) (quo complex128)
 
 func getcallerpc() uintptr
+func getcallersp() uintptr
 
 // race detection
 func racefuncenter(uintptr)

src/cmd/compile/internal/typecheck/universe.go

@@ -354,9 +354,4 @@ func DeclareUniverse() {
 		s1.Def = s.Def
 		s1.Block = s.Block
 	}
-
-	ir.RegFP = NewName(Lookup(".fp"))
-	ir.RegFP.SetType(types.Types[types.TINT32])
-	ir.RegFP.Class = ir.PPARAM
-	ir.RegFP.SetUsed(true)
 }

src/cmd/compile/internal/walk/expr.go

@@ -158,7 +158,14 @@ func walkExpr1(n ir.Node, init *ir.Nodes) ir.Node {
 
 	case ir.ORECOVER:
 		n := n.(*ir.CallExpr)
-		return mkcall("gorecover", n.Type(), init, typecheck.NodAddr(ir.RegFP))
+		// Call gorecover with the FP of this frame.
+		// FP is equal to caller's SP plus FixedFrameSize().
+		var fp ir.Node = mkcall("getcallersp", types.Types[types.TUINTPTR], init)
+		if off := base.Ctxt.FixedFrameSize(); off != 0 {
+			fp = ir.NewBinaryExpr(fp.Pos(), ir.OADD, fp, ir.NewInt(off))
+		}
+		fp = ir.NewConvExpr(fp.Pos(), ir.OCONVNOP, types.NewPtr(types.Types[types.TINT32]), fp)
+		return mkcall("gorecover", n.Type(), init, fp)
 
 	case ir.OCFUNC:
 		return n