cmd/compile: don't bother compiling functions named "_"

They can't be used, so we don't need code generated for them. We just
need to report errors in their bodies.

The compiler currently has a bunch of special cases sprinkled about
for "_" functions, because we never generate a linker symbol for them.
Instead, abort compilation earlier so we never reach any of that
special-case code.

Fixes #29870

Change-Id: I3530c9c353deabcf75ce9072c0b740e992349ee5
Reviewed-on: https://go-review.googlesource.com/c/158845
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -176,11 +176,6 @@ func (pp *Progs) settext(fn *Node) {
 	ptxt := pp.Prog(obj.ATEXT)
 	pp.Text = ptxt
 
-	if fn.Func.lsym == nil {
-		// func _() { }
-		return
-	}
-
 	fn.Func.lsym.Func.Text = ptxt
 	ptxt.From.Type = obj.TYPE_MEM
 	ptxt.From.Name = obj.NAME_EXTERN

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

@@ -258,7 +258,6 @@ func compile(fn *Node) {
 	// be types of stack objects. We need to do this here
 	// because symbols must be allocated before the parallel
 	// phase of the compiler.
-	if fn.Func.lsym != nil { // not func _(){}
 	for _, n := range fn.Func.Dcl {
 		switch n.Class() {
 		case PPARAM, PPARAMOUT, PAUTO:
@@ -272,7 +271,6 @@ func compile(fn *Node) {
 			}
 		}
 	}
-	}
 
 	if compilenow() {
 		compileSSA(fn, 0)

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

@@ -1426,7 +1426,7 @@ func liveness(e *ssafn, f *ssa.Func, pp *Progs) LivenessMap {
 	}
 
 	// Emit the live pointer map data structures
-	if ls := e.curfn.Func.lsym; ls != nil {
+	ls := e.curfn.Func.lsym
 	ls.Func.GCArgs, ls.Func.GCLocals, ls.Func.GCRegs = lv.emit()
 
 	p := pp.Prog(obj.AFUNCDATA)
@@ -1446,6 +1446,6 @@ func liveness(e *ssafn, f *ssa.Func, pp *Progs) LivenessMap {
 	p.To.Type = obj.TYPE_MEM
 	p.To.Name = obj.NAME_EXTERN
 	p.To.Sym = ls.Func.GCRegs
-	}
+
 	return lv.livenessMap
 }

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

@@ -5175,10 +5175,7 @@ func genssa(f *ssa.Func, pp *Progs) {
 				}
 			case ssa.OpInlMark:
 				p := thearch.Ginsnop(s.pp)
-				if pp.curfn.Func.lsym != nil {
-					// lsym is nil if the function name is "_".
 				pp.curfn.Func.lsym.Func.AddInlMark(p, v.AuxInt32())
-				}
 				// TODO: if matching line number, merge somehow with previous instruction?
 
 			default: