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 <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Josh Bleecher Snyder <[email protected]>

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

@@ -258,17 +258,15 @@ 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:
- if livenessShouldTrack(n) && n.Addrtaken() {
- dtypesym(n.Type)
- // Also make sure we allocate a linker symbol
- // for the stack object data, for the same reason.
- if fn.Func.lsym.Func.StackObjects == nil {
- fn.Func.lsym.Func.StackObjects = lookup(fmt.Sprintf("%s.stkobj", fn.funcname())).Linksym()
- }
+ for _, n := range fn.Func.Dcl {
+ switch n.Class() {
+ case PPARAM, PPARAMOUT, PAUTO:
+ if livenessShouldTrack(n) && n.Addrtaken() {
+ dtypesym(n.Type)
+ // Also make sure we allocate a linker symbol
+ // for the stack object data, for the same reason.
+ if fn.Func.lsym.Func.StackObjects == nil {
+ fn.Func.lsym.Func.StackObjects = lookup(fmt.Sprintf("%s.stkobj", fn.funcname())).Linksym()
  }
  }
  }

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

@@ -1426,26 +1426,26 @@ 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.Func.GCArgs, ls.Func.GCLocals, ls.Func.GCRegs = lv.emit()
-
-  p := pp.Prog(obj.AFUNCDATA)
-  Addrconst(&p.From, objabi.FUNCDATA_ArgsPointerMaps)
-  p.To.Type = obj.TYPE_MEM
-  p.To.Name = obj.NAME_EXTERN
-  p.To.Sym = ls.Func.GCArgs
-
-  p = pp.Prog(obj.AFUNCDATA)
-  Addrconst(&p.From, objabi.FUNCDATA_LocalsPointerMaps)
-  p.To.Type = obj.TYPE_MEM
-  p.To.Name = obj.NAME_EXTERN
-  p.To.Sym = ls.Func.GCLocals
-
-  p = pp.Prog(obj.AFUNCDATA)
-  Addrconst(&p.From, objabi.FUNCDATA_RegPointerMaps)
-  p.To.Type = obj.TYPE_MEM
-  p.To.Name = obj.NAME_EXTERN
-  p.To.Sym = ls.Func.GCRegs
- }
+ ls := e.curfn.Func.lsym
+ ls.Func.GCArgs, ls.Func.GCLocals, ls.Func.GCRegs = lv.emit()
+
+ p := pp.Prog(obj.AFUNCDATA)
+ Addrconst(&p.From, objabi.FUNCDATA_ArgsPointerMaps)
+ p.To.Type = obj.TYPE_MEM
+ p.To.Name = obj.NAME_EXTERN
+ p.To.Sym = ls.Func.GCArgs
+
+ p = pp.Prog(obj.AFUNCDATA)
+ Addrconst(&p.From, objabi.FUNCDATA_LocalsPointerMaps)
+ p.To.Type = obj.TYPE_MEM
+ p.To.Name = obj.NAME_EXTERN
+ p.To.Sym = ls.Func.GCLocals
+
+ p = pp.Prog(obj.AFUNCDATA)
+ Addrconst(&p.From, objabi.FUNCDATA_RegPointerMaps)
+ 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())
- }
+ pp.curfn.Func.lsym.Func.AddInlMark(p, v.AuxInt32())
  // TODO: if matching line number, merge somehow with previous instruction?
 
  default: