cmd/compile: rework reporting of oversized stack frames

We don't support stack frames over 2GB.
Rather than detect this during backend compilation,
check for it at the end of compilation.
This is arguably a more accurate check anyway,
since it takes into account the full frame,
including local stack, arguments, and arch-specific
rounding, although it's unlikely anyone would ever notice.

Also, rather than reporting the error right away,
take note of it and report it later, at the top level.
This is not relevant now, but it will help with making
the backend concurrent, as the append to the list of
oversized functions can be cheaply protected by a plain mutex.

Updates #15756
Updates #19250

Change-Id: Id3fa21906616d62e9dc66e27a17fd5f83304e96e
Reviewed-on: https://go-review.googlesource.com/38972
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -523,6 +523,9 @@ func Main(archInit func(*Arch)) {
 		if compiling_runtime {
 			checknowritebarrierrec()
 		}
+		for _, largePos := range largeStackFrames {
+			yyerrorl(largePos, "stack frame too large (>2GB)")
+		}
 	}
 
 	// Phase 9: Check external declarations.

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

@@ -235,10 +235,6 @@ func (s *ssafn) AllocFrame(f *ssa.Func) {
 		if thearch.LinkArch.InFamily(sys.MIPS, sys.MIPS64, sys.ARM, sys.ARM64, sys.PPC64, sys.S390X) {
 			s.stksize = Rnd(s.stksize, int64(Widthptr))
 		}
-		if s.stksize >= 1<<31 {
-			yyerrorl(s.curfn.Pos, "stack frame too large (>2GB)")
-		}
-
 		n.Xoffset = -s.stksize
 	}
 
@@ -289,6 +285,10 @@ func compile(fn *Node) {
 	pp := newProgs(fn)
 	genssa(ssafn, pp)
 	fieldtrack(pp.Text.From.Sym, fn.Func.FieldTrack)
+	if pp.Text.To.Offset >= 1<<31 {
+		largeStackFrames = append(largeStackFrames, fn.Pos)
+		return
+	}
 	pp.Flush()
 }
 

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

@@ -27,6 +27,8 @@ type Error struct {
 
 var errors []Error
 
+var largeStackFrames []src.XPos // positions of functions whose stack frames are too large (rare)
+
 func errorexit() {
 	flusherrors()
 	if outfile != "" {