[dev.regabi] cmd/compile: cleanup label handling

- The use of a label's Name.Defn to point at the named for/select/switch
  means that any rewrite of the for/select/switch must overwrite the original
  or else the pointer will dangle. Remove that pointer by adding the label
  name directly to the for/select/switch representation instead.

- The only uses of a label's Sym.Label were ephemeral values during
  markbreak and escape analysis. Use a map for each. Although in general
  we are not going to replace all computed fields with maps (too slow),
  the one in markbreak is only for labeled for/select/switch, and the one
  in escape is for all labels, but even so, labels are fairly rare.

In theory this cleanup should make it easy to allow labeled for/select/switch
in inlined bodies, but this CL does not attempt that. It's only concerned
with cleanup to enable a new Node representation.

Passes buildall w/ toolstash -cmp.

Change-Id: I7e36ee98d2ea40dbae94e6722d585f007b7afcfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/274086
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 2 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/types/sym.go b/src/cmd/compile/internal/types/sym.go
index 046104d0dc..7272f1f786 100644
--- a/src/cmd/compile/internal/types/sym.go
+++ b/src/cmd/compile/internal/types/sym.go
@@ -33,13 +33,12 @@ type Sym struct {
 	Name string // object name
 
 	// saved and restored by dcopy
-	Def        IRNode    // definition: ONAME OTYPE OPACK or OLITERAL
+	Def        IRNode   // definition: ONAME OTYPE OPACK or OLITERAL
 	Block      int32    // blocknumber to catch redeclaration
 	Lastlineno src.XPos // last declaration for diagnostic
 
 	flags   bitset8
-	Label   IRNode // corresponding label (ephemeral)
-	Origpkg *Pkg  // original package for . import
+	Origpkg *Pkg // original package for . import
 }
 
 const (