code review 7395052: exp/ssa: reimplement logic for field selection.

exp/ssa: reimplement logic for field selection.

The previous approach desugared the ast.SelectorExpr
to make implicit field selections explicit.  But:
1) it was clunky since it required allocating temporary
   syntax trees.
2) it was not thread-safe since it required poking
   types into the shared type map for the new ASTs.
3) the desugared syntax had no place to represent the
   package lexically enclosing each implicit field
   selection, so it was as if they all occurred in the
   same package as the explicit field selection.
   This meant unexported field names changed meaning.

This CL does what I should have done all along: just
generate the SSA instructions directly from the original
AST and the promoted field information.

Also:
- add logStack util for paired start/end log messages.
  Useful for debugging crashes.

[adonovan, 2013-02-23 04:59:50]

Hello gri@golang.org (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go/

[gri, 2013-02-26 00:05:06]

LGTM

I am not claiming to have understood the details.

https://codereview.appspot.com/7395052/diff/6001/src/pkg/exp/ssa/builder.go
File src/pkg/exp/ssa/builder.go (right):

https://codereview.appspot.com/7395052/diff/6001/src/pkg/exp/ssa/builder.go#n...
src/pkg/exp/ssa/builder.go:548: // applies to it any implicit field selections
from path, then selects
s/then/and then/

https://codereview.appspot.com/7395052/diff/6001/src/pkg/exp/ssa/builder.go#n...
src/pkg/exp/ssa/builder.go:549: // the field #index of type fieldType; its
address is returned.
s/; its/. Its/ ?

https://codereview.appspot.com/7395052/diff/6001/src/pkg/exp/ssa/builder.go#n...
src/pkg/exp/ssa/builder.go:580: // the field #index of type fieldType; its value
is returned.
same changes as above?

https://codereview.appspot.com/7395052/diff/6001/src/pkg/exp/ssa/util.go
File src/pkg/exp/ssa/util.go (right):

https://codereview.appspot.com/7395052/diff/6001/src/pkg/exp/ssa/util.go#newc...
src/pkg/exp/ssa/util.go:217: // Don't forget trailing parens!
cute!

[adonovan, 2013-02-26 18:32:25]

*** Submitted as https://code.google.com/p/go/source/detail?r=81bda3810efb ***

exp/ssa: reimplement logic for field selection.

The previous approach desugared the ast.SelectorExpr
to make implicit field selections explicit.  But:
1) it was clunky since it required allocating temporary
   syntax trees.
2) it was not thread-safe since it required poking
   types into the shared type map for the new ASTs.
3) the desugared syntax had no place to represent the
   package lexically enclosing each implicit field
   selection, so it was as if they all occurred in the
   same package as the explicit field selection.
   This meant unexported field names changed meaning.

This CL does what I should have done all along: just
generate the SSA instructions directly from the original
AST and the promoted field information.

Also:
- add logStack util for paired start/end log messages.
  Useful for debugging crashes.

R=gri
CC=golang-dev
https://codereview.appspot.com/7395052