[lto] C++. streaming, front-end specific tree nodes, IR types, and assembler names

["Bill Maddox" <maddox at google dot com>]

I am continuing to make progress with the LTO streamer on C++ code,
and finding a considerable amount  of front-end specific crud hanging
off of the GIMPLE, particularly related to templates.  This is not
surprising given the current strategy for generating debug information
(late, in the back-end).   The LTO IR must be language-independent,
hower, and the current short-term plan of record, as I understand it,
is to strip out left-over front-end specific parts of the tree in
tree.c:free_lang_specifics, accepting that this will break debug info
generation until a satisfactory alternative is devised.

Finding that DECL_CONTEXT might include portions of what looked like
template definitions, and surmising that  lexical nesting was a
language artifact that should no longer be of significance beyond the
front-end, I attempted to clear DECL_CONTEXT in free_lang_specifics.
This resulted in an assertion failure in
cp/mangle.c:write_unscoped_name.  It appears that the name mangling
code, as one would expect, is very interested in such nesting, and
examines DECL_CONTEXT in multiple locations.  None of this would be of
any consequence if the name mangling were complete upon entry to the
middle-end, but it can in fact be invoked via a langhook from
DECL_ASSEMBLER_NAME.  Setting the assembler name is performed lazily
in the back-end, requiring nesting relationships reflected in
DECL_CONTEXT to be preserved to this point.

My assertion is that the langhook must go, and that mangled names
should be generated before invoking the middle-end.  Given the baroque
treatment of the assembler name, however, I don't think it will be
sufficient simply to make a pass over the declarations in
free_lang_specifics setting the assembler name.

Any advice from someone who deeply understands this stuff?

--Bill