Re: Add VIEW_CONVERT_EXPR to operand_equal_p

[Jan Hubicka <hubicka@ucw.cz>]

> >And AFAIK nobody answered the question: what do we gain by making this
> >change?
> >So far I have only seen breakages, suspicious fixes and code
> >duplication...
> 
> Honza wants the structural equality predicate (operand_equal_p) complete (optimistically) for GIMPLE.

There are two independent things - operand_equal_p changes and
useless_type_conversion changes.

operand_equal_p
===============

What I want to do is to merge logic of ipa-icf-gimple's
func_checker::compare_operand and operand_equal_p so we don't have two
incomplette and duplicated ways to say if two gimple operands are equal, but
one that does it right.

Main problem of func_checker::compare_operand is that it is confused about
matching types, producing too many false negatives.  Because ipa-icf works as a
propagation engine, gving up on one equivalnce leads to a cascaded effect.

operand_equal_p does handle types better, but on the other hand it does not handle
few trees that we need to match.
 - CONSTRUCTOR
 - VIEW_CONVERT_EXPR
 - OBJ_TYPE_REF

The plan is to add these into operand_equal_p and implement interface for valueizing
hook that can be used by ipa-icf-gimple to match objects cross function boundary
(for example say that two SSA_NAMEs are equal) and drop func_checker::compare_operand

usless_type_conversion
======================

I was only tracking one isse I hit: Fortran/C interoperability nees LTO to produce
same TYPE_CANONICAl for signed and unsigned version of size_t. Doing so broke
useless_type_conversion because it used TYPE_CANONICAL. We discussed the topic on
the GNU Cauldron and decided that it is cleaner to drop TYPE_CANONICAL from
useless_type_conversion because it does not really belong there.

That is only change I plan into the area. The decision to drop comparsion of TYPE_MODE
from the aggregate path was decision of the discussion about this particular patch
and I do not really insist on it.

Having fewer VCE expressions in the code is not a bad thing, but I do not really
see it as an important change. I am sorry for the breakage in move expansion that
I hoped to not be as important. I am willing to continue fixing the fallout (and
be more cureful about it - obviously I originally underestimated the issue).
I am also happy with simply adding back the mode checking and drop the changes
we did to expr.c so far.

Honza