operand swapping in get_expr_operands.

[Jeffrey A Law]

On Wed, 2005-05-04 at 10:28 -0400, Andrew MacLeod wrote:
> Why is it we try to swap operands in get_expr_operands, where we are
> otherwise simply parsing not modifying?
> 
>         /* If it would be profitable to swap the operands, then do so to
>            canonicalize the statement, enabling better optimization.
>            By placing canonicalization of such expressions here we
>            transparently keep statements in canonical form, even
>            when the statement is modified.  */
> 
> 
> Is this statement still true? and if so, who is counting on it?  DOM? 
> And does it need to be done in update_stmt?  Its been a minor wart
> everytime I turn around that we are modifying the stmt when in the
> middle of building operands. I'm curious if we still need to do it here,
> now its making comparing operands more difficult.
Any code which is hashing expressions is probably dependent on this
to some extent.

> I believe it basically attempts to have the first operand of a pair be
> the lower ssa_name index... ie, so commutative or comparision pairs are
> sorted. 
That's its primary purpose -- to ensure that SSA_NAMEs appear in a 
predictable order within an expression.

An alternate approach would be to twiddle the hashing code -- that works
OK for commutative operands, but not necessarily for things like LT_EXPR
where putting things into canonical form involves changing the tree 
code.

jeff