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
