Understanding tree_swap_operands_p wrt SSA name versions

[Michael Ploujnikov <michael.ploujnikov@oracle.com>]

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Hi everyone,

(I hope this is the right place to ask, if not my apologies; please
point me in the right direction)

I'm trying to get a better understanding of the following part in
tree_swap_operands_p():

  /* It is preferable to swap two SSA_NAME to ensure a canonical form
     for commutative and comparison operators.  Ensuring a canonical
     form allows the optimizers to find additional redundancies without
     having to explicitly check for both orderings.  */
  if (TREE_CODE (arg0) == SSA_NAME
      && TREE_CODE (arg1) == SSA_NAME
      && SSA_NAME_VERSION (arg0) > SSA_NAME_VERSION (arg1))
    return 1;

My questions in no particular order: It looks like this was added in
2004. I couldn't find any info other than what's in the corresponding
commit (cc0bdf913) so I'm wondering if the canonical form/order still
relevant/needed today? Does the ordering have to be done based on the
name versions specifically? Or can it be based on something more
intrinsic to the input source code rather than a GCC internal value, eg:
would alphabetic sort order of the variable names be a reasonable
replacement?

Thanks,
- Michael


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