On 06/19/2018 12:30 PM, Michael Ploujnikov wrote: > 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? Canonicalization is still important and useful.
However, canonicalizing on SSA_NAMEs is problematical due to the way we recycle nodes and re-pack them. I think defining additional rules for canonicalization prior to using SSA_NAME_VERSION as the fallback would be looked upon favorably. Note however, that many of the _DECL nodes referenced by SSA_NAMEs are temporaries generated by the compiler and do not correspond to any declared/defined object in the original source. So you'll still need the SSA_NAME_VERSION (or something as stable or better) canonicalization to handle those cases. Jeff
