On Fri, Dec 05, 2014 at 01:40:56PM -0700, Jeff Law wrote: > My first thought would be to allow both and have combine swap the order > in the vector if recog doesn't recognize the pattern. One could argue > we could go through a full permutation of ordering in the vector, but > that's probably above and beyond the call of duty.
Combine also expects a certain ordering for its *inputs*. It might not be the only pass that does this either. All targets (where this works) have the compare first in all their patterns. This goes back, what, 20+ years? It might not be "officially" canonical, but it's the only ordering that works everywhere. Why can the compare-elim pass not simply swap the two elts of the parallel around? The alternative is to 1) modify all machine descriptions: this is used many hundreds of times, if not thousands, and modifications are not trivial (match_dups change location, for example); and 2) read all RTL code to identify all the places where these implicit assumptions are made. Segher
