> The actual problem (or maybe my misunderstanding) is that it > combines the two original insns, then does some substitutions and tries > to match the combined and transformed insn against those defined in the > machine description. If it can't find anything there it reverts > everything and proceeds with the next insn pair. It never tries out the > straight forward option in the first place (which is not to transform > the combination).
The combiner pass does some form of canonicalization to better combine, see expand_compound_operation and make_compound_operation. > Is the scenario above intended behavior of the combine pass or an > accident? Or maybe even something else wrong in the machine description > that makes it behave like that? See how the i386 back-end copes with this for its "test" instruction. -- Eric Botcazou
