On 2 February 2015 at 21:54, Paul Shortis <pshor...@dataworx.com.au> wrote: > I could have avoided the expander and used a single instruction pattern for > a)b)c) if if could have found a way to have alternative dependent clobbers > in an instruction pattern. I investigated attributes but couldn't see how I > would be able to achieve what I needed. Also tried clobber (match_dup 2) but > when one of the alternatives has a constant for operands[2] the clobber is > accepted silently by the .md compiler but doesn't actually clobber the > non-constant alternatives.
You can clobber one or more match_scratch with suitable constraint alternatives and let the register allocator / reload reserve the hard registers. Well, not really for cc, you'll just have to say you clobber it, and split away the unused clobber post-reload. That will not really give you less source code, but more uniform patterns prior to register allocation. That gives the rtl optimizers a better handle on the code. The effect is even more pronounced when you replace all hard register usage with pseudo register usage and appropriate constraints.
