On 10/22/18 2:17 PM, Segher Boessenkool wrote: > On most targets every function starts with moves from the parameter > passing (hard) registers into pseudos. Similarly, after every call > there is a move from the return register into a pseudo. These moves > usually combine with later instructions (leaving pretty much the same > instruction, just with a hard reg instead of a pseudo). > > This isn't a good idea. Register allocation can get rid of unnecessary > moves just fine, and moving the parameter passing registers into many > later instructions tends to prevent good register allocation. This > patch disallows combining moves from a hard (non-fixed) register. > > This also avoid the problem mentioned in PR87600 #c3 (combining hard > registers into inline assembler is problematic). > > Because the register move can often be combined with other instructions > *itself*, for example for setting some condition code, this patch adds > extra copies via new pseudos after every copy-from-hard-reg. > > On some targets this reduces average code size. On others it increases > it a bit, 0.1% or 0.2% or so. (I tested this on all *-linux targets). > > I'll commit this to trunk now. If there are problems, please don't > hesitate to let me know! Thanks. > > > Segher > > > 2018-10-22 Segher Boessenkool <seg...@kernel.crashing.org> > > PR rtl-optimization/87600 > * combine.c: Add include of expr.h. > (cant_combine_insn_p): Do not combine moves from any hard non-fixed > register to a pseudo. > (make_more_copies): New function, add a copy to a new pseudo after > the moves from hard registers into pseudos. > (rest_of_handle_combine): Declare rebuild_jump_labels_after_combine > later. Call make_more_copies. I know we've gone back and forth on this stuff through the years, particularly for targets with likely-to-spilled classes that are used for register passing/return values.
I'm certainly willing to go with this as general guidance. I wouldn't be surprised if we find that things like CSE, fwprop and other passes need twiddling over time to mimick what you're doing in combine. jeff
