I don't know enough to say whether the legitimize_address hook is sufficient for the intended purpose, but I am sure that RISC-V's concerns are not unique: other GCC targets have to cope with offset-size constraints that are coupled to register-allocation constraints.
On Sat, Oct 26, 2019 at 11:21 AM Jeff Law <l...@redhat.com> wrote: > > On 10/25/19 11:39 AM, Craig Blackmore wrote: > > This patch aims to allow more load/store instructions to be compressed by > > replacing a load/store of 'base register + large offset' with a new > > load/store > > of 'new base + small offset'. If the new base gets stored in a compressed > > register, then the new load/store can be compressed. Since there is an > > overhead > > in creating the new base, this change is only attempted when 'base > > register' is > > referenced in at least 4 load/stores in a basic block. > > > > The optimization is implemented in a new RISC-V specific pass called > > shorten_memrefs which is enabled for RVC targets. It has been developed for > > the > > 32-bit lw/sw instructions but could also be extended to 64-bit ld/sd in > > future. > > > > Tested on bare metal rv32i, rv32iac, rv32im, rv32imac, rv32imafc, rv64imac, > > rv64imafdc via QEMU. No regressions. > > > > gcc/ChangeLog: > > > > * config.gcc: Add riscv-shorten-memrefs.o to extra_objs for riscv. > > * config/riscv/riscv-passes.def: New file. > > * config/riscv/riscv-protos.h (make_pass_shorten_memrefs): Declare. > > * config/riscv/riscv-shorten-memrefs.c: New file. > > * config/riscv/riscv.c (tree-pass.h): New include. > > (riscv_compressed_reg_p): New Function > > (riscv_compressed_lw_offset_p): Likewise. > > (riscv_compressed_lw_address_p): Likewise. > > (riscv_shorten_lw_offset): Likewise. > > (riscv_legitimize_address): Attempt to convert base + large_offset > > to compressible new_base + small_offset. > > (riscv_address_cost): Make anticipated compressed load/stores > > cheaper for code size than uncompressed load/stores. > > (riscv_register_priority): Move compressed register check to > > riscv_compressed_reg_p. > > * config/riscv/riscv.h (RISCV_MAX_COMPRESSED_LW_OFFSET): Define. > > * config/riscv/riscv.opt (mshorten-memefs): New option. > > * config/riscv/t-riscv (riscv-shorten-memrefs.o): New rule. > > (PASSES_EXTRA): Add riscv-passes.def. > > * doc/invoke.texi: Document -mshorten-memrefs. > This has traditionally been done via the the legitimize_address hook. > Is there some reason that hook is insufficient for this case? > > The hook, IIRC, is called out explow.c. > > Jeff >
