Re: [PATCH v2 1/2] RISC-V: Add shorten_memrefs pass

[Andrew Waterman <andrew@sifive.com>]

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 <law@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
>