Re: [PATCH] RISC-V: Reject table-based CRC optabs for size

LGTM :)

Jojo R <[email protected]> 於 2026年8月19日週三 下午3:02寫道：
>
> The profitability check added for PR middle-end/124900 asks the target
> whether a CRC optab is suitable for both speed and size.  The RISC-V
> reversed CRC optab is always present, however, and falls back to a
> 256-entry lookup table when CLMUL expansion is unavailable.  This
> allows the transformation under -Os and -Oz even though it
> significantly increases code size.
>
> Implement TARGET_OPTAB_SUPPORTED_P for crc_rev_optab.  Keep the table
> fallback available when optimizing for speed, but report the optab as
> suitable for both speed and size only when Zbc, Zbkc, or Zvbc can
> provide the CLMUL expansion and the result mode is narrower than
> word_mode.
>
> Add an RV32/RV64 regression test for the no-CLMUL -Os and -Oz paths.
>
> gcc/ChangeLog:
>
>         * config/riscv/riscv.cc (riscv_optab_supported_p): New function.
>         (TARGET_OPTAB_SUPPORTED_P): Define.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.target/riscv/crc-size-no-table.c: New test.
> ---
>  gcc/config/riscv/riscv.cc                     | 18 +++++++++++
>  .../gcc.target/riscv/crc-size-no-table.c      | 31 +++++++++++++++++++
>  2 files changed, 49 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.target/riscv/crc-size-no-table.c
>
> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
> index a2a51c019ec..5557cf1476d 100644
> --- a/gcc/config/riscv/riscv.cc
> +++ b/gcc/config/riscv/riscv.cc
> @@ -13197,6 +13197,21 @@ riscv_scalar_mode_supported_p (scalar_mode mode)
>      return default_scalar_mode_supported_p (mode);
>  }
>
> +/* Implement TARGET_OPTAB_SUPPORTED_P.  */
> +
> +static bool
> +riscv_optab_supported_p (int op, machine_mode, machine_mode result_mode,
> +                        optimization_type opt_type)
> +{
> +  /* The second CRC optab mode is the result mode.  The CLMUL expansion
> +     requires room for a quotient wider than the CRC value itself.  */
> +  if (op == crc_rev_optab && opt_type != OPTIMIZE_FOR_SPEED)
> +    return ((TARGET_ZBKC || TARGET_ZBC || TARGET_ZVBC)
> +           && result_mode < word_mode);
> +
> +  return true;
> +}
> +
>  /* Implement TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P - return TRUE
>     if MODE is HFmode or BFmode, and punt to the generic implementation
>     otherwise.  */
> @@ -16859,6 +16874,9 @@ riscv_memtag_tag_bitsize ()
>  #undef TARGET_SCALAR_MODE_SUPPORTED_P
>  #define TARGET_SCALAR_MODE_SUPPORTED_P riscv_scalar_mode_supported_p
>
> +#undef TARGET_OPTAB_SUPPORTED_P
> +#define TARGET_OPTAB_SUPPORTED_P riscv_optab_supported_p
> +
>  #undef TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P
>  #define TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P                                \
>    riscv_libgcc_floating_mode_supported_p
> diff --git a/gcc/testsuite/gcc.target/riscv/crc-size-no-table.c b/gcc/testsuite/gcc.target/riscv/crc-size-no-table.c
> new file mode 100644
> index 00000000000..1e6d8c45529
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/riscv/crc-size-no-table.c
> @@ -0,0 +1,31 @@
> +/* { dg-do compile } */
> +/* { dg-options "-march=rv32gc -mabi=ilp32 -fdump-tree-crc-details -fdisable-tree-phiopt2 -fdisable-tree-phiopt3" { target { rv32 } } } */
> +/* { dg-options "-march=rv64gc -mabi=lp64 -fdump-tree-crc-details -fdisable-tree-phiopt2 -fdisable-tree-phiopt3" { target { rv64 } } } */
> +/* { dg-skip-if "" { *-*-* } { "*" } { "-Os" "-Oz" } } */
> +
> +typedef unsigned char uint8_t;
> +typedef unsigned int uint32_t;
> +
> +uint32_t
> +crc32r (const uint8_t *data, uint32_t size)
> +{
> +  uint32_t crc = 0xffffffff;
> +
> +  for (uint32_t i = 0; i != size; i++)
> +    {
> +      crc ^= data[i];
> +      for (int j = 0; j < 8; j++)
> +       if (crc & 1)
> +         crc = (crc >> 1) ^ 0xedb88320;
> +       else
> +         crc >>= 1;
> +    }
> +
> +  return ~crc;
> +}
> +
> +/* The CRC loop must be recognized, but retained because rv32gc/rv64gc has no
> +   CRC optab that is smaller than the original loop.  */
> +/* { dg-final { scan-tree-dump "calculates CRC!" "crc" } } */
> +/* { dg-final { scan-tree-dump "Couldn't generate faster CRC code." "crc" } } */
> +/* { dg-final { scan-tree-dump-not {\.CRC_REV} "crc" } } */
> --
> 2.47.1
>