[PATCH v3] riscv: Resolve symbols directly for symbols with STO_RISCV_VARIANT_CC.

[Palmer Dabbelt]

On Tue, 14 Mar 2023 09:25:12 PDT (-0700), libc-alpha@sourceware.org wrote:
> From: Hsiangkai Wang <kai.wang@sifive.com>
>
> In some cases, we do not want to go through the resolver for function
> calls. For example, functions with vector arguments will use vector
> registers to pass arguments. In the resolver, we do not save/restore the
> vector argument registers for lazy binding efficiency. To avoid ruining
> the vector arguments, functions with vector arguments will not go
> through the resolver.
>
> To achieve the goal, we will annotate the function symbols with
> STO_RISCV_VARIANT_CC flag and add DT_RISCV_VARIANT_CC tag in the dynamic
> section. In the first pass on PLT relocations, we do not set up to call
> _dl_runtime_resolve. Instead, we resolve the functions directly.
>
> Signed-off-by: Hsiangkai Wang <kai.wang@sifive.com>
> Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
>
> ---
>
> Changes:
>
> - Update copyright year.
> - Seperated from the original patchset, it could be used for vector ISA support,
>   but NOT only used for vector support.
> ---
>  elf/elf.h                    |  7 +++++++
>  manual/platform.texi         |  6 ++++++
>  sysdeps/riscv/dl-dtprocnum.h | 21 +++++++++++++++++++++
>  sysdeps/riscv/dl-machine.h   | 26 ++++++++++++++++++++++++++
>  4 files changed, 60 insertions(+)
>  create mode 100644 sysdeps/riscv/dl-dtprocnum.h
>
> diff --git a/elf/elf.h b/elf/elf.h
> index 4bc0e4299c..1eef1a7184 100644
> --- a/elf/elf.h
> +++ b/elf/elf.h
> @@ -3932,6 +3932,13 @@ enum
>
>  #define R_TILEGX_NUM		130
>
> +/* RISC-V specific values for the Dyn d_tag field.  */
> +#define DT_RISCV_VARIANT_CC	(DT_LOPROC + 1)
> +#define DT_RISCV_NUM		2
> +
> +/* RISC-V specific values for the st_other field.  */
> +#define STO_RISCV_VARIANT_CC 0x80
> +
>  /* RISC-V ELF Flags */
>  #define EF_RISCV_RVC 			0x0001
>  #define EF_RISCV_FLOAT_ABI 		0x0006
> diff --git a/manual/platform.texi b/manual/platform.texi
> index d5fdc5bd05..a1a740f381 100644
> --- a/manual/platform.texi
> +++ b/manual/platform.texi
> @@ -121,6 +121,12 @@ when it is not allowed, the priority is set to medium.
>  @node RISC-V
>  @appendixsec RISC-V-specific Facilities
>
> +Functions that are lazily bound must be compatible with the standard calling
> +convention. When a function is annotated with STO_RISCV_VARIANT_CC, it means
> +this function is not compatible with the standard calling convention. The
> +dynamic linker will directly resolve it instead of using the lazy binding
> +mechanism.
> +
>  Cache management facilities specific to RISC-V systems that implement the Linux
>  ABI are declared in @file{sys/cachectl.h}.
>
> diff --git a/sysdeps/riscv/dl-dtprocnum.h b/sysdeps/riscv/dl-dtprocnum.h
> new file mode 100644
> index 0000000000..281c5aadeb
> --- /dev/null
> +++ b/sysdeps/riscv/dl-dtprocnum.h
> @@ -0,0 +1,21 @@
> +/* Configuration of lookup functions.  RISC-V version.
> +   Copyright (C) 2023 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library.  If not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +/* Number of extra dynamic section entries for this architecture.  By
> +   default there are none.  */
> +#define DT_THISPROCNUM	DT_RISCV_NUM
> diff --git a/sysdeps/riscv/dl-machine.h b/sysdeps/riscv/dl-machine.h
> index c0c9bd93ad..0e6c0bb155 100644
> --- a/sysdeps/riscv/dl-machine.h
> +++ b/sysdeps/riscv/dl-machine.h
> @@ -53,6 +53,9 @@
>       || (__WORDSIZE == 64 && (type) == R_RISCV_TLS_TPREL64)))	\
>     | (ELF_RTYPE_CLASS_COPY * ((type) == R_RISCV_COPY)))
>
> +//* Translate a processor specific dynamic tag to the index in l_info array.  */
> +#define DT_RISCV(x) (DT_RISCV_##x - DT_LOPROC + DT_NUM)
> +
>  /* Return nonzero iff ELF header is compatible with the running host.  */
>  static inline int __attribute_used__
>  elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
> @@ -281,6 +284,29 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
>    /* Check for unexpected PLT reloc type.  */
>    if (__glibc_likely (r_type == R_RISCV_JUMP_SLOT))
>      {
> +      if (__glibc_unlikely (map->l_info[DT_RISCV (VARIANT_CC)] != NULL))
> +	{
> +          /* Check the symbol table for variant CC symbols.  */
> +          const Elf_Symndx symndx = ELFW(R_SYM) (reloc->r_info);
> +          const ElfW(Sym) *symtab =
> +            (const void *)D_PTR (map, l_info[DT_SYMTAB]);
> +          const ElfW(Sym) *sym = &symtab[symndx];
> +          if (__glibc_unlikely (sym->st_other & STO_RISCV_VARIANT_CC))
> +            {
> +              /* Avoid lazy resolution of variant CC symbols.  */
> +              const struct r_found_version *version = NULL;
> +              if (map->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
> +                {
> +                  const ElfW(Half) *vernum =
> +                    (const void *)D_PTR (map, l_info[VERSYMIDX (DT_VERSYM)]);
> +                  version = &map->l_versions[vernum[symndx] & 0x7fff];
> +                }
> +              elf_machine_rela (map, scope, reloc, sym, version, reloc_addr,
> +                                skip_ifunc);
> +              return;
> +            }
> +	}
> +
>        if (__glibc_unlikely (map->l_mach.plt == 0))
>  	{
>  	  if (l_addr)

Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>