Re: [PATCH v2 2/2] riscv: Resolve symbols directly for symbols with STO_RISCV_VARIANT_CC.

[Kito Cheng <kito.cheng@sifive.com>]

>As per the ratified psABI v1.0, the V calling convention doesn't allow
> use of V reg for functions args, so this is not needed for now.

We don't have one for now, but we could expect the future will have one,
so I think we could accept that on upstream first?

On Fri, Dec 9, 2022 at 12:11 PM Vineet Gupta <vineetg@rivosinc.com> wrote:
>
> On 1/17/22 20:31, Vincent Chen 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.
>
> As per the ratified psABI v1.0, the V calling convention doesn't allow
> use of V reg for functions args, so this is not needed for now.
>
> -Vineet
>
> >
> > Signed-off-by: Hsiangkai Wang <kai.wang@sifive.com>
> > Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
> > ---
> >   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 0735f6b579..9c95544050 100644
> > --- a/elf/elf.h
> > +++ b/elf/elf.h
> > @@ -3911,6 +3911,13 @@ 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..f189fd700a
> > --- /dev/null
> > +++ b/sysdeps/riscv/dl-dtprocnum.h
> > @@ -0,0 +1,21 @@
> > +/* Configuration of lookup functions.  RISC-V version.
> > +   Copyright (C) 2019-2021 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 1d3e2e588c..cdbaca6533 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)
> > @@ -305,6 +308,29 @@ 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)
>