Re: [PATCH v3 4/6] btf: add -fprune-btf option

[Richard Biener <richard.guenther@gmail.com>]

On Thu, May 30, 2024 at 11:34 PM David Faust <david.faust@oracle.com> wrote:
>
> This patch adds a new option, -fprune-btf, to control BTF debug info
> generation.

Can you name it -gprune-btf instead?

> As the name implies, this option enables a kind of "pruning" of the BTF
> information before it is emitted.  When enabled, rather than emitting
> all type information translated from DWARF, only information for types
> directly used in the source program is emitted.
>
> The primary purpose of this pruning is to reduce the amount of
> unnecessary BTF information emitted, especially for BPF programs.  It is
> very common for BPF programs to incldue Linux kernel internal headers in
> order to have access to kernel data structures.  However, doing so often
> has the side effect of also adding type definitions for a large number
> of types which are not actually used by nor relevant to the program.
> In these cases, -fprune-btf commonly reduces the size of the resulting
> BTF information by 10x or more, as seen on average when compiling Linux
> kernel BPF selftests.  This both slims down the size of the resulting
> object and reduces the time required by the BPF loader to verify the
> program and its BTF information.
>
> Note that the pruning implemented in this patch follows the same rules
> as the BTF pruning performed unconditionally by LLVM's BPF backend when
> generating BTF.  In particular, the main sources of pruning are:
>
>   1) Only generate BTF for types used by variables and functions at
>      the file scope.  Note that with or without pruning, BTF_KIND_VAR
>      entries are only generated for variables present in the final
>      object - unused static variables or variables completely optimized
>      away must not have VAR entries in BTF.
>
>   2) Avoid emitting full BTF for struct and union types which are only
>      pointed-to by members of other struct/union types.  In these cases,
>      the full BTF_KIND_STRUCT or BTF_KIND_UNION which would normally
>      be emitted is replaced with a BTF_KIND_FWD, as though the
>      underlying type was a forward-declared struct or union type.
>
> gcc/
>         * btfout.cc (btf_used_types): New hash set.
>         (struct btf_fixup): New.
>         (fixups, forwards): New vecs.
>         (btf_output): Calculate num_types depending on flag_prune_btf.
>         (btf_early_finsih): New initialization for flag_prune_btf.
>         (btf_add_used_type): New function.
>         (btf_used_type_list_cb): Likewise.
>         (btf_late_collect_pruned_types): Likewise.
>         (btf_late_add_vars): Handle special case for variables in ".maps"
>         section when generating BTF for BPF CO-RE target.
>         (btf_late_finish): Use btf_late_collect_pruned_types when
>         flag_prune_btf in effect.  Move some initialization to btf_early_finish.
>         (btf_finalize): Additional deallocation for flag_prune_btf.
>         * common.opt (fprune-btf): New flag.
>         * ctfc.cc (init_ctf_strtable): Make non-static.
>         * ctfc.h (struct ctf_dtdef): Add visited_children_p boolean flag.
>         (init_ctf_strtable, ctfc_delete_strtab): Make extern.
>         * doc/invoke.texi (Debugging Options): Document -fprune-btf.
>
> gcc/testsuite/
>         * gcc.dg/debug/btf/btf-prune-1.c: New test.
>         * gcc.dg/debug/btf/btf-prune-2.c: Likewise.
>         * gcc.dg/debug/btf/btf-prune-3.c: Likewise.
>         * gcc.dg/debug/btf/btf-prune-maps.c: Likewise.
> ---
>  gcc/btfout.cc                                 | 359 +++++++++++++++++-
>  gcc/common.opt                                |   4 +
>  gcc/ctfc.cc                                   |   2 +-
>  gcc/ctfc.h                                    |   3 +
>  gcc/doc/invoke.texi                           |  20 +
>  gcc/testsuite/gcc.dg/debug/btf/btf-prune-1.c  |  25 ++
>  gcc/testsuite/gcc.dg/debug/btf/btf-prune-2.c  |  33 ++
>  gcc/testsuite/gcc.dg/debug/btf/btf-prune-3.c  |  35 ++
>  .../gcc.dg/debug/btf/btf-prune-maps.c         |  20 +
>  9 files changed, 494 insertions(+), 7 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/debug/btf/btf-prune-1.c
>  create mode 100644 gcc/testsuite/gcc.dg/debug/btf/btf-prune-2.c
>  create mode 100644 gcc/testsuite/gcc.dg/debug/btf/btf-prune-3.c
>  create mode 100644 gcc/testsuite/gcc.dg/debug/btf/btf-prune-maps.c
>
> diff --git a/gcc/btfout.cc b/gcc/btfout.cc
> index 32fda14f704b..a7da164f6b31 100644
> --- a/gcc/btfout.cc
> +++ b/gcc/btfout.cc
> @@ -828,7 +828,10 @@ output_btf_types (ctf_container_ref ctfc)
>  {
>    size_t i;
>    size_t num_types;
> -  num_types = ctfc->ctfc_types->elements ();
> +  if (flag_prune_btf)
> +    num_types = max_translated_id;
> +  else
> +    num_types = ctfc->ctfc_types->elements ();
>
>    if (num_types)
>      {
> @@ -957,6 +960,212 @@ btf_early_add_func_records (ctf_container_ref ctfc)
>      }
>  }
>
> +/* The set of types used directly in the source program, and any types manually
> +   marked as used.  This is the set of types which will be emitted when
> +   flag_prune_btf is set.  */
> +static GTY (()) hash_set<ctf_dtdef_ref> *btf_used_types;
> +
> +/* Fixup used to avoid unnecessary pointer chasing for types.  A fixup is
> +   created when a structure or union member is a pointer to another struct
> +   or union type.  In such cases, avoid emitting full type information for
> +   the pointee struct or union type (which may be quite large), unless that
> +   type is used directly elsewhere.  */
> +struct btf_fixup
> +{
> +  ctf_dtdef_ref pointer_dtd; /* Type node to which the fixup is applied.  */
> +  ctf_dtdef_ref pointee_dtd; /* Original type node referred to by pointer_dtd.
> +                               If this concrete type is not otherwise used,
> +                               then a forward is created.  */
> +};
> +
> +/* Stores fixups while processing types.  */
> +static vec<struct btf_fixup> fixups;
> +
> +/* For fixups where the underlying type is not used in the end, a BTF_KIND_FWD
> +   is created and emitted.  This vector stores them.  */
> +static GTY (()) vec<ctf_dtdef_ref, va_gc> *forwards;
> +
> +/* Recursively add type DTD and any types it references to the used set.
> +   Return a type that should be used for references to DTD - usually DTD itself,
> +   but may be NULL if DTD corresponds to a type which will not be emitted.
> +   CHECK_PTR is true if one of the predecessors in recursive calls is a struct
> +   or union member.  SEEN_PTR is true if CHECK_PTR is true AND one of the
> +   predecessors was a pointer type.  These two flags are used to avoid chasing
> +   pointers to struct/union only used from pointer members.  For such types, we
> +   will emit a forward instead of the full type information, unless
> +   CREATE_FIXUPS is false.  */
> +
> +static ctf_dtdef_ref
> +btf_add_used_type (ctf_container_ref ctfc, ctf_dtdef_ref dtd,
> +                  bool check_ptr, bool seen_ptr, bool create_fixups)
> +{
> +  if (dtd == NULL)
> +    return NULL;
> +
> +  uint32_t ctf_kind = CTF_V2_INFO_KIND (dtd->dtd_data.ctti_info);
> +  uint32_t kind = get_btf_kind (ctf_kind);
> +
> +  /* Check whether the type has already been added.  */
> +  if (btf_used_types->contains (dtd))
> +    {
> +      /* It's possible the type was already added as a fixup, but that we now
> +        have a concrete use of it.  */
> +      switch (kind)
> +       {
> +       case BTF_KIND_PTR:
> +       case BTF_KIND_TYPEDEF:
> +       case BTF_KIND_CONST:
> +       case BTF_KIND_VOLATILE:
> +       case BTF_KIND_RESTRICT:
> +         if (check_ptr)
> +           /* Type was previously added as a fixup, and that's OK.  */
> +           return dtd;
> +         else
> +           {
> +             /* The type was previously added as a fixup, but now we have
> +                a concrete use of it.  Remove the fixup.  */
> +             for (size_t i = 0; i < fixups.length (); i++)
> +               if (fixups[i].pointer_dtd == dtd)
> +                 fixups.unordered_remove (i);
> +
> +             /* Add the concrete base type.  */
> +             dtd->ref_type = btf_add_used_type (ctfc, dtd->ref_type, check_ptr,
> +                                                seen_ptr, create_fixups);
> +             return dtd;
> +           }
> +       default:
> +         return dtd;
> +       }
> +    }
> +
> +  if (ctf_kind == CTF_K_SLICE)
> +    {
> +      /* Bitfield.  Add the underlying type to the used set, but leave
> +        the reference to the bitfield.  The slice type won't be emitted,
> +        but we need the information in it when writing out the bitfield
> +        encoding.  */
> +      btf_add_used_type (ctfc, dtd->dtd_u.dtu_slice.cts_type,
> +                        check_ptr, seen_ptr, create_fixups);
> +      return dtd;
> +    }
> +
> +  /* Skip redundant definitions of void and types with no BTF encoding.  */
> +  if ((kind == BTF_KIND_INT && dtd->dtd_data.ctti_size == 0)
> +      || (kind == BTF_KIND_UNKN))
> +    return NULL;
> +
> +  /* Add the type itself, and assign its id.
> +     Do this before recursing to handle things like linked list structures.  */
> +  gcc_assert (ctfc->ctfc_nextid <= BTF_MAX_TYPE);
> +  dtd->dtd_type = ctfc->ctfc_nextid++;
> +  btf_used_types->add (dtd);
> +  ctf_add_string (ctfc, dtd->dtd_name, &(dtd->dtd_data.ctti_name), CTF_STRTAB);
> +  ctfc->ctfc_num_types++;
> +  ctfc->ctfc_num_vlen_bytes += btf_calc_num_vbytes (dtd);
> +
> +  /* Recursively add types referenced by this type.  */
> +  switch (kind)
> +    {
> +    case BTF_KIND_INT:
> +    case BTF_KIND_FLOAT:
> +    case BTF_KIND_FWD:
> +      /* Leaf kinds which do not refer to any other types.  */
> +      break;
> +
> +    case BTF_KIND_FUNC:
> +    case BTF_KIND_VAR:
> +      /* Root kinds; no type we are visiting may refer to these.  */
> +      gcc_unreachable ();
> +
> +    case BTF_KIND_PTR:
> +    case BTF_KIND_TYPEDEF:
> +    case BTF_KIND_CONST:
> +    case BTF_KIND_VOLATILE:
> +    case BTF_KIND_RESTRICT:
> +      {
> +       /* These type kinds refer to exactly one other type.  */
> +       if (check_ptr && !seen_ptr)
> +         seen_ptr = (kind == BTF_KIND_PTR);
> +
> +       /* Try to avoid chasing pointers to struct/union types if the
> +          underlying type isn't used.  */
> +       if (check_ptr && seen_ptr && create_fixups)
> +         {
> +           ctf_dtdef_ref ref = dtd->ref_type;
> +           uint32_t ref_kind = btf_dtd_kind (ref);
> +
> +           if ((ref_kind == BTF_KIND_STRUCT || ref_kind == BTF_KIND_UNION)
> +               && !btf_used_types->contains (ref))
> +             {
> +               struct btf_fixup fixup;
> +               fixup.pointer_dtd = dtd;
> +               fixup.pointee_dtd = ref;
> +               fixups.safe_push (fixup);
> +               break;
> +             }
> +         }
> +
> +       /* Add the type to which this type refers.  */
> +       dtd->ref_type = btf_add_used_type (ctfc, dtd->ref_type, check_ptr,
> +                                          seen_ptr, create_fixups);
> +       break;
> +      }
> +    case BTF_KIND_ARRAY:
> +      {
> +       /* Add element and index types.  */
> +       ctf_arinfo_t *arr = &(dtd->dtd_u.dtu_arr);
> +       arr->ctr_contents = btf_add_used_type (ctfc, arr->ctr_contents, false,
> +                                              false, create_fixups);
> +       arr->ctr_index = btf_add_used_type (ctfc, arr->ctr_index, false, false,
> +                                           create_fixups);
> +       break;
> +      }
> +    case BTF_KIND_STRUCT:
> +    case BTF_KIND_UNION:
> +    case BTF_KIND_ENUM:
> +    case BTF_KIND_ENUM64:
> +      {
> +       /* Add members.  */
> +       ctf_dmdef_t *dmd;
> +       for (dmd = dtd->dtd_u.dtu_members;
> +            dmd != NULL; dmd = (ctf_dmdef_t *) ctf_dmd_list_next (dmd))
> +         {
> +           /* Add member type for struct/union members.  For enums, only the
> +              enumerator names are needed.  */
> +           if (kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION)
> +             dmd->dmd_type = btf_add_used_type (ctfc, dmd->dmd_type, true,
> +                                                false, create_fixups);
> +           ctf_add_string (ctfc, dmd->dmd_name, &(dmd->dmd_name_offset),
> +                           CTF_STRTAB);
> +         }
> +       break;
> +      }
> +    case BTF_KIND_FUNC_PROTO:
> +      {
> +       /* Add return type.  */
> +       dtd->ref_type = btf_add_used_type (ctfc, dtd->ref_type, false, false,
> +                                          create_fixups);
> +
> +       /* Add arg types.  */
> +       ctf_func_arg_t * farg;
> +       for (farg = dtd->dtd_u.dtu_argv;
> +            farg != NULL; farg = (ctf_func_arg_t *) ctf_farg_list_next (farg))
> +         {
> +           farg->farg_type = btf_add_used_type (ctfc, farg->farg_type, false,
> +                                                false, create_fixups);
> +           /* Note: argument names are stored in the auxilliary string table,
> +              since CTF does not include arg names.  That table has not been
> +              cleared, so no need to re-add argument names here.  */
> +         }
> +       break;
> +      }
> +    default:
> +      return NULL;
> +    }
> +
> +  return dtd;
> +}
> +
>  /* Initial entry point of BTF generation, called at early_finish () after
>     CTF information has possibly been output.  Translate all CTF information
>     to BTF, and do any processing that must be done early, such as creating
> @@ -972,6 +1181,27 @@ btf_early_finish (void)
>
>    btf_early_add_const_void (tu_ctfc);
>    btf_early_add_func_records (tu_ctfc);
> +
> +  /* Note: from here on, destructive changes are made to the TU CTFC to
> +     translate CTF to BTF.  These fields are reset to count BTF types etc.  */
> +  tu_ctfc->ctfc_num_types = 0;
> +  tu_ctfc->ctfc_num_vlen_bytes = 0;
> +  tu_ctfc->ctfc_vars_list_count = 0;
> +
> +  if (flag_prune_btf)
> +    {
> +      btf_used_types
> +       = hash_set<ctf_dtdef_ref>::create_ggc (tu_ctfc->ctfc_types->elements ());
> +      tu_ctfc->ctfc_nextid = 1;
> +      fixups.create (1);
> +
> +      /* Empty the string table, which was already populated with strings for
> +        all types translated from DWARF.  We may only need a very small subset
> +        of these strings; those will be re-added below.  */
> +      ctfc_delete_strtab (&tu_ctfc->ctfc_strtable);
> +      init_ctf_strtable (&tu_ctfc->ctfc_strtable);
> +      tu_ctfc->ctfc_strlen++;
> +    }
>  }
>
>  /* Push a BTF datasec entry ENTRY into the datasec named SECNAME,
> @@ -1154,6 +1384,25 @@ btf_late_add_vars (ctf_container_ref ctfc)
>
>        /* Add a BTF_KIND_DATASEC entry for the variable.  */
>        btf_datasec_add_var (ctfc, var, dvd);
> +
> +      const char *section = var->get_section ();
> +      if (section && (strcmp (section, ".maps") == 0) && flag_prune_btf)
> +       {
> +         /* The .maps section has special meaning in BTF: it is used for BPF
> +            map definitions.  These definitions should be structs.  We must
> +            collect pointee types used in map members as though they are used
> +            directly, effectively ignoring (from the pruning perspective) that
> +            they are struct members.  */
> +         ctf_dtdef_ref dtd = dvd->dvd_type;
> +         uint32_t kind = btf_dtd_kind (dvd->dvd_type);
> +         if (kind == BTF_KIND_STRUCT)
> +           {
> +             ctf_dmdef_t *dmd;
> +             for (dmd = dtd->dtd_u.dtu_members;
> +                  dmd != NULL; dmd = (ctf_dmdef_t *) ctf_dmd_list_next (dmd))
> +               btf_add_used_type (ctfc, dmd->dmd_type, false, false, true);
> +           }
> +       }
>      }
>  }
>
> @@ -1252,6 +1501,86 @@ btf_late_assign_datasec_ids (ctf_container_ref ctfc)
>      }
>  }
>
> +/* Callback used for assembling the only-used-types list.  Note that this is
> +   the same as btf_type_list_cb above, but the hash_set traverse requires a
> +   different function signature.  */
> +
> +static bool
> +btf_used_type_list_cb (const ctf_dtdef_ref& dtd, ctf_container_ref ctfc)
> +{
> +  ctfc->ctfc_types_list[dtd->dtd_type] = dtd;
> +  return true;
> +}
> +
> +/* Collect the set of types reachable from global variables and functions.
> +   This is the minimal set of types, used when generating pruned BTF.  */
> +
> +static void
> +btf_late_collect_pruned_types (ctf_container_ref ctfc)
> +{
> +  vec_alloc (forwards, 1);
> +
> +  /* Add types used from functions.  */
> +  ctf_dtdef_ref dtd;
> +  size_t i;
> +  FOR_EACH_VEC_ELT (*funcs, i, dtd)
> +    {
> +      btf_add_used_type (ctfc, dtd->ref_type, false, false, true);
> +      ctf_add_string (ctfc, dtd->dtd_name, &(dtd->dtd_data.ctti_name),
> +                     CTF_STRTAB);
> +    }
> +
> +  /* Add types used from global variables.  */
> +  for (i = 0; i < ctfc->ctfc_vars_list_count; i++)
> +    {
> +      ctf_dvdef_ref dvd = ctfc->ctfc_vars_list[i];
> +      btf_add_used_type (ctfc, dvd->dvd_type, false, false, true);
> +      ctf_add_string (ctfc, dvd->dvd_name, &(dvd->dvd_name_offset), CTF_STRTAB);
> +    }
> +
> +  /* Process fixups.  If the base type was never added, create a forward for it
> +     and adjust the reference to point to that.  If it was added, then nothing
> +     needs to change.  */
> +  for (i = 0; i < fixups.length (); i++)
> +    {
> +      struct btf_fixup *fx = &fixups[i];
> +      if (!btf_used_types->contains (fx->pointee_dtd))
> +       {
> +         /* The underlying type is not used.  Create a forward.  */
> +         ctf_dtdef_ref fwd = ggc_cleared_alloc<ctf_dtdef_t> ();
> +         ctf_id_t id = ctfc->ctfc_nextid++;
> +         gcc_assert (id <= BTF_MAX_TYPE);
> +
> +         bool union_p = (btf_dtd_kind (fx->pointee_dtd) == BTF_KIND_UNION);
> +
> +         fwd->dtd_name = fx->pointee_dtd->dtd_name;
> +         fwd->dtd_data.ctti_info = CTF_TYPE_INFO (CTF_K_FORWARD, union_p, 0);
> +         fwd->dtd_type = id;
> +         ctfc->ctfc_num_types++;
> +         ctfc->ctfc_num_vlen_bytes += btf_calc_num_vbytes (fwd);
> +         ctf_add_string (ctfc, fwd->dtd_name, &(fwd->dtd_data.ctti_name),
> +                         CTF_STRTAB);
> +
> +         /* Update the pointer to point to the forward.  */
> +         fx->pointer_dtd->ref_type = fwd;
> +         vec_safe_push (forwards, fwd);
> +       }
> +    }
> +
> +  /* Construct the resulting pruned type list.  */
> +  ctfc->ctfc_types_list
> +    = ggc_vec_alloc<ctf_dtdef_ref> (btf_used_types->elements () + 1
> +                                   + vec_safe_length (forwards));
> +
> +  btf_used_types->traverse<ctf_container_ref, btf_used_type_list_cb> (ctfc);
> +
> +  /* Insert the newly created forwards into the regular types list too.  */
> +  FOR_EACH_VEC_ELT (*forwards, i, dtd)
> +    ctfc->ctfc_types_list[dtd->dtd_type] = dtd;
> +
> +  max_translated_id = btf_used_types->elements () + vec_safe_length (forwards);
> +}
> +
>  /* Late entry point for BTF generation, called from dwarf2out_finish ().
>     Complete and emit BTF information.  */
>
> @@ -1263,13 +1592,22 @@ btf_finish (void)
>
>    datasecs.create (0);
>
> -  tu_ctfc->ctfc_num_types = 0;
> -  tu_ctfc->ctfc_num_vlen_bytes = 0;
> -  tu_ctfc->ctfc_vars_list_count = 0;
> -
>    btf_late_add_vars (tu_ctfc);
> -  btf_late_collect_translated_types (tu_ctfc);
> +  if (flag_prune_btf)
> +    {
> +      /* Collect pruned set of BTF types and prepare for emission.
> +        This includes only types directly used in file-scope variables and
> +        function return/argument types.  */
> +      btf_late_collect_pruned_types (tu_ctfc);
> +    }
> +  else
> +    {
> +      /* Collect all BTF types and prepare for emission.
> +        This includes all types translated from DWARF.  */
> +      btf_late_collect_translated_types (tu_ctfc);
> +    }
>    btf_late_add_func_datasec_entries (tu_ctfc);
> +
>    btf_late_assign_var_ids (tu_ctfc);
>    btf_late_assign_func_ids (tu_ctfc);
>    btf_late_assign_datasec_ids (tu_ctfc);
> @@ -1302,6 +1640,15 @@ btf_finalize (void)
>    func_map->empty ();
>    func_map = NULL;
>
> +  if (flag_prune_btf)
> +    {
> +      btf_used_types->empty ();
> +      btf_used_types = NULL;
> +
> +      fixups.release ();
> +      forwards = NULL;
> +    }
> +
>    ctf_container_ref tu_ctfc = ctf_get_tu_ctfc ();
>    ctfc_delete_container (tu_ctfc);
>    tu_ctfc = NULL;
> diff --git a/gcc/common.opt b/gcc/common.opt
> index 2c078fdd1f86..3d4a55d02c26 100644
> --- a/gcc/common.opt
> +++ b/gcc/common.opt
> @@ -2588,6 +2588,10 @@ fpatchable-function-entry=
>  Common Var(flag_patchable_function_entry) Joined Optimization
>  Insert NOP instructions at each function entry.
>
> +fprune-btf
> +Common Var(flag_prune_btf) Init(0)
> +Generate minimal BTF debug info.
> +
>  frandom-seed
>  Common Var(common_deferred_options) Defer
>
> diff --git a/gcc/ctfc.cc b/gcc/ctfc.cc
> index 678ca2a072cf..27fc3a6d5851 100644
> --- a/gcc/ctfc.cc
> +++ b/gcc/ctfc.cc
> @@ -913,7 +913,7 @@ ctfc_get_dvd_srcloc (ctf_dvdef_ref dvd, ctf_srcloc_ref loc)
>  /* Initialize the CTF string table.
>     The first entry in the CTF string table (empty string) is added.  */
>
> -static void
> +void
>  init_ctf_strtable (ctf_strtable_t * strtab)
>  {
>    strtab->ctstab_head = NULL;
> diff --git a/gcc/ctfc.h b/gcc/ctfc.h
> index d0b724817a7f..29267dc036d1 100644
> --- a/gcc/ctfc.h
> +++ b/gcc/ctfc.h
> @@ -369,6 +369,9 @@ extern unsigned int ctfc_get_num_ctf_vars (ctf_container_ref);
>
>  extern ctf_strtable_t * ctfc_get_strtab (ctf_container_ref, int);
>
> +extern void init_ctf_strtable (ctf_strtable_t *);
> +extern void ctfc_delete_strtab (ctf_strtable_t *);
> +
>  /* Get the length of the specified string table in the CTF container.  */
>
>  extern size_t ctfc_get_strtab_len (ctf_container_ref, int);
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 517a782987d9..6dac00d14381 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -534,6 +534,7 @@ Objective-C and Objective-C++ Dialects}.
>  -gvms -gz@r{[}=@var{type}@r{]}
>  -gsplit-dwarf  -gdescribe-dies  -gno-describe-dies
>  -fdebug-prefix-map=@var{old}=@var{new}  -fdebug-types-section
> +-fprune-btf -fno-prune-btf
>  -fno-eliminate-unused-debug-types
>  -femit-struct-debug-baseonly  -femit-struct-debug-reduced
>  -femit-struct-debug-detailed@r{[}=@var{spec-list}@r{]}
> @@ -12292,6 +12293,25 @@ compressed debug sections, the option is rejected.  Otherwise, if the
>  assembler does not support them, @option{-gz} is silently ignored when
>  producing object files.
>
> +@opindex fprune-btf
> +@opindex fno-prune-btf
> +@item -fprune-btf
> +@itemx -fno-prune-btf
> +Prune BTF information before emission.  When pruning, only type
> +information for types used by global variables and file-scope functions
> +will be emitted.  If compiling for the BPF target with BPF CO-RE
> +enabled, type information will also be emitted for types used in BPF
> +CO-RE relocations.  In addition, struct and union types which are only
> +referred to via pointers from members of other struct or union types
> +shall be pruned and replaced with BTF_KIND_FWD, as though those types
> +were only present in the input as forward declarations.
> +
> +This option substantially reduces the size of produced BTF information,
> +but at significant loss in the amount of detailed type information.
> +It is primarily useful when compiling for the BPF target, to minimize
> +the size of the resulting object, and to eliminate BTF information
> +which is not immediately relevant to the BPF program loading process.
> +
>  @opindex femit-struct-debug-baseonly
>  @item -femit-struct-debug-baseonly
>  Emit debug information for struct-like types
> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-prune-1.c b/gcc/testsuite/gcc.dg/debug/btf/btf-prune-1.c
> new file mode 100644
> index 000000000000..3c9b59a07ecf
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-prune-1.c
> @@ -0,0 +1,25 @@
> +/* Simple test of -fprune-btf option operation.
> +   Since 'struct foo' is not used, no BTF shall be emitted for it.  */
> +
> +/* { dg-do compile } */
> +/* { dg-options "-gbtf -fprune-btf -dA" } */
> +
> +/* No BTF info for 'struct foo' nor types used only by it.  */
> +/* { dg-final { scan-assembler-not "BTF_KIND_STRUCT 'foo'" } } */
> +/* { dg-final { scan-assembler-not "BTF_KIND_INT 'char'" } } */
> +
> +/* We should get BTF info for 'struct bar' since it is used.  */
> +/* { dg-final { scan-assembler "BTF_KIND_STRUCT 'bar'"} } */
> +
> +struct foo {
> +  int a;
> +  char c;
> +};
> +
> +struct bar {
> +  int x;
> +  long z[4];
> +};
> +
> +struct bar a_bar;
> +
> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-prune-2.c b/gcc/testsuite/gcc.dg/debug/btf/btf-prune-2.c
> new file mode 100644
> index 000000000000..20183dffcc7f
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-prune-2.c
> @@ -0,0 +1,33 @@
> +/* Test that -fprune-btf does not chase pointer-to-struct members.  */
> +
> +/* { dg-do compile } */
> +/* { dg-options "-gbtf -fprune-btf -dA" } */
> +
> +/* Only use of B is via a pointer member of C.
> +   Full BTF for B is replaced with a forward.  */
> +/* { dg-final { scan-assembler-not "BTF_KIND_STRUCT 'B'" } } */
> +/* { dg-final { scan-assembler-times "TYPE \[0-9\]+ BTF_KIND_FWD 'B'" 1 } } */
> +
> +/* Detailed info for B is omitted, and A is otherwise unused.  */
> +/* { dg-final { scan-assembler-not "BTF_KIND_\[A-Z\]+ 'A'" } } */
> +
> +/* { dg-final { scan-assembler "BTF_KIND_STRUCT 'C'" } } */
> +
> +struct A;
> +
> +struct B {
> +  int x;
> +  int (*do_A_thing) (int, int);
> +  struct A *other;
> +};
> +
> +struct C {
> +  unsigned int x;
> +  struct B * a;
> +};
> +
> +int
> +foo (struct C *c)
> +{
> +  return c->x;
> +}
> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-prune-3.c b/gcc/testsuite/gcc.dg/debug/btf/btf-prune-3.c
> new file mode 100644
> index 000000000000..57a079cf0b4d
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-prune-3.c
> @@ -0,0 +1,35 @@
> +/* Test that -fprune-btf */
> +
> +/* { dg-do compile } */
> +/* { dg-options "-gbtf -fprune-btf -dA" } */
> +
> +/* We expect full BTF information each struct.  */
> +/* { dg-final { scan-assembler "TYPE \[0-9\]+ BTF_KIND_FWD 'file'" } } */
> +/* { dg-final { scan-assembler "TYPE \[0-9\]+ BTF_KIND_STRUCT 'A'" } } */
> +/* { dg-final { scan-assembler "TYPE \[0-9\]+ BTF_KIND_STRUCT 'B'" } } */
> +/* { dg-final { scan-assembler "TYPE \[0-9\]+ BTF_KIND_STRUCT 'C'" } } */
> +
> +struct file;
> +
> +struct A {
> +  void *private;
> +  long (*read)(struct file *, char *, unsigned long);
> +  long (*write)(struct file *, const char *, unsigned long);
> +};
> +
> +struct B {
> +  unsigned int x;
> +  struct A **as;
> +};
> +
> +struct C {
> +  struct A *arr_a[4];
> +  struct A *lone_a;
> +  unsigned int z;
> +};
> +
> +unsigned int
> +foo (struct B *b, struct C *c)
> +{
> +  return b->x + c->z;
> +}
> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-prune-maps.c b/gcc/testsuite/gcc.dg/debug/btf/btf-prune-maps.c
> new file mode 100644
> index 000000000000..bf3a25e984ff
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-prune-maps.c
> @@ -0,0 +1,20 @@
> +/* Test special meaning of .maps section for BTF when pruning.  For global
> +   variables of struct type placed in this section, we must treat members as
> +   though they are used directly, always collecting pointee types.
> +   Therefore, full type information for struct keep_me should be emitted.  */
> +
> +/* { dg-do compile } */
> +/* { dg-options "-gbtf -fprune-btf -dA" } */
> +
> +/* { dg-final { scan-assembler-not "BTF_KIND_FWD 'keep_me'" } } */
> +/* { dg-final { scan-assembler "BTF_KIND_STRUCT 'keep_me'" } } */
> +
> +struct keep_me {
> +  int a;
> +  char c;
> +};
> +
> +struct {
> +  int *key;
> +  struct keep_me *value;
> +} my_map __attribute__((section (".maps")));
> --
> 2.43.0
>