> Am 31.05.2024 um 17:58 schrieb David Faust <david.fa...@oracle.com>:
>
>
>
>> On 5/31/24 00:07, Richard Biener wrote:
>>> On Thu, May 30, 2024 at 11:34 PM David Faust <david.fa...@oracle.com> wrote:
>>>
>>> This patch adds a new option, -fprune-btf, to control BTF debug info
>>> generation.
>>
>> Can you name it -gprune-btf instead?
>
> Yes, sure.
>
> I think I followed -feliminate-unused-debug-types,
Unfortunately GCC is full of historical mistakes…
> but I see the large
> majority of options controlling e.g. DWARF use -g instead.
>
> Happy to change it to -gprune-btf.
Please.
Richard
>>
>>> 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
>>>