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, but I see the large
majority of options controlling e.g. DWARF use -g instead.
Happy to change it to -gprune-btf.
>
>> 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
>>
