Hi Faust.
Thanks for the patch.
Please see a question below.
> This patch enables -gprune-btf by default in the BPF backend when
> generating BTF information, and fixes BPF CO-RE generation when using
> -gprune-btf.
>
> When generating BPF CO-RE information, we must ensure that types used
> in CO-RE relocations always have sufficient BTF information emited so
> that the CO-RE relocations can be processed by a BPF loader. The BTF
> pruning algorithm on its own does not have sufficient information to
> determine which types are used in a BPF CO-RE relocation, so this
> information must be supplied by the BPF backend, using a new
> btf_mark_type_used function.
>
> Co-authored-by: Cupertino Miranda <cupertino.mira...@oracle.com>
>
> gcc/
> * btfout.cc (btf_mark_type_used): New.
> * ctfc.h (btf_mark_type_used): Declare it here.
> * config/bpf/bpf.cc (bpf_option_override): Enable -gprune-btf
> by default if -gbtf is enabled.
> * config/bpf/core-builtins.cc (extra_fn): New typedef.
> (compute_field_expr): Add callback parameter, and call it if supplied.
> Fix computation for MEM_REF.
> (mark_component_type_as_used): New.
> (bpf_mark_types_as_used): Likewise.
> (bpf_expand_core_builtin): Call here.
> * doc/invoke.texi (Debugging Options): Note that -gprune-btf is
> enabled by default for BPF target when generating BTF.
>
> gcc/testsuite/
> * gcc.dg/debug/btf/btf-variables-5.c: Adjust one test for bpf-*-*
> target.
> ---
> gcc/btfout.cc | 22 ++++++
> gcc/config/bpf/bpf.cc | 5 ++
> gcc/config/bpf/core-builtins.cc | 71 +++++++++++++++++--
> gcc/ctfc.h | 1 +
> gcc/doc/invoke.texi | 3 +
> .../gcc.dg/debug/btf/btf-variables-5.c | 6 +-
> 6 files changed, 100 insertions(+), 8 deletions(-)
>
> diff --git a/gcc/btfout.cc b/gcc/btfout.cc
> index 34d8cec0a2e3..083ca48d6279 100644
> --- a/gcc/btfout.cc
> +++ b/gcc/btfout.cc
> @@ -1503,6 +1503,28 @@ btf_assign_datasec_ids (ctf_container_ref ctfc)
> }
> }
>
> +
> +/* Manually mark that type T is used to ensure it will not be pruned.
> + Used by the BPF backend when generating BPF CO-RE to mark types used
> + in CO-RE relocations. */
> +
> +void
> +btf_mark_type_used (tree t)
> +{
> + /* If we are not going to prune anyway, this is a no-op. */
> + if (!debug_prune_btf)
> + return;
> +
> + gcc_assert (TYPE_P (t));
> + ctf_container_ref ctfc = ctf_get_tu_ctfc ();
> + ctf_dtdef_ref dtd = ctf_lookup_tree_type (ctfc, t);
> +
> + if (!dtd)
> + return;
> +
> + btf_add_used_type (ctfc, dtd, false, false, true);
> +}
> +
> /* 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. */
> diff --git a/gcc/config/bpf/bpf.cc b/gcc/config/bpf/bpf.cc
> index dd1bfe38d29b..c62af7a6efa7 100644
> --- a/gcc/config/bpf/bpf.cc
> +++ b/gcc/config/bpf/bpf.cc
> @@ -221,6 +221,11 @@ bpf_option_override (void)
> && !(target_flags_explicit & MASK_BPF_CORE))
> target_flags |= MASK_BPF_CORE;
>
> + /* -gbtf implies -gprune-btf for BPF target. */
> + if (btf_debuginfo_p ())
> + SET_OPTION_IF_UNSET (&global_options, &global_options_set,
> + debug_prune_btf, true);
> +
> /* Determine available features from ISA setting (-mcpu=). */
> if (bpf_has_jmpext == -1)
> bpf_has_jmpext = (bpf_isa >= ISA_V2);
> diff --git a/gcc/config/bpf/core-builtins.cc b/gcc/config/bpf/core-builtins.cc
> index 232bebcadbd5..86e2e9d6e39f 100644
> --- a/gcc/config/bpf/core-builtins.cc
> +++ b/gcc/config/bpf/core-builtins.cc
> @@ -624,13 +624,20 @@ bpf_core_get_index (const tree node, bool *valid)
>
> ALLOW_ENTRY_CAST is an input arguments and specifies if the function
> should
> consider as valid expressions in which NODE entry is a cast expression (or
> - tree code nop_expr). */
> + tree code nop_expr).
> +
> + EXTRA_FN is a callback function to allow extra functionality with this
> + function traversal. Currently used for marking used type during expand
> + pass. */
> +
> +typedef void (*extra_fn) (tree);
>
> static unsigned char
> compute_field_expr (tree node, unsigned int *accessors,
> bool *valid,
> tree *access_node,
> - bool allow_entry_cast = true)
> + bool allow_entry_cast = true,
> + extra_fn callback = NULL)
> {
> unsigned char n = 0;
> unsigned int fake_accessors[MAX_NR_ACCESSORS];
> @@ -647,6 +654,9 @@ compute_field_expr (tree node, unsigned int *accessors,
>
> *access_node = node;
>
> + if (callback != NULL)
> + callback (node);
> +
> switch (TREE_CODE (node))
> {
> case INDIRECT_REF:
> @@ -664,17 +674,19 @@ compute_field_expr (tree node, unsigned int *accessors,
> case COMPONENT_REF:
> n = compute_field_expr (TREE_OPERAND (node, 0), accessors,
> valid,
> - access_node, false);
> + access_node, false, callback);
> accessors[n] = bpf_core_get_index (TREE_OPERAND (node, 1), valid);
> return n + 1;
> case ARRAY_REF:
> case ARRAY_RANGE_REF:
> - case MEM_REF:
> n = compute_field_expr (TREE_OPERAND (node, 0), accessors,
> valid,
> - access_node, false);
> + access_node, false, callback);
> accessors[n++] = bpf_core_get_index (node, valid);
> return n;
> + case MEM_REF:
> + accessors[0] = bpf_core_get_index (node, valid);
> + return 1;
> case NOP_EXPR:
> if (allow_entry_cast == true)
> {
> @@ -683,7 +695,7 @@ compute_field_expr (tree node, unsigned int *accessors,
> }
> n = compute_field_expr (TREE_OPERAND (node, 0), accessors,
> valid,
> - access_node, false);
> + access_node, false, callback);
> return n;
>
> case ADDR_EXPR:
> @@ -1549,6 +1561,51 @@ bpf_resolve_overloaded_core_builtin (location_t loc,
> tree fndecl,
> return construct_builtin_core_reloc (loc, fndecl, args, argsvec->length
> ());
> }
>
> +/* Callback function for bpf_mark_field_expr_types_as_used. */
> +
> +static void
> +mark_component_type_as_used (tree node)
> +{
> + if (TREE_CODE (node) == COMPONENT_REF)
> + btf_mark_type_used (TREE_TYPE (TREE_OPERAND (node, 0)));
> +}
This means that the callback is only marking as used these types
reachable from the CO-RE builtin arguments that are referenced by
indexation or field name or pointer?
> +
> +/* Mark types needed for BPF CO-RE relocations as used. Doing so ensures
> that
> + these types do not get pruned from the BTF information. */
> +
> +static void
> +bpf_mark_types_as_used (struct cr_builtins *data)
> +{
> + tree expr = data->expr;
> + switch (data->kind)
> + {
> + case BPF_RELO_FIELD_BYTE_OFFSET:
> + case BPF_RELO_FIELD_BYTE_SIZE:
> + case BPF_RELO_FIELD_EXISTS:
> + case BPF_RELO_FIELD_SIGNED:
> + case BPF_RELO_FIELD_LSHIFT_U64:
> + case BPF_RELO_FIELD_RSHIFT_U64:
> + if (TREE_CODE (expr) == ADDR_EXPR)
> + expr = TREE_OPERAND (expr, 0);
> +
> + expr = root_for_core_field_info (expr);
> + compute_field_expr (data->expr, NULL, NULL, NULL, false,
> + mark_component_type_as_used);
> + break;
> + case BPF_RELO_TYPE_ID_LOCAL:
> + case BPF_RELO_TYPE_ID_TARGET:
> + case BPF_RELO_TYPE_EXISTS:
> + case BPF_RELO_TYPE_SIZE:
> + case BPF_RELO_ENUMVAL_EXISTS:
> + case BPF_RELO_ENUMVAL_VALUE:
> + case BPF_RELO_TYPE_MATCHES:
> + btf_mark_type_used (data->type);
> + break;
> + default:
> + gcc_unreachable ();
> + }
> +}
> +
> /* Used in bpf_expand_builtin. This function is called in RTL expand stage
> to
> convert the internal __builtin_core_reloc in unspec:UNSPEC_CORE_RELOC RTL,
> which will contain a third argument that is the index in the vec collected
> @@ -1567,6 +1624,8 @@ bpf_expand_core_builtin (tree exp, enum bpf_builtins
> code)
> tree index = CALL_EXPR_ARG (exp, 0);
> struct cr_builtins *data = get_builtin_data (TREE_INT_CST_LOW (index));
>
> + bpf_mark_types_as_used (data);
> +
> rtx v = expand_normal (data->default_value);
> rtx i = expand_normal (index);
> return gen_rtx_UNSPEC (DImode,
> diff --git a/gcc/ctfc.h b/gcc/ctfc.h
> index 29267dc036d1..41e1169f271d 100644
> --- a/gcc/ctfc.h
> +++ b/gcc/ctfc.h
> @@ -457,6 +457,7 @@ extern ctf_dtdef_ref ctf_lookup_tree_type
> (ctf_container_ref, const tree);
>
> typedef bool (*funcs_traverse_callback) (ctf_dtdef_ref, void *);
> bool traverse_btf_func_types (funcs_traverse_callback, void *);
> +extern void btf_mark_type_used (tree);
>
> /* CTF section does not emit location information; at this time, location
> information is needed for BTF CO-RE use-cases. */
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 8479fd5cf2b8..0afd686733d0 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -12023,6 +12023,9 @@ 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.
>
> +This option is enabled by default for the BPF target when generating
> +BTF information.
> +
> @opindex gctf
> @item -gctf
> @itemx -gctf@var{level}
> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-variables-5.c
> b/gcc/testsuite/gcc.dg/debug/btf/btf-variables-5.c
> index 8aae76cacabd..a08130cfc072 100644
> --- a/gcc/testsuite/gcc.dg/debug/btf/btf-variables-5.c
> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-variables-5.c
> @@ -11,9 +11,11 @@
> /* { dg-final { scan-assembler-times "\[\t \]0xe000000\[\t
> \]+\[^\n\]*btv_info" 1 } } */
> /* { dg-final { scan-assembler-times "\[\t \]0x1\[\t \]+\[^\n\]*btv_linkage"
> 1 } } */
>
> -/* Expect 2 array types, one of which is unsized. */
> +/* Expect 2 array types, one of which is unsized. For BPF target,
> -gprune-btf
> + is the default and will remove the unsized array type. */
> /* { dg-final { scan-assembler-times "\[\t \]0x4\[\t \]+\[^\n\]*bta_nelems"
> 1 } } */
> -/* { dg-final { scan-assembler-times "\[\t \]0\[\t \]+\[^\n\]*bta_nelems" 1
> } } */
> +/* { dg-final { scan-assembler-times "\[\t \]0\[\t \]+\[^\n\]*bta_nelems" 1
> { target { !bpf-*-* } } } } */
> +/* { dg-final { scan-assembler-times "\[\t \]0\[\t \]+\[^\n\]*bta_nelems" 0
> { target { bpf-*-* } } } } */
>
> extern const char FOO[];
> const char FOO[] = "foo";
