[PATCH v2 6/6] bpf,btf: enable BTF pruning by default for BPF

[David Faust <david.faust@oracle.com>]

This patch enables -fprune-btf by default in the BPF backend when
generating BTF information, and fixes BPF CO-RE generation when using
-fprune-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.miranda@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 -fprune-btf
	by default if -gbtf is enabled.
	* config/bpf/bcore-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 -fprune-btf is
	enabled by default for BPF target when generating BTF.

gcc/testsuite/
	* gcc.dg/debug/btf/btf-variables-5.c: Add -fno-prune-btf to dg-options.
---
 gcc/btfout.cc                                 | 22 ++++++
 gcc/config/bpf/bpf.cc                         |  5 ++
 gcc/config/bpf/core-builtins.cc               | 70 +++++++++++++++++--
 gcc/ctfc.h                                    |  1 +
 gcc/doc/invoke.texi                           |  3 +
 .../gcc.dg/debug/btf/btf-variables-5.c        |  2 +-
 6 files changed, 96 insertions(+), 7 deletions(-)

diff --git a/gcc/btfout.cc b/gcc/btfout.cc
index 93d56492bbe..da2c9d35be9 100644
--- a/gcc/btfout.cc
+++ b/gcc/btfout.cc
@@ -1539,6 +1539,28 @@ btf_late_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 (!flag_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_minimal_add_type (ctfc, dtd, false, false);
+}
+
 /* 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 e6ea211a2c6..75303ce8f46 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 -fprune-btf for BPF target.  */
+  if (btf_debuginfo_p ())
+    SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+			 flag_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 d5a7de825ad..1b91b1c0d25 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 +1561,50 @@ 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)));
+}
+
+/* Find and mark used struct/union types for BTF pruning.  */
+
+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 +1623,8 @@ 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 bd3d98ffed9..99873a0b15c 100644
--- a/gcc/ctfc.h
+++ b/gcc/ctfc.h
@@ -458,6 +458,7 @@ 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 2908afff7c6..f8778cdf7cc 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -12254,6 +12254,9 @@ 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 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-variables-5.c b/gcc/testsuite/gcc.dg/debug/btf/btf-variables-5.c
index 8aae76cacab..c146b90abf9 100644
--- a/gcc/testsuite/gcc.dg/debug/btf/btf-variables-5.c
+++ b/gcc/testsuite/gcc.dg/debug/btf/btf-variables-5.c
@@ -5,7 +5,7 @@
    with 'global' linkage. However two array types will be generated.  */
 
 /* { dg-do compile } */
-/* { dg-options "-O0 -gbtf -dA" } */
+/* { dg-options "-O0 -gbtf -dA -fno-prune-btf" } */
 
 /* Expect 1 variable with global (1) linkage.  */
 /* { dg-final { scan-assembler-times "\[\t \]0xe000000\[\t \]+\[^\n\]*btv_info" 1 } } */
-- 
2.43.0