[PATCH v3 5/6] 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/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 -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               | 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 a7da164f6b31..35d2875e3f61 100644
--- a/gcc/btfout.cc
+++ b/gcc/btfout.cc
@@ -1501,6 +1501,28 @@ 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_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..775730700eba 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 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)));
+}
+
+/* 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 6dac00d14381..8edf0914fddb 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -12312,6 +12312,9 @@ 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 8aae76cacabd..7cd8728ecaf6 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, -fprune-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";
-- 
2.43.0