Symver attribute
Jan Hubicka
hubicka@ucw.cz
Fri Nov 15 09:07:00 GMT 2019
Hi,
this patch implements symver attribute which can be use din place of
.symver directive and is LTO friendly. I would welcome feedback for
this, in particular some evidence that codebases can be easily converted
to use it.
There is some discussion in the PR and its duplicate filled by Carlos.
He suggests bit more elaborate syntax then just passing name@nodename
as a string. I can implement it easily if that is preferred based on
the current infrastructure (basically handle_symver_attribute will need
to parse the parameters and produce the actual assembler name).
I found the PR only after drafing the patch and my goal was to keep
things similar to .symver GAS directive. I am not sure which variant is
preferred. I sort of like to keep symver attribute name same as the gas
directive and i think users are used to the node@nodename strings since
they appear everywhere else.
Internally the patch tries to mimic what happens in ELF. In particular
__attribute__ ((__symver__ ("foo@VERS_1"))) int
foo_v1 (void)
{
}
creates a special form of alias with symver flag. Its assembler name is
foo@VERS_1 since it is what happens in ELF too (normally @ is not
allowed in symbol name). Rest of GCC and LTO path handles is as normal
alias (since it seems what binutils does as well) and instead of
outputting it via .set we use .symver directive at the end.
I added some sanity checks: for duplicated directives, for versions of
weakrefs and commons (which are not accepted by gas) and for versions of
symbols not exported to final symbol table (which are accepted by gas
but seems to have no effect).
I am by no means expert in the area so feedback is welcome. I would
like to get the patch to trunk early next week if it works well.
Also I wonder for testsuite bits, I think I need to implement
dl-require-symver and then use it to gate the individual tests? Or do we
have some common way to check for ELF?
Honza
PR lto/48200
* c-family/c-attribs.c (handle_symver_attribute): New.
(c_common_attribytes): Add symver.
* cgraph.h (symtab_node): Add symver flag.
* cgraphunit.c (process_symver_attribute): New.
(process_common_attributes): Use it.
(cgraph_node::assemble_thunks_and_aliases): Assemble symvers.
* config/elfos.h (ASM_OUTPUT_SYMVER_DIRECTIVE): New.
* lto-cgraph.c (lto_output_node, lto_output_varpool_node,
input_overwrite_node, input_varpool_node): Stream symver attributes.
* output.h (do_assemble_symver): Declare.
* symtab.c (symtab_node::dump_base): Dump symver flag.
(symtab_node::verify_base): Sanity check symver.
* varasm.c (do_assemble_symver): New.
* varpool.c (varpool_node::assemble_aliases): Output symver.
* doc/extend.texi (symver): Document new attribute.
Index: c-family/c-attribs.c
===================================================================
--- c-family/c-attribs.c (revision 278220)
+++ c-family/c-attribs.c (working copy)
@@ -149,6 +149,7 @@ static tree handle_designated_init_attri
static tree handle_fallthrough_attribute (tree *, tree, tree, int, bool *);
static tree handle_patchable_function_entry_attribute (tree *, tree, tree,
int, bool *);
+static tree handle_symver_attribute (tree *, tree, tree, int, bool *);
static tree handle_copy_attribute (tree *, tree, tree, int, bool *);
/* Helper to define attribute exclusions. */
@@ -476,6 +477,8 @@ const struct attribute_spec c_common_att
NULL },
{ "nocf_check", 0, 0, false, true, true, true,
handle_nocf_check_attribute, NULL },
+ { "symver", 1, -1, true, false, false, false,
+ handle_symver_attribute, NULL},
{ "copy", 1, 1, false, false, false, false,
handle_copy_attribute, NULL },
{ "noinit", 0, 0, true, false, false, false,
@@ -2332,6 +2335,58 @@ handle_noplt_attribute (tree *node, tree
return NULL_TREE;
}
+static tree
+handle_symver_attribute (tree *node, tree ARG_UNUSED (name), tree args,
+ int ARG_UNUSED (flags), bool *no_add_attrs)
+{
+ tree symver;
+ const char *symver_str;
+ unsigned n;
+
+ if (TREE_CODE (*node) != FUNCTION_DECL && TREE_CODE (*node) != VAR_DECL)
+ {
+ warning (OPT_Wattributes,
+ "symver attribute is only applicable on functions and variables");
+ *no_add_attrs = true;
+ return NULL_TREE;
+ }
+
+ if (!decl_in_symtab_p (*node))
+ {
+ warning (OPT_Wattributes,
+ "symver attribute is only applicable to symbols");
+ *no_add_attrs = true;
+ return NULL_TREE;
+ }
+
+ for (; args; args = TREE_CHAIN (args))
+ {
+ symver = TREE_VALUE (args);
+ if (TREE_CODE (symver) != STRING_CST)
+ {
+ error ("symver attribute argument not a string constant");
+ *no_add_attrs = true;
+ return NULL_TREE;
+ }
+
+ symver_str = TREE_STRING_POINTER (symver);
+
+ int ats = 0;
+ for (n = 0; n < TREE_STRING_LENGTH (symver); n++)
+ if (symver_str[n] == '@')
+ ats++;
+
+ if (ats != 1 && ats != 2)
+ {
+ error ("symver attribute argument must have format %<name@nodename%>");
+ *no_add_attrs = true;
+ return NULL_TREE;
+ }
+ }
+
+ return NULL_TREE;
+}
+
/* Handle an "alias" or "ifunc" attribute; arguments as in
struct attribute_spec.handler, except that IS_ALIAS tells us
whether this is an alias as opposed to ifunc attribute. */
Index: cgraph.h
===================================================================
--- cgraph.h (revision 278220)
+++ cgraph.h (working copy)
@@ -497,6 +497,8 @@ public:
and their visibility needs to be copied from their "masters" at
the end of parsing. */
unsigned cpp_implicit_alias : 1;
+ /* The alias is a symbol version. */
+ unsigned symver : 1;
/* Set once the definition was analyzed. The list of references and
other properties are built during analysis. */
unsigned analyzed : 1;
Index: cgraphunit.c
===================================================================
--- cgraphunit.c (revision 278220)
+++ cgraphunit.c (working copy)
@@ -711,6 +711,89 @@ symbol_table::process_same_body_aliases
cpp_implicit_aliases_done = true;
}
+/* Process a symver attribute. */
+
+static void
+process_symver_attribute (symtab_node *n)
+{
+ tree value = lookup_attribute ("symver", DECL_ATTRIBUTES (n->decl));
+
+ if (!value)
+ return;
+ if (lookup_attribute ("symver", TREE_CHAIN (value)))
+ {
+ error_at (DECL_SOURCE_LOCATION (n->decl),
+ "multiple versions for one symbol");
+ return;
+ }
+ tree symver = get_identifier_with_length
+ (TREE_STRING_POINTER (TREE_VALUE (TREE_VALUE (value))),
+ TREE_STRING_LENGTH (TREE_VALUE (TREE_VALUE (value))));
+ symtab_node *def = symtab_node::get_for_asmname (symver);
+
+ if (def)
+ {
+ error_at (DECL_SOURCE_LOCATION (n->decl),
+ "duplicate definition of a symbol version");
+ inform (DECL_SOURCE_LOCATION (def->decl),
+ "same version was previously defined here");
+ return;
+ }
+ if (!n->definition)
+ {
+ error_at (DECL_SOURCE_LOCATION (n->decl),
+ "symbol needs to be defined to have a version");
+ return;
+ }
+ if (DECL_COMMON (n->decl))
+ {
+ error_at (DECL_SOURCE_LOCATION (n->decl),
+ "common symbol can't be versioned");
+ return;
+ }
+ if (DECL_COMDAT (n->decl))
+ {
+ error_at (DECL_SOURCE_LOCATION (n->decl),
+ "comdat symbol can't be versioned");
+ return;
+ }
+ if (n->weakref)
+ {
+ error_at (DECL_SOURCE_LOCATION (n->decl),
+ "weakref can't be versioned");
+ return;
+ }
+ if (!TREE_PUBLIC (n->decl))
+ {
+ error_at (DECL_SOURCE_LOCATION (n->decl),
+ "versioned symbol must be public");
+ return;
+ }
+ if (DECL_VISIBILITY (n->decl) != VISIBILITY_DEFAULT)
+ {
+ error_at (DECL_SOURCE_LOCATION (n->decl),
+ "versioned symbol must have default visibility");
+ return;
+ }
+
+ /* Create new symbol table entry represneting the version. */
+ tree new_decl = copy_node (n->decl);
+
+ DECL_INITIAL (new_decl) = NULL_TREE;
+ if (TREE_CODE (new_decl) == FUNCTION_DECL)
+ DECL_STRUCT_FUNCTION (new_decl) = NULL;
+ SET_DECL_ASSEMBLER_NAME (new_decl, symver);
+ TREE_PUBLIC (new_decl) = 1;
+ DECL_ATTRIBUTES (new_decl) = NULL;
+
+ symtab_node *symver_node = symtab_node::get_create (new_decl);
+ symver_node->alias = true;
+ symver_node->definition = true;
+ symver_node->symver = true;
+ symver_node->create_reference (n, IPA_REF_ALIAS, NULL);
+ symver_node->analyzed = true;
+}
+
/* Process attributes common for vars and functions. */
static void
@@ -730,6 +813,7 @@ process_common_attributes (symtab_node *
if (lookup_attribute ("no_reorder", DECL_ATTRIBUTES (decl)))
node->no_reorder = 1;
+ process_symver_attribute (node);
}
/* Look for externally_visible and used attributes and mark cgraph nodes
@@ -2137,8 +2221,12 @@ cgraph_node::assemble_thunks_and_aliases
/* Force assemble_alias to really output the alias this time instead
of buffering it in same alias pairs. */
TREE_ASM_WRITTEN (decl) = 1;
- do_assemble_alias (alias->decl,
- DECL_ASSEMBLER_NAME (decl));
+ if (alias->symver)
+ do_assemble_symver (alias->decl,
+ DECL_ASSEMBLER_NAME (decl));
+ else
+ do_assemble_alias (alias->decl,
+ DECL_ASSEMBLER_NAME (decl));
alias->assemble_thunks_and_aliases ();
TREE_ASM_WRITTEN (decl) = saved_written;
}
Index: config/elfos.h
===================================================================
--- config/elfos.h (revision 278220)
+++ config/elfos.h (working copy)
@@ -248,6 +248,17 @@ see the files COPYING3 and COPYING.RUNTI
} \
while (0)
+#define ASM_OUTPUT_SYMVER_DIRECTIVE(FILE, NAME, NAME2) \
+ do \
+ { \
+ fputs ("\t.symver\t", (FILE)); \
+ assemble_name ((FILE), (NAME)); \
+ fputs (", ", (FILE)); \
+ assemble_name ((FILE), (NAME2)); \
+ fputc ('\n', (FILE)); \
+ } \
+ while (0)
+
/* The following macro defines the format used to output the second
operand of the .type assembler directive. Different svr4 assemblers
expect various different forms for this operand. The one given here
Index: doc/extend.texi
===================================================================
--- doc/extend.texi (revision 278220)
+++ doc/extend.texi (working copy)
@@ -3640,6 +3640,29 @@ Function Attributes}, @ref{PowerPC Funct
@ref{Nios II Function Attributes}, and @ref{S/390 Function Attributes}
for details.
+@item symver ("@var{name2}@@@var{nodename}")
+On ELF targets this attribute creates symbol version. The @var{name2} part of
+the parameter is the actual name of the symbol by which it will be externally
+referenced. The @code{nodename} portion of the alias should be the name of a
+node specified in the version script supplied to the linker when building a
+shared library. Versioned symbol must be defined and must be exported with
+default visibility.
+
+@smallexample
+__attribute__ ((__symver__ ("foo@@VERS_1"))) int
+foo_v1 (void)
+@{
+@}
+@end smallexample
+
+Will produce @code{.symver foo_v1, foo@@VERS_1} directive in the assembler
+output.
+
+
+
+@smallexample
+@end smallexample
+
@item target_clones (@var{options})
@cindex @code{target_clones} function attribute
The @code{target_clones} attribute is used to specify that a function
Index: lto-cgraph.c
===================================================================
--- lto-cgraph.c (revision 278220)
+++ lto-cgraph.c (working copy)
@@ -526,6 +526,7 @@ lto_output_node (struct lto_simple_outpu
bp_pack_value (&bp, node->alias, 1);
bp_pack_value (&bp, node->transparent_alias, 1);
bp_pack_value (&bp, node->weakref, 1);
+ bp_pack_value (&bp, node->symver, 1);
bp_pack_value (&bp, node->frequency, 2);
bp_pack_value (&bp, node->only_called_at_startup, 1);
bp_pack_value (&bp, node->only_called_at_exit, 1);
@@ -606,6 +607,7 @@ lto_output_varpool_node (struct lto_simp
bp_pack_value (&bp, node->alias, 1);
bp_pack_value (&bp, node->transparent_alias, 1);
bp_pack_value (&bp, node->weakref, 1);
+ bp_pack_value (&bp, node->symver, 1);
bp_pack_value (&bp, node->analyzed && (!boundary_p || node->alias), 1);
gcc_assert (node->definition || !node->analyzed);
/* Constant pool initializers can be de-unified into individual ltrans units.
@@ -1170,6 +1172,7 @@ input_overwrite_node (struct lto_file_de
node->alias = bp_unpack_value (bp, 1);
node->transparent_alias = bp_unpack_value (bp, 1);
node->weakref = bp_unpack_value (bp, 1);
+ node->symver = bp_unpack_value (bp, 1);
node->frequency = (enum node_frequency)bp_unpack_value (bp, 2);
node->only_called_at_startup = bp_unpack_value (bp, 1);
node->only_called_at_exit = bp_unpack_value (bp, 1);
@@ -1363,6 +1366,7 @@ input_varpool_node (struct lto_file_decl
node->alias = bp_unpack_value (&bp, 1);
node->transparent_alias = bp_unpack_value (&bp, 1);
node->weakref = bp_unpack_value (&bp, 1);
+ node->symver = bp_unpack_value (&bp, 1);
node->analyzed = bp_unpack_value (&bp, 1);
node->used_from_other_partition = bp_unpack_value (&bp, 1);
node->in_other_partition = bp_unpack_value (&bp, 1);
Index: output.h
===================================================================
--- output.h (revision 278220)
+++ output.h (working copy)
@@ -167,6 +167,7 @@ extern int decode_reg_name (const char *
extern int decode_reg_name_and_count (const char *, int *);
extern void do_assemble_alias (tree, tree);
+extern void do_assemble_symver (tree, tree);
extern void default_assemble_visibility (tree, int);
Index: params.opt
===================================================================
--- params.opt (revision 278220)
+++ params.opt (working copy)
@@ -483,7 +483,7 @@ Common Joined UInteger Var(param_max_inl
The maximum number of instructions in a single function eligible for inlining with -O3 and -Ofast.
-param=max-inline-insns-single-O2=
-Common Joined UInteger Var(param_max_inline_insns_single_o2) Init(30) Param
+Common Joined UInteger Var(param_max_inline_insns_single_o2) Init(70) Param
The maximum number of instructions in a single function eligible for inlining.
-param=max-inline-insns-size=
Index: symtab.c
===================================================================
--- symtab.c (revision 278220)
+++ symtab.c (working copy)
@@ -848,6 +848,8 @@ symtab_node::dump_base (FILE *f)
fprintf (f, " transparent_alias");
if (weakref)
fprintf (f, " weakref");
+ if (symver)
+ fprintf (f, " symver");
if (cpp_implicit_alias)
fprintf (f, " cpp_implicit_alias");
if (alias_target)
@@ -1145,6 +1147,11 @@ symtab_node::verify_base (void)
error ("node is transparent_alias but not an alias");
error_found = true;
}
+ if (symver && !alias)
+ {
+ error ("node is symver but not alias");
+ error_found = true;
+ }
if (same_comdat_group)
{
symtab_node *n = same_comdat_group;
Index: varasm.c
===================================================================
--- varasm.c (revision 278220)
+++ varasm.c (working copy)
@@ -5960,6 +5961,23 @@ do_assemble_alias (tree decl, tree targe
#endif
}
+/* Output .symver directive. */
+
+void
+do_assemble_symver (tree decl, tree target)
+{
+ tree id = DECL_ASSEMBLER_NAME (decl);
+ ultimate_transparent_alias_target (&id);
+ ultimate_transparent_alias_target (&target);
+#ifdef ASM_OUTPUT_SYMVER_DIRECTIVE
+ ASM_OUTPUT_SYMVER_DIRECTIVE (asm_out_file,
+ IDENTIFIER_POINTER (target),
+ IDENTIFIER_POINTER (id));
+#else
+ error ("symver is only supported on ELF platforms");
+#endif
+}
+
/* Emit an assembler directive to make the symbol for DECL an alias to
the symbol for TARGET. */
Index: varpool.c
===================================================================
--- varpool.c (revision 278220)
+++ varpool.c (working copy)
@@ -540,7 +540,10 @@ varpool_node::assemble_aliases (void)
FOR_EACH_ALIAS (this, ref)
{
varpool_node *alias = dyn_cast <varpool_node *> (ref->referring);
- if (!alias->transparent_alias)
+ if (alias->symver)
+ do_assemble_symver (alias->decl,
+ DECL_ASSEMBLER_NAME (decl));
+ else if (!alias->transparent_alias)
do_assemble_alias (alias->decl,
DECL_ASSEMBLER_NAME (decl));
alias->assemble_aliases ();
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