Re: [PATCH] c++, TLS: Support cross-tu static initialization for targets without alias support [PR106435].

[Jason Merrill <jason@redhat.com>]

On 12/7/22 10:39, Iain Sandoe wrote:
>   This has been tested on x86_64 and arm64 Darwin and on x86_64 linux gnu.
>   The basic patch is live in the homebrew macOS support and so has had quite
>   wide coverage on non-trivial codebases.
>   
>   OK for master?
>   Iain
>   
>   Since this actually fixes wrong code, I wonder if we should also consider
>   back-porting.
>   
>   --- >8 ---
> 
> The description below relates to the code path when TARGET_SUPPORTS_ALIASES is
> false; current operation is maintained for targets with alias support and any
> new support code should be DCEd in that case.
> 
> --
> 
> Currently, cross-tu static initialisation is not supported for targets without
> alias support.
> 
> The patch adds support by building a shim function in place of the alias for
> these targets; the shim simply calls the generic initialiser.  Although this is
> slightly less efficient than the alias, in practice (for targets that allow
> sibcalls) the penalty is a single jump when code is optimised.
> 
>  From the perspective of a TU referencing an extern TLS variable, there is no
> way to determine if it requires a guarded dynamic init.  So, in the referencing
> TU, we build a weak reference to the potential init and check at runtime if the
> init is present before calling it.  This strategy is fine for targets that have
> ELF semantics, but fails at link time for Mach-O (which does not permit the
> reference to be undefined in the static link).
> 
> The actual initialiser call is contained in a wrapper function, and to resolve
> the Mach-O linker issue, in the TU that is referencing the var, we now generate
> both the wrapper _and_ a weak definition of a dummy init function.  In the case
> that there _is_ a dynamic init (in a different TU), that version will be non-weak
> and will be override the weak dummy one.

IIUC, this isn't reliable in general; in specific, I believe that the 
glibc dynamic loader no longer prefers strong definitions to weak ones.

Perhaps on targets that don't allow weakrefs to be unbound, we should 
unconditionally emit the init function where the variable is defined, 
even if it does nothing, and unconditionally call it from the wrapper?

> In the case that we have a trivial
> static init (so no init in any other TU) the weak-defined dummy init will be
> called (a single return insn for optimised code).  We mitigate the call to
> the dummy init by reworking the wrapper code-gen path to remove the test for
> the weak reference function (as it will always be true) since the static linker
> will now determine the function to be called.
> 
> Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
> 
> 	PR c++/106435
> 
> gcc/c-family/ChangeLog:
> 
> 	* c-opts.cc (c_common_post_options): Allow fextern-tls-init for targets
> 	without alias support.
> 
> gcc/cp/ChangeLog:
> 
> 	* decl2.cc (get_tls_init_fn): Allow targets without alias support.
> 	(handle_tls_init): Emit aliases for single init functions where the
> 	target supporst this, otherwise emit a stub function that calls the
> 	main tls init function.  (generate_tls_dummy_init): New.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/pr106435-b.cc: New file.
> 	* g++.dg/cpp0x/pr106435.C: New test.
> 	* g++.dg/cpp0x/pr106435.h: New file.
> ---
>   gcc/c-family/c-opts.cc                   |  2 +-
>   gcc/cp/decl2.cc                          | 80 ++++++++++++++++++++----
>   gcc/testsuite/g++.dg/cpp0x/pr106435-b.cc | 22 +++++++
>   gcc/testsuite/g++.dg/cpp0x/pr106435.C    | 24 +++++++
>   gcc/testsuite/g++.dg/cpp0x/pr106435.h    | 27 ++++++++
>   5 files changed, 142 insertions(+), 13 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/pr106435-b.cc
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/pr106435.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/pr106435.h
> 
> diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
> index 70745aa4e7c..064645f980d 100644
> --- a/gcc/c-family/c-opts.cc
> +++ b/gcc/c-family/c-opts.cc
> @@ -1070,7 +1070,7 @@ c_common_post_options (const char **pfilename)
>   
>     if (flag_extern_tls_init)
>       {
> -      if (!TARGET_SUPPORTS_ALIASES || !SUPPORTS_WEAK)
> +      if (!SUPPORTS_WEAK)
>   	{
>   	  /* Lazy TLS initialization for a variable in another TU requires
>   	     alias and weak reference support.  */
> diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
> index f95529a5c9a..c6550c0c2fc 100644
> --- a/gcc/cp/decl2.cc
> +++ b/gcc/cp/decl2.cc
> @@ -3672,9 +3672,8 @@ get_tls_init_fn (tree var)
>     if (!flag_extern_tls_init && DECL_EXTERNAL (var))
>       return NULL_TREE;
>   
> -  /* If the variable is internal, or if we can't generate aliases,
> -     call the local init function directly.  */
> -  if (!TREE_PUBLIC (var) || !TARGET_SUPPORTS_ALIASES)
> +  /* If the variable is internal call the local init function directly.  */
> +  if (!TREE_PUBLIC (var))
>       return get_local_tls_init_fn (DECL_SOURCE_LOCATION (var));
>   
>     tree sname = mangle_tls_init_fn (var);
> @@ -3811,8 +3810,12 @@ generate_tls_wrapper (tree fn)
>     if (tree init_fn = get_tls_init_fn (var))
>       {
>         tree if_stmt = NULL_TREE;
> -      /* If init_fn is a weakref, make sure it exists before calling.  */
> -      if (lookup_attribute ("weak", DECL_ATTRIBUTES (init_fn)))
> +
> +      /* If init_fn is a weakref, make sure it exists before calling.
> +	 If the target does not support aliases, then we will have generated
> +	 a dummy weak function, so there is no need to test its existence.  */
> +      if (TARGET_SUPPORTS_ALIASES &&
> +	  lookup_attribute ("weak", DECL_ATTRIBUTES (init_fn)))
>   	{
>   	  if_stmt = begin_if_stmt ();
>   	  tree addr = cp_build_addr_expr (init_fn, tf_warning_or_error);
> @@ -3837,6 +3840,25 @@ generate_tls_wrapper (tree fn)
>     expand_or_defer_fn (finish_function (/*inline_p=*/false));
>   }
>   
> +/* A dummy init function to act as a weak placeholder for a (possibly non-
> +   existent) dynamic init.  */
> +static void
> +generate_tls_dummy_init (tree fn)
> +{
> +  tree var = DECL_BEFRIENDING_CLASSES (fn);
> +  tree init_fn = get_tls_init_fn (var);
> +  /* If have no init fn, or it is non-weak, then we do not need to make a
> +     dummy.  */
> +  if (!init_fn || !lookup_attribute ("weak", DECL_ATTRIBUTES (init_fn)))
> +    return;
> +  start_preparsed_function (init_fn, NULL_TREE, SF_DEFAULT | SF_PRE_PARSED);
> +  tree body = begin_function_body ();
> +  declare_weak (init_fn);
> +  finish_return_stmt (NULL_TREE);
> +  finish_function_body (body);
> +  expand_or_defer_fn (finish_function (/*inline_p=*/false));
> +}
> +
>   /* Start a global constructor or destructor function.  */
>   
>   static tree
> @@ -4626,22 +4648,24 @@ handle_tls_init (void)
>     finish_expr_stmt (cp_build_modify_expr (loc, guard, NOP_EXPR,
>   					  boolean_true_node,
>   					  tf_warning_or_error));
> +  auto_vec<tree> direct_calls;
>     for (; vars; vars = TREE_CHAIN (vars))
>       {
>         tree var = TREE_VALUE (vars);
>         tree init = TREE_PURPOSE (vars);
>         one_static_initialization_or_destruction (/*initp=*/true, var, init);
>   
> -      /* Output init aliases even with -fno-extern-tls-init.  */
> -      if (TARGET_SUPPORTS_ALIASES && TREE_PUBLIC (var))
> +      /* Output inits even with -fno-extern-tls-init.
> +	 We save the list here and output either an alias or a stub function
> +	 below.  */
> +      if (TREE_PUBLIC (var))
>   	{
>             tree single_init_fn = get_tls_init_fn (var);
>   	  if (single_init_fn == NULL_TREE)
>   	    continue;
> -	  cgraph_node *alias
> -	    = cgraph_node::get_create (fn)->create_same_body_alias
> -		(single_init_fn, fn);
> -	  gcc_assert (alias != NULL);
> +	  if (single_init_fn == fn)
> +	    continue;
> +	  direct_calls.safe_push (single_init_fn);
>   	}
>       }
>   
> @@ -4649,6 +4673,30 @@ handle_tls_init (void)
>     finish_if_stmt (if_stmt);
>     finish_function_body (body);
>     expand_or_defer_fn (finish_function (/*inline_p=*/false));
> +
> +  /* For each TLS var that we have an init function, we either emit an alias
> +     between that and the tls_init, or a stub function that just calls the
> +     tls_init.  */
> +  while (!direct_calls.is_empty())
> +    {
> +      tree single_init_fn = direct_calls.pop ();
> +      if (TARGET_SUPPORTS_ALIASES)
> +	{
> +	  cgraph_node *alias
> +	     = cgraph_node::get_create (fn)->create_same_body_alias
> +		(single_init_fn, fn);
> +	  gcc_assert (alias != NULL);
> +	}
> +      else
> +	{
> +	  start_preparsed_function (single_init_fn, NULL_TREE, SF_PRE_PARSED);
> +	  tree body = begin_function_body ();
> +	  tree r = build_call_expr (fn, 0);
> +	  finish_expr_stmt (r);
> +	  finish_function_body (body);
> +	  expand_or_defer_fn (finish_function (/*inline_p=*/false));
> +	}
> +    }
>   }
>   
>   /* We're at the end of compilation, so generate any mangling aliases that
> @@ -5058,7 +5106,15 @@ c_parse_final_cleanups (void)
>   	    }
>   
>   	  if (!DECL_INITIAL (decl) && decl_tls_wrapper_p (decl))
> -	    generate_tls_wrapper (decl);
> +	    {
> +	      generate_tls_wrapper (decl);
> +	      if (!TARGET_SUPPORTS_ALIASES)
> +	      /* The wrapper might have a weak reference to an init, we provide
> +		 a dummy function to satisfy that here.  The linker/dynamic
> +		 loader will override this with the actual init, if one is
> +		 required.  */
> +		generate_tls_dummy_init (decl);
> +	    }
>   
>   	  if (!DECL_SAVED_TREE (decl))
>   	    continue;
> diff --git a/gcc/testsuite/g++.dg/cpp0x/pr106435-b.cc b/gcc/testsuite/g++.dg/cpp0x/pr106435-b.cc
> new file mode 100644
> index 00000000000..bd586286647
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/pr106435-b.cc
> @@ -0,0 +1,22 @@
> +// PR c++/106435
> +#include "pr106435.h"
> +
> +//#include <iostream>
> +
> +Foo::Foo() {
> +  ++num_calls;
> +//  std::cout << "Foo::Foo(this=" << this << ")\n";
> +}
> +
> +int Foo::func() {
> +//  std::cout << "Foo::func(this=" << this << ")\n";
> +  return num_calls;
> +}
> +
> +thread_local Foo Bar::foo;
> +thread_local Foo Bar::baz;
> +
> +thread_local Foo F = Foo{5};
> +
> +thread_local void* tl = (void*)&F;
> +thread_local void* tl1 = nullptr;
> diff --git a/gcc/testsuite/g++.dg/cpp0x/pr106435.C b/gcc/testsuite/g++.dg/cpp0x/pr106435.C
> new file mode 100644
> index 00000000000..9071e032697
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/pr106435.C
> @@ -0,0 +1,24 @@
> +// PR c++/106435
> +// { dg-do run { target c++11 } }
> +// { dg-additional-sources "pr106435-b.cc" }
> +
> +#include "pr106435.h"
> +
> +int num_calls = 0;
> +
> +thread_local Foo Bar::bat;
> +
> +int main() {
> +  int v = Bar::foo.func();
> +  if (v != 2)
> +    __builtin_abort ();
> +  v = Bar::bat.func();
> +  if (v != 3)
> +    __builtin_abort ();
> +  if (F.getX() != 5)
> +    __builtin_abort();
> +  if  (gt_tl () != (void*)&F)
> +    __builtin_abort();
> +  if  (gt_tl1 ())
> +    __builtin_abort();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp0x/pr106435.h b/gcc/testsuite/g++.dg/cpp0x/pr106435.h
> new file mode 100644
> index 00000000000..06d3e60484f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/pr106435.h
> @@ -0,0 +1,27 @@
> +// PR c++/106435
> +#pragma once
> +
> +extern int num_calls;
> +struct Foo {
> +  Foo();
> +  Foo(int _x) : x(_x) {}
> +  int func();
> +  int getX () { return x; }
> +  int x;
> +};
> +
> +struct Bar {
> +  thread_local static Foo foo;
> +  thread_local static Foo baz;
> +  thread_local static Foo bat;
> +};
> +
> +extern thread_local void* tl;
> +
> +inline void* gt_tl () {return tl;}
> +
> +extern thread_local Foo F;
> +
> +extern thread_local void* tl1;
> +
> +inline void* gt_tl1 () {return tl1;}