[PATCH v2] ipa-visibility: Optimize TLS access [PR99619]

[Alexander Monakov <amonakov@ispras.ru>]

From: Artem Klimov <jakmobius@gmail.com>

Fix PR99619, which asks to optimize TLS model based on visibility.
The fix is implemented as an IPA optimization: this allows to take
optimized visibility status into account (as well as avoid modifying
all language frontends).

2022-04-17  Artem Klimov  <jakmobius@gmail.com>

gcc/ChangeLog:

	* ipa-visibility.cc (function_and_variable_visibility): Promote
	TLS access model afer visibility optimizations.
	* varasm.cc (have_optimized_refs): New helper.
	(optimize_dyn_tls_for_decl_p): New helper. Use it ...
	(decl_default_tls_model): ... here in place of 'optimize' check.

gcc/testsuite/ChangeLog:

	* gcc.dg/tls/vis-attr-gd.c: New test.
	* gcc.dg/tls/vis-attr-hidden-gd.c: New test.
	* gcc.dg/tls/vis-attr-hidden.c: New test.
	* gcc.dg/tls/vis-flag-hidden-gd.c: New test.
	* gcc.dg/tls/vis-flag-hidden.c: New test.
	* gcc.dg/tls/vis-pragma-hidden-gd.c: New test.
	* gcc.dg/tls/vis-pragma-hidden.c: New test.

Co-Authored-By:  Alexander Monakov  <amonakov@gcc.gnu.org>
Signed-off-by: Artem Klimov <jakmobius@gmail.com>
---

v2: run the new loop in ipa-visibility only in the whole-program IPA pass;
    in decl_default_tls_model, check if any referring function is optimized
    when 'optimize == 0' (when running in LTO mode)


Note for reviewers: I noticed there's a place which tries to avoid TLS
promotion, but the comment seems wrong and I could not find a testcase.
I'd suggest we remove it. The compiler can only promote general-dynamic
to local-dynamic and initial-exec to local-exec. The comment refers to
promoting x-dynamic to y-exec, but that cannot happen AFAICT:
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=8e1ba78f1b8eedd6c65c6f0e6d6d09a801de5d3d


 gcc/ipa-visibility.cc                         | 19 +++++++++++
 gcc/testsuite/gcc.dg/tls/vis-attr-gd.c        | 12 +++++++
 gcc/testsuite/gcc.dg/tls/vis-attr-hidden-gd.c | 13 ++++++++
 gcc/testsuite/gcc.dg/tls/vis-attr-hidden.c    | 12 +++++++
 gcc/testsuite/gcc.dg/tls/vis-flag-hidden-gd.c | 13 ++++++++
 gcc/testsuite/gcc.dg/tls/vis-flag-hidden.c    | 12 +++++++
 .../gcc.dg/tls/vis-pragma-hidden-gd.c         | 17 ++++++++++
 gcc/testsuite/gcc.dg/tls/vis-pragma-hidden.c  | 16 ++++++++++
 gcc/varasm.cc                                 | 32 ++++++++++++++++++-
 9 files changed, 145 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/tls/vis-attr-gd.c
 create mode 100644 gcc/testsuite/gcc.dg/tls/vis-attr-hidden-gd.c
 create mode 100644 gcc/testsuite/gcc.dg/tls/vis-attr-hidden.c
 create mode 100644 gcc/testsuite/gcc.dg/tls/vis-flag-hidden-gd.c
 create mode 100644 gcc/testsuite/gcc.dg/tls/vis-flag-hidden.c
 create mode 100644 gcc/testsuite/gcc.dg/tls/vis-pragma-hidden-gd.c
 create mode 100644 gcc/testsuite/gcc.dg/tls/vis-pragma-hidden.c

diff --git a/gcc/ipa-visibility.cc b/gcc/ipa-visibility.cc
index 8a27e7bcd..3ed2b7cf6 100644
--- a/gcc/ipa-visibility.cc
+++ b/gcc/ipa-visibility.cc
@@ -873,6 +873,25 @@ function_and_variable_visibility (bool whole_program)
 	}
     }
 
+  if (symtab->state >= IPA_SSA)
+    {
+      FOR_EACH_VARIABLE (vnode)
+	{
+	  tree decl = vnode->decl;
+
+	  /* Upgrade TLS access model based on optimized visibility status,
+	     unless it was specified explicitly or no references remain.  */
+	  if (DECL_THREAD_LOCAL_P (decl)
+	      && !lookup_attribute ("tls_model", DECL_ATTRIBUTES (decl))
+	      && vnode->ref_list.referring.length ())
+	    {
+	      enum tls_model new_model = decl_default_tls_model (decl);
+	      gcc_checking_assert (new_model >= decl_tls_model (decl));
+	      set_decl_tls_model (decl, new_model);
+	    }
+	}
+    }
+
   if (dump_file)
     {
       fprintf (dump_file, "\nMarking local functions:");
diff --git a/gcc/varasm.cc b/gcc/varasm.cc
index 4db8506b1..de149e82c 100644
--- a/gcc/varasm.cc
+++ b/gcc/varasm.cc
@@ -6679,6 +6679,36 @@ init_varasm_once (void)
 #endif
 }
 
+/* Determine whether SYMBOL is used in any optimized function.  */
+
+static bool
+have_optimized_refs (struct symtab_node *symbol)
+{
+  struct ipa_ref *ref;
+
+  for (int i = 0; symbol->iterate_referring (i, ref); i++)
+    {
+      cgraph_node *cnode = dyn_cast <cgraph_node *> (ref->referring);
+
+      if (cnode && opt_for_fn (cnode->decl, optimize))
+	return true;
+    }
+
+  return false;
+}
+
+/* Check if promoting general-dynamic TLS access model to local-dynamic is
+   desirable for DECL.  */
+
+static bool
+optimize_dyn_tls_for_decl_p (const_tree decl)
+{
+  if (optimize)
+    return true;
+  return symtab->state >= IPA && have_optimized_refs (symtab_node::get (decl));
+}
+
+
 enum tls_model
 decl_default_tls_model (const_tree decl)
 {
@@ -6696,7 +6726,7 @@ decl_default_tls_model (const_tree decl)
 
   /* Local dynamic is inefficient when we're not combining the
      parts of the address.  */
-  else if (optimize && is_local)
+  else if (is_local && optimize_dyn_tls_for_decl_p (decl))
     kind = TLS_MODEL_LOCAL_DYNAMIC;
   else
     kind = TLS_MODEL_GLOBAL_DYNAMIC;
diff --git a/gcc/testsuite/gcc.dg/tls/vis-attr-gd.c b/gcc/testsuite/gcc.dg/tls/vis-attr-gd.c
new file mode 100644
index 000000000..89a248a80
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tls/vis-attr-gd.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target fpic } */
+/* { dg-require-effective-target tls } */
+/* { dg-options "-O2 -fPIC -fdump-ipa-whole-program" } */
+
+// tls_model should be global-dynamic due to explicitly specified attribute
+__attribute__((tls_model("global-dynamic")))
+__thread int x;
+
+void reference() { x++; }
+
+/* { dg-final { scan-ipa-dump "Varpool flags: tls-global-dynamic" "whole-program" } } */
diff --git a/gcc/testsuite/gcc.dg/tls/vis-attr-hidden-gd.c b/gcc/testsuite/gcc.dg/tls/vis-attr-hidden-gd.c
new file mode 100644
index 000000000..e32565588
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tls/vis-attr-hidden-gd.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target fpic } */
+/* { dg-require-effective-target tls } */
+/* { dg-options "-O2 -fPIC -fdump-ipa-whole-program" } */
+
+// tls_model should be global-dynamic due to explicitly specified attribute
+__attribute__((visibility("hidden")))
+__attribute__((tls_model("global-dynamic")))
+__thread int x;
+
+void reference() { x++; }
+
+/* { dg-final { scan-ipa-dump "Varpool flags: tls-global-dynamic" "whole-program" } } */
diff --git a/gcc/testsuite/gcc.dg/tls/vis-attr-hidden.c b/gcc/testsuite/gcc.dg/tls/vis-attr-hidden.c
new file mode 100644
index 000000000..0d43fc565
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tls/vis-attr-hidden.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target fpic } */
+/* { dg-require-effective-target tls } */
+/* { dg-options "-O2 -fPIC -fdump-ipa-whole-program" } */
+
+//tls_model should be local-dynamic due to visibility("hidden")
+__attribute__((visibility("hidden")))
+__thread int x;
+
+void reference() { x++; }
+
+/* { dg-final { scan-ipa-dump "Varpool flags: tls-local-dynamic" "whole-program" } } */
diff --git a/gcc/testsuite/gcc.dg/tls/vis-flag-hidden-gd.c b/gcc/testsuite/gcc.dg/tls/vis-flag-hidden-gd.c
new file mode 100644
index 000000000..cad41e0c8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tls/vis-flag-hidden-gd.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target fpic } */
+/* { dg-require-effective-target tls } */
+/* { dg-options "-O2 -fPIC -fdump-ipa-whole-program -fvisibility=hidden" } */
+
+
+// tls_model should be global-dynamic due to explicitly specified attribute
+__attribute__((tls_model("global-dynamic")))
+__thread int x;
+
+void reference() { x++; }
+
+/* { dg-final { scan-ipa-dump "Varpool flags: tls-global-dynamic" "whole-program" } } */
diff --git a/gcc/testsuite/gcc.dg/tls/vis-flag-hidden.c b/gcc/testsuite/gcc.dg/tls/vis-flag-hidden.c
new file mode 100644
index 000000000..a15df092d
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tls/vis-flag-hidden.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target fpic } */
+/* { dg-require-effective-target tls } */
+/* { dg-options "-O2 -fPIC -fdump-ipa-whole-program -fvisibility=hidden" } */
+
+
+// tls_model should be local-dynamic due to -fvisibility=hidden
+__thread int x;
+
+void reference() { x++; }
+
+/* { dg-final { scan-ipa-dump "Varpool flags: tls-local-dynamic" "whole-program" } } */
diff --git a/gcc/testsuite/gcc.dg/tls/vis-pragma-hidden-gd.c b/gcc/testsuite/gcc.dg/tls/vis-pragma-hidden-gd.c
new file mode 100644
index 000000000..3b3598134
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tls/vis-pragma-hidden-gd.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target fpic } */
+/* { dg-require-effective-target tls } */
+/* { dg-options "-O2 -fPIC -fdump-ipa-whole-program" } */
+
+
+#pragma GCC visibility push(hidden)
+
+// tls_model should be global-dynamic due to explicitly specified attribute
+__attribute__((tls_model("global-dynamic")))
+__thread int x;
+
+#pragma GCC visibility pop
+
+void reference() { x++; }
+
+/* { dg-final { scan-ipa-dump "Varpool flags: tls-global-dynamic" "whole-program" } } */
diff --git a/gcc/testsuite/gcc.dg/tls/vis-pragma-hidden.c b/gcc/testsuite/gcc.dg/tls/vis-pragma-hidden.c
new file mode 100644
index 000000000..1be976442
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tls/vis-pragma-hidden.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target fpic } */
+/* { dg-require-effective-target tls } */
+/* { dg-options "-O2 -fPIC -fdump-ipa-whole-program" } */
+
+
+#pragma GCC visibility push(hidden)
+
+// tls_model should be local-dynamic due to a pragma
+__thread int x;
+
+#pragma GCC visibility pop
+
+void reference() { x++; }
+
+/* { dg-final { scan-ipa-dump "Varpool flags: tls-local-dynamic" "whole-program" } } */
-- 
2.35.1