[PATCH v3] c++: extend cold, hot attributes to classes

[PATCH v3] c++: extend cold, hot attributes to classes

[Javier Martinez]

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Hi Jason,

Regarding the initialization example - no, the set of classes that we
consider cold is more loosely defined.

On Thu, Aug 10, 2023 at 11:01 PM Jason Merrill <jason@redhat.com> wrote:
> Yes, but that's because the implicit op== isn't declared lazily like
> some other special member functions (CLASSTYPE_LAZY_*/lazily_declare_fn)
> which can happen after the class is complete.

I see, thanks. I have fixed this now by injecting it directly from
lazily_declare_fn, works well. Doing it from grokclassfn instead seems to
be a nuisance because the explicit method attribute might be processed
after the class-propagated attribute is injected, which is the wrong way
around for the desired precedence.

> I think it would work to check for (flags & (ATTR_FLAG_FUNCTION_NEXT |
> ATTR_FLAG_DECL_NEXT)) and return without warning in that case.  You'd
> still set *no_add_attr.

Correct, done.

I have added the patch as an attachment, if it garbles it then I will use
git-send-email next time.

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From 684ee3b19463fe7f447fbaa96a7b44522f1ce594 Mon Sep 17 00:00:00 2001
From: Javier Martinez <javier.martinez.bugzilla@gmail.com>
Date: Thu, 10 Aug 2023 17:08:27 +0200
Subject: [PATCH v3] c++: extend cold, hot attributes to classes

Signed-off-by: Javier Martinez <javier.martinez.bugzilla@gmail.com>

gcc/c-family/ChangeLog:

        * c-attribs.cc (handle_hot_attribute): remove warning on RECORD_TYPE
        and UNION_TYPE when in c_dialect_xx.
        (handle_cold_attribute): Likewise

gcc/cp/ChangeLog:

        * class.cc (propagate_class_warmth_attribute): New function.
        (finish_struct): propagate hot and cold attributes to all
        FUNCTION_DECL when the record is marked hot or cold.
        * cp-tree.h (maybe_propagate_warmth_attributes): New function.
        * decl2.cc (maybe_propagate_warmth_attributes): New function.
        * method.cc (lazily_declare_fn): propagate hot and cold
        attributes to lazily declared functions when the record is
        marked hot or cold.

gcc/testsuite/ChangeLog:

        * g++.dg/ext/attr-hotness.C: New test.

---
 gcc/c-family/c-attribs.cc               | 48 +++++++++++++++++++++++--
 gcc/cp/class.cc                         | 29 +++++++++++++++
 gcc/cp/cp-tree.h                        |  1 +
 gcc/cp/decl2.cc                         | 37 +++++++++++++++++++
 gcc/cp/method.cc                        |  6 ++++
 gcc/testsuite/g++.dg/ext/attr-hotness.C | 16 +++++++++
 6 files changed, 135 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/ext/attr-hotness.C

diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
index e2792ca6898..bfd2ff194b5 100644
--- a/gcc/c-family/c-attribs.cc
+++ b/gcc/c-family/c-attribs.cc
@@ -452,10 +452,10 @@ const struct attribute_spec c_common_attribute_table[] =
   { "alloc_size",	      1, 2, false, true, true, false,
 			      handle_alloc_size_attribute,
 	                      attr_alloc_exclusions },
-  { "cold",                   0, 0, true,  false, false, false,
+  { "cold",		      0, 0, false,  false, false, false,
 			      handle_cold_attribute,
 	                      attr_cold_hot_exclusions },
-  { "hot",                    0, 0, true,  false, false, false,
+  { "hot",		      0, 0, false,  false, false, false,
 			      handle_hot_attribute,
 	                      attr_cold_hot_exclusions },
   { "no_address_safety_analysis",
@@ -1110,6 +1110,28 @@ handle_hot_attribute (tree *node, tree name, tree ARG_UNUSED (args),
     {
       /* Attribute hot processing is done later with lookup_attribute.  */
     }
+  else if ((TREE_CODE (*node) == RECORD_TYPE
+	    || TREE_CODE (*node) == UNION_TYPE)
+	  && c_dialect_cxx ())
+    {
+      /* Check conflict here as decl_attributes will otherwise only catch
+	 it late at the function when the attribute is used on a class.  */
+      tree cold_attr = lookup_attribute ("cold", TYPE_ATTRIBUTES (*node));
+      if (cold_attr)
+	{
+	  warning (OPT_Wattributes, "ignoring attribute %qE because it "
+			"conflicts with attribute %qs", name, "cold");
+	  *no_add_attrs = true;
+	}
+    }
+  else if (flags & ((int) ATTR_FLAG_FUNCTION_NEXT
+		    | (int) ATTR_FLAG_DECL_NEXT))
+    {
+	/* Avoid applying the attribute to a function return type when
+	   used as:  void __attribute ((hot)) foo (void).  It will be
+	   passed to the function.  */
+	*no_add_attrs = true;
+    }
   else
     {
       warning (OPT_Wattributes, "%qE attribute ignored", name);
@@ -1131,6 +1153,28 @@ handle_cold_attribute (tree *node, tree name, tree ARG_UNUSED (args),
     {
       /* Attribute cold processing is done later with lookup_attribute.  */
     }
+  else if ((TREE_CODE (*node) == RECORD_TYPE
+	    || TREE_CODE (*node) == UNION_TYPE)
+	  && c_dialect_cxx ())
+    {
+      /* Check conflict here as decl_attributes will otherwise only catch
+	 it late at the function when the attribute is used on a class.  */
+      tree hot_attr = lookup_attribute ("hot", TYPE_ATTRIBUTES (*node));
+      if (hot_attr)
+	{
+	  warning (OPT_Wattributes, "ignoring attribute %qE because it "
+			"conflicts with attribute %qs", name, "hot");
+	  *no_add_attrs = true;
+	}
+    }
+  else if (flags & ((int) ATTR_FLAG_FUNCTION_NEXT
+		    | (int) ATTR_FLAG_DECL_NEXT))
+    {
+	/* Avoid applying the attribute to a function return type when
+	   used as:  void __attribute ((cold)) foo (void).  It will be
+	   passed to the function.  */
+	*no_add_attrs = true;
+    }
   else
     {
       warning (OPT_Wattributes, "%qE attribute ignored", name);
diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 778759237dc..56573d01ec1 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -205,6 +205,7 @@ static tree get_vcall_index (tree, tree);
 static bool type_maybe_constexpr_default_constructor (tree);
 static bool type_maybe_constexpr_destructor (tree);
 static bool field_poverlapping_p (tree);
+static void propagate_class_warmth_attribute (tree);
 
 /* Set CURRENT_ACCESS_SPECIFIER based on the protection of DECL.  */
 
@@ -7733,6 +7734,30 @@ unreverse_member_declarations (tree t)
     }
 }
 
+/* Classes, structs or unions T marked with hotness attributes propagate
+   the attribute to all methods.  */
+
+void
+propagate_class_warmth_attribute (tree t)
+{
+
+  if (t == NULL_TREE
+      || !(TREE_CODE (t) == RECORD_TYPE
+	   || TREE_CODE (t) == UNION_TYPE))
+    return;
+
+  tree class_has_cold_attr = lookup_attribute ("cold",
+   				TYPE_ATTRIBUTES (t));
+  tree class_has_hot_attr = lookup_attribute ("hot",
+   				TYPE_ATTRIBUTES (t));
+
+  if (class_has_cold_attr || class_has_hot_attr)
+    for (tree f = TYPE_FIELDS (t); f; f = DECL_CHAIN (f))
+      if (TREE_CODE (f) == FUNCTION_DECL)
+	maybe_propagate_warmth_attributes (f, t);
+
+}
+
 tree
 finish_struct (tree t, tree attributes)
 {
@@ -7866,6 +7891,10 @@ finish_struct (tree t, tree attributes)
       && !LAMBDA_TYPE_P (t))
     add_stmt (build_min (TAG_DEFN, t));
 
+  /* This must be done after all lazily declared special member functions
+     have been injected.  */
+  propagate_class_warmth_attribute (t);
+
   return t;
 }
 \f
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index d051ee85f70..4931a45607e 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7033,6 +7033,7 @@ extern int parm_index                           (tree);
 extern tree vtv_start_verification_constructor_init_function (void);
 extern tree vtv_finish_verification_constructor_init_function (tree);
 extern void cp_check_const_attributes (tree);
+extern void maybe_propagate_warmth_attributes (tree, tree);
 
 /* in error.cc */
 extern const char *type_as_string		(tree, int);
diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index b402befba6d..b7c43e10a9d 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -1604,6 +1604,43 @@ cp_check_const_attributes (tree attributes)
     }
 }
 
+/* Copies hot or cold attributes to a function FN if present on the
+   encapsulating class, struct, or union TYPE.  */
+
+void
+maybe_propagate_warmth_attributes (tree fn, tree type)
+{
+  if (fn == NULL_TREE || type == NULL_TREE
+      || !(TREE_CODE (type) == RECORD_TYPE
+	   || TREE_CODE (type) == UNION_TYPE))
+    return;
+
+  tree has_cold_attr = lookup_attribute ("cold", TYPE_ATTRIBUTES (type));
+  tree has_hot_attr = lookup_attribute ("hot", TYPE_ATTRIBUTES (type));
+
+  if (has_cold_attr || has_hot_attr)
+    {
+
+      /* Transparently ignore the new warmth attribute if it
+	 conflicts with a present function attribute.  Otherwise
+	 decl_attribute would still honour the present attribute,
+	 but producing an undesired warning in the process.  */
+
+      if (has_cold_attr)
+	{
+	  if (lookup_attribute ("hot", DECL_ATTRIBUTES (fn)) == NULL)
+	    decl_attributes (&fn,
+		tree_cons (get_identifier ("cold"), NULL, NULL), 0);
+	}
+      else if (has_hot_attr)
+	{
+	  if (lookup_attribute ("cold", DECL_ATTRIBUTES (fn)) == NULL)
+	    decl_attributes (&fn,
+		tree_cons (get_identifier ("hot"), NULL, NULL), 0);
+	}
+    }
+}
+
 /* Return the last pushed declaration for the symbol DECL or NULL
    when no such declaration exists.  */
 
diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 4ba00175048..741d1049725 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -3592,6 +3592,12 @@ lazily_declare_fn (special_function_kind sfk, tree type)
     /* Create appropriate clones.  */
     clone_cdtor (fn, /*update_methods=*/true);
 
+/* Classes, structs or unions TYPE marked with hotness attributes propagate
+   the attribute to all methods.  This is typically done in finish_struct,
+   but we must also inject them for deferred lazily_declared functions.  */
+
+  maybe_propagate_warmth_attributes (fn, type);
+
   return fn;
 }
 
diff --git a/gcc/testsuite/g++.dg/ext/attr-hotness.C b/gcc/testsuite/g++.dg/ext/attr-hotness.C
new file mode 100644
index 00000000000..f9a6930304d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attr-hotness.C
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O0 -Wattributes -fdump-tree-gimple" } */
+
+
+struct __attribute((cold)) A { __attribute((noinline, used)) void foo(void) { } };
+
+struct __attribute((hot)) B { __attribute((noinline, used)) void foo(void) { } };
+
+struct __attribute((hot, cold)) C { __attribute((noinline, used)) void foo(void) { } }; /* { dg-warning "ignoring attribute .cold. because it conflicts with attribute .hot." } */
+
+struct __attribute((cold, hot)) D { __attribute((noinline, used)) void foo(void) { } }; /* { dg-warning "ignoring attribute .hot. because it conflicts with attribute .cold." } */
+
+
+/* { dg-final { scan-tree-dump-times "cold" 2 "gimple" } } */
+/* { dg-final { scan-tree-dump-times "hot" 2 "gimple" } } */
+
-- 
2.34.1

Re: [PATCH v3] c++: extend cold, hot attributes to classes

[Jason Merrill]

On 8/11/23 09:18, Javier Martinez wrote:
> Hi Jason,
> 
> Regarding the initialization example - no, the set of classes that we 
> consider cold is more loosely defined.
> 
> On Thu, Aug 10, 2023 at 11:01 PM Jason Merrill <jason@redhat.com 
> <mailto:jason@redhat.com>> wrote:
>  > Yes, but that's because the implicit op== isn't declared lazily like
>  > some other special member functions (CLASSTYPE_LAZY_*/lazily_declare_fn)
>  > which can happen after the class is complete.
> 
> I see, thanks. I have fixed this now by injecting it directly from 
> lazily_declare_fn, works well. Doing it from grokclassfn instead seems 
> to be a nuisance because the explicit method attribute might be 
> processed after the class-propagated attribute is injected, which is the 
> wrong way around for the desired precedence.
> 
>  > I think it would work to check for (flags & (ATTR_FLAG_FUNCTION_NEXT |
>  > ATTR_FLAG_DECL_NEXT)) and return without warning in that case.  You'd
>  > still set *no_add_attr.
> 
> Correct, done.
> 
> I have added the patch as an attachment, if it garbles it then I will 
> use git-send-email next time.

That worked fine, thanks.

> @@ -1110,6 +1110,28 @@ handle_hot_attribute (tree *node, tree name, tree ARG_UNUSED (args),
>      {
>        /* Attribute hot processing is done later with lookup_attribute.  */
>      }
> +  else if ((TREE_CODE (*node) == RECORD_TYPE
> +	    || TREE_CODE (*node) == UNION_TYPE)
> +	  && c_dialect_cxx ())

I think you also want to check for ATTR_FLAG_TYPE_IN_PLACE.

> @@ -7866,6 +7891,10 @@ finish_struct (tree t, tree attributes)
>        && !LAMBDA_TYPE_P (t))
>      add_stmt (build_min (TAG_DEFN, t));
>  
> +  /* This must be done after all lazily declared special member functions
> +     have been injected.  */
> +  propagate_class_warmth_attribute (t);

Maybe call this in check_bases_and_members instead?

Jason