Implement the "persistent" attribute

The "persistent" attribute is used for variables that are initialized
by the program loader, but are not initialized by the runtime startup
code. "persistent" variables are placed in a non-volatile area of
memory, which allows their value to "persist" between processor resets.

gcc/c-family/ChangeLog:

	* c-attribs.c (handle_special_var_sec_attribute): New.
	(handle_noinit_attribute): Remove.
	(attr_noinit_exclusions): Rename to...
	(attr_section_exclusions): ...this, and add "persistent" attribute
	exclusion.
	(c_common_attribute_table): Add "persistent" attribute.

gcc/ChangeLog:

	* doc/extend.texi (Common Variable Attributes): Document the
	"persistent" variable attribute.
	* doc/sourcebuild.texi (Effective-Target Keywords): Document
	the "persistent" effective target keyword.
	* tree.h (DECL_PERSISTENT_P): Define.
	* varasm.c (bss_initializer_p): Return false for a
	DECL_PERSISTENT_P decl initialized to zero.
	(default_section_type_flags): Handle the ".persistent" section.
	(default_elf_select_section): Likewise.
	(default_unique_section): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/execute/noinit-attribute.c: Moved to...
	* c-c++-common/torture/attr-noinit-main.inc: ...here.
	* lib/target-supports.exp (check_effective_target_persistent): New.
	* c-c++-common/torture/attr-noinit-1.c: New test.
	* c-c++-common/torture/attr-noinit-2.c: New test.
	* c-c++-common/torture/attr-noinit-3.c: New test.
	* c-c++-common/torture/attr-noinit-invalid.c: New test.
	* c-c++-common/torture/attr-persistent-1.c: New test.
	* c-c++-common/torture/attr-persistent-2.c: New test.
	* c-c++-common/torture/attr-persistent-3.c: New test.
	* c-c++-common/torture/attr-persistent-invalid.c: New test.
	* c-c++-common/torture/attr-persistent-main.inc: New test.

1 files changed, 18 insertions, 1 deletions

diff --git a/gcc/varasm.c b/gcc/varasm.c
index da7d0d7..b92da26 100644
--- a/gcc/varasm.c
+++ b/gcc/varasm.c
@@ -1057,7 +1057,11 @@ bss_initializer_p (const_tree decl, bool named)
 	      || (DECL_INITIAL (decl) == error_mark_node
 	          && !in_lto_p)
 	      || (flag_zero_initialized_in_bss
-	          && initializer_zerop (DECL_INITIAL (decl)))));
+		  && initializer_zerop (DECL_INITIAL (decl))
+		  /* A decl with the "persistent" attribute applied and
+		     explicitly initialized to 0 should not be treated as a BSS
+		     variable.  */
+		  && !DECL_PERSISTENT_P (decl))));
 }
 
 /* Compute the alignment of variable specified by DECL.
@@ -6680,6 +6684,9 @@ default_section_type_flags (tree decl, const char *name, int reloc)
   if (strcmp (name, ".noinit") == 0)
     flags |= SECTION_WRITE | SECTION_BSS | SECTION_NOTYPE;
 
+  if (strcmp (name, ".persistent") == 0)
+    flags |= SECTION_WRITE | SECTION_NOTYPE;
+
   /* Various sections have special ELF types that the assembler will
      assign by default based on the name.  They are neither SHT_PROGBITS
      nor SHT_NOBITS, so when changing sections we don't want to print a
@@ -7023,6 +7030,11 @@ default_elf_select_section (tree decl, int reloc,
       sname = ".sdata2";
       break;
     case SECCAT_DATA:
+      if (DECL_P (decl) && DECL_PERSISTENT_P (decl))
+	{
+	  sname = ".persistent";
+	  break;
+	}
       return data_section;
     case SECCAT_DATA_REL:
       sname = ".data.rel";
@@ -7093,6 +7105,11 @@ default_unique_section (tree decl, int reloc)
       break;
     case SECCAT_DATA:
       prefix = one_only ? ".d" : ".data";
+      if (DECL_P (decl) && DECL_PERSISTENT_P (decl))
+	{
+	  prefix = one_only ? ".p" : ".persistent";
+	  break;
+	}
       break;
     case SECCAT_DATA_REL:
       prefix = one_only ? ".d.rel" : ".data.rel";