Re: [PATCH] Add generic support for "noinit" attribute

[Christophe Lyon <christophe.lyon@linaro.org>]

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Hi,

Thanks for the useful feedback.


On Tue, 16 Jul 2019 at 11:54, Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Thanks for doing this in a generic way.
>
> Christophe Lyon <christophe.lyon@linaro.org> writes:
> > @@ -2224,6 +2234,50 @@ handle_weak_attribute (tree *node, tree name,
> >    return NULL_TREE;
> >  }
> >
> > +/* Handle a "noinit" attribute; arguments as in struct
> > +   attribute_spec.handler.  Check whether the attribute is allowed
> > +   here and add the attribute to the variable decl tree or otherwise
> > +   issue a diagnostic.  This function checks NODE is of the expected
> > +   type and issues diagnostics otherwise using NAME.  If it is not of
> > +   the expected type *NO_ADD_ATTRS will be set to true.  */
> > +
> > +static tree
> > +handle_noinit_attribute (tree * node,
> > +               tree   name,
> > +               tree   args,
> > +               int    flags ATTRIBUTE_UNUSED,
> > +               bool *no_add_attrs)
> > +{
> > +  const char *message = NULL;
> > +
> > +  gcc_assert (DECL_P (*node));
> > +  gcc_assert (args == NULL);
> > +
> > +  if (TREE_CODE (*node) != VAR_DECL)
> > +    message = G_("%qE attribute only applies to variables");
> > +
> > +  /* Check that it's possible for the variable to have a section.  */
> > +  if ((TREE_STATIC (*node) || DECL_EXTERNAL (*node) || in_lto_p)
> > +      && DECL_SECTION_NAME (*node))
> > +    message = G_("%qE attribute cannot be applied to variables "
> > +              "with specific sections");
> > +
> > +  /* If this var is thought to be common, then change this.  Common
> > +     variables are assigned to sections before the backend has a
> > +     chance to process them.  */
> > +  if (DECL_COMMON (*node))
> > +    DECL_COMMON (*node) = 0;
> > +
> > +  if (message)
> > +    {
> > +      warning (OPT_Wattributes, message, name);
> > +      *no_add_attrs = true;
> > +    }
> > +
> > +  return NULL_TREE;
> > +}
>
> This might cause us to clear DECL_COMMON even when rejecting the
> attribute.  Also, the first message should win over the others
> (e.g. for a function in a particular section).
>
> So I think the "/* Check that it's possible ..." should be an else-if
> and the DECL_COMMON stuff should be specific to !message.

You are right, thanks.

Jozef, this is true for msp430_data_attr() too. I'll let you update it.

>
> Since this is specific to ELF targets, I think we should also
> warn if !targetm.have_switchable_bss_sections.
>
OK

> > @@ -2338,6 +2336,8 @@ msp430_select_section (tree decl, int reloc, unsigned HOST_WIDE_INT align)
> >      {
> >        if (TREE_CODE (decl) == FUNCTION_DECL)
> >       return text_section;
> > +      /* FIXME: ATTR_NOINIT is handled generically in
> > +      default_elf_select_section.  */
> >        else if (has_attr (ATTR_NOINIT, decl))
> >       return noinit_section;
> >        else if (has_attr (ATTR_PERSIST, decl))
>
> I guess adding a FIXME is OK.  It's very tempting to remove
> the noinit stuff and use default_elf_select_section instead of
> default_select_section, but I realise that'd be difficult to test.

I added that as suggested by Jozef, it's best if he handles the
change you propose, I guess he can do proper testing.


> > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> > index f2619e1..850153e 100644
> > --- a/gcc/doc/extend.texi
> > +++ b/gcc/doc/extend.texi
> > @@ -7129,6 +7129,12 @@ The @code{visibility} attribute is described in
> >  The @code{weak} attribute is described in
> >  @ref{Common Function Attributes}.
> >
> > +@item noinit
> > +@cindex @code{noinit} variable attribute
> > +Any data with the @code{noinit} attribute will not be initialized by
> > +the C runtime startup code, or the program loader.  Not initializing
> > +data in this way can reduce program startup times.
> > +
> >  @end table
> >
> >  @node ARC Variable Attributes
>
> Would be good to mention that the attribute is specific to ELF targets.
> (Yeah, we don't seem to do that consistently for other attributes.)
> It might also be worth saying that it requires specific linker support.
OK, done.

>
> > diff --git a/gcc/testsuite/gcc.c-torture/execute/noinit-attribute.c b/gcc/testsuite/gcc.c-torture/execute/noinit-attribute.c
> > new file mode 100644
> > index 0000000..f33c0d0
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.c-torture/execute/noinit-attribute.c
> > @@ -0,0 +1,59 @@
> > +/* { dg-do run } */
> > +/* { dg-require-effective-target noinit */
> > +/* { dg-options "-O2" } */
> > +
> > +/* This test checks that noinit data is handled correctly.  */
> > +
> > +extern void _start (void) __attribute__ ((noreturn));
> > +extern void abort (void) __attribute__ ((noreturn));
> > +extern void exit (int) __attribute__ ((noreturn));
> > +
> > +int var_common;
> > +int var_zero = 0;
> > +int var_one = 1;
> > +int __attribute__((noinit)) var_noinit;
> > +int var_init = 2;
> > +
> > +int __attribute__((noinit)) func(); /* { dg-warning "attribute only applies to variables" } */
> > +int __attribute__((section ("mysection"), noinit)) var_section1; /* { dg-warning "because it conflicts with attribute" } */
> > +int __attribute__((noinit, section ("mysection"))) var_section2; /* { dg-warning "because it conflicts with attribute" } */
> > +
> > +
> > +int
> > +main (void)
> > +{
> > +  /* Make sure that the C startup code has correctly initialised the ordinary variables.  */
>
> initialized (alas).  Same for the rest of the file.

That was a copy-and-paste from msp430 testcase; Jozef, you have 3
typos to fix :-)

> > +  if (var_common != 0)
> > +    abort ();
> > +
> > +  /* Initialised variables are not re-initialised during startup, so
> > +     check their original values only during the first run of this
> > +     test.  */
> > +  if (var_init == 2)
> > +    if (var_zero != 0 || var_one != 1)
> > +      abort ();
> > +
> > +  switch (var_init)
> > +    {
> > +    case 2:
> > +      /* First time through - change all the values.  */
> > +      var_common = var_zero = var_one = var_noinit = var_init = 3;
> > +      break;
> > +
> > +    case 3:
> > +      /* Second time through - make sure that d has not been reset.  */
> > +      if (var_noinit != 3)
> > +     abort ();
> > +      exit (0);
> > +
> > +    default:
> > +      /* Any other value for var_init is an error.  */
> > +      abort ();
> > +    }
> > +
> > +  /* Simulate a processor reset by calling the C startup code.  */
> > +  _start ();
> > +
> > +  /* Should never reach here.  */
> > +  abort ();
> > +}
> > diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
> > index 815e837..ae05c0a 100644
> > --- a/gcc/testsuite/lib/target-supports.exp
> > +++ b/gcc/testsuite/lib/target-supports.exp
> > @@ -364,6 +364,18 @@ proc check_weak_override_available { } {
> >      return [check_weak_available]
> >  }
> >
> > +# The noinit attribute is only supported by some targets.
> > +# This proc returns 1 if it's supported, 0 if it's not.
> > +
> > +proc check_effective_target_noinit { } {
> > +    if { [istarget arm*-*-eabi]
> > +      || [istarget msp430-*-*] } {
> > +     return 1
> > +    }
> > +
> > +    return 0
> > +}
> > +
>
> Should be documented in sourcebuild.texi.  (Sometimes wonder how many
> people actually use that instead of just reading this file.)
>
Sigh..... I keep forgetting this.

> > diff --git a/gcc/varasm.c b/gcc/varasm.c
> > index 626a4c9..7740e88 100644
> > --- a/gcc/varasm.c
> > +++ b/gcc/varasm.c
> > @@ -6428,6 +6428,9 @@ default_section_type_flags (tree decl, const char *name, int reloc)
> >        || strncmp (name, ".gnu.linkonce.tb.", 17) == 0)
> >      flags |= SECTION_TLS | SECTION_BSS;
> >
> > +  if (strcmp (name, ".noinit") == 0)
> > +    flags |= SECTION_WRITE | SECTION_BSS | 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
> > @@ -6748,11 +6751,14 @@ decl_readonly_section (const_tree decl, int reloc)
> >
> >  /* Select a section based on the above categorization.  */
> >
> > +static section *noinit_section = NULL;
> > +
> >  section *
> >  default_elf_select_section (tree decl, int reloc,
> >                           unsigned HOST_WIDE_INT align)
> >  {
> >    const char *sname;
> > +
> >    switch (categorize_decl_for_section (decl, reloc))
> >      {
> >      case SECCAT_TEXT:
> > @@ -6790,6 +6796,14 @@ default_elf_select_section (tree decl, int reloc,
> >        sname = ".tdata";
> >        break;
> >      case SECCAT_BSS:
> > +      if (DECL_P (decl)
> > +       && lookup_attribute ("noinit", DECL_ATTRIBUTES (decl)) != NULL_TREE)
> > +     {
> > +       if (noinit_section == NULL)
> > +         noinit_section = get_named_section (decl, ".noinit", reloc);
> > +       return noinit_section;
> > +     }
> > +
>
> I don't think the special global for noinit_section is worth it, since
> gen_named_section does its own caching.  So IMO we should just have:
>
>   name = ".noinit";
>   break;
OK

> Did you consider supporting .noinit.*, e.g. for -fdata-sections?
Not so far. I don't think we have received such a request yet?

Thanks,

Christophe

> Thanks,
> Richard

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commit 833e8ee8a5ca1ccc53121c7fd86b37e0eadc0086
Author: Christophe Lyon <christophe.lyon@linaro.org>
Date:   Thu Jul 4 14:46:00 2019 +0000

    Add generic support for noinit attribute.
    
    Similar to what already exists for TI msp430 or in TI compilers for
    arm, this patch adds support for the "noinit" attribute.
    
    It is convenient for embedded targets where the user wants to keep the
    value of some data when the program is restarted: such variables are
    not zero-initialized. It is mostly a helper/shortcut to placing
    variables in a dedicated section.
    
    It's probably desirable to add the following chunk to the GNU linker:
    diff --git a/ld/emulparams/armelf.sh b/ld/emulparams/armelf.sh
    index 272a8bc..9555cec 100644
    --- a/ld/emulparams/armelf.sh
    +++ b/ld/emulparams/armelf.sh
    @@ -10,7 +10,19 @@ OTHER_TEXT_SECTIONS='*(.glue_7t) *(.glue_7)
    *(.vfp11_veneer) *(.v4_bx)'
     OTHER_BSS_SYMBOLS="${CREATE_SHLIB+PROVIDE (}__bss_start__ =
    .${CREATE_SHLIB+)};"
     OTHER_BSS_END_SYMBOLS="${CREATE_SHLIB+PROVIDE (}_bss_end__ =
    .${CREATE_SHLIB+)}; ${CREATE_SHLIB+PROVIDE (}__bss_end__ =
    .${CREATE_SHLIB+)};"
     OTHER_END_SYMBOLS="${CREATE_SHLIB+PROVIDE (}__end__ = .${CREATE_SHLIB+)};"
     -OTHER_SECTIONS='.note.gnu.arm.ident 0 : { KEEP (*(.note.gnu.arm.ident)) }'
     +OTHER_SECTIONS='
     +.note.gnu.arm.ident 0 : { KEEP (*(.note.gnu.arm.ident)) }
     +  /* This section contains data that is not initialised during load
     +     *or* application reset.  */
     +   .noinit (NOLOAD) :
     +   {
     +     . = ALIGN(2);
     +     PROVIDE (__noinit_start = .);
     +     *(.noinit)
     +     . = ALIGN(2);
     +     PROVIDE (__noinit_end = .);
     +   }
     +'
    
    so that the noinit section has the "NOLOAD" flag.
    
    I'll submit that part separately to the binutils project if OK.
    
    However, I'm not sure for which other targets (beyond arm), I should
    update the linker scripts.
    
    I added a testcase if gcc.c-torture/execute, gated by the new noinit
    effective-target.
    
    Finally, I tested on arm-eabi, but not on msp430 for which I do not
    have the environment, so advice from msp430 maintainers is
    appreciated.
    
    Thanks,
    
    Christophe
    
    gcc/ChangeLog:
    
    2019-07-04  Christophe Lyon  <christophe.lyon@linaro.org>
    
    	* doc/extend.texi: Add "noinit" attribute documentation.
    	* doc/sourcebuild.texi: Add noinit effective target documentation.
    	* varasm.c
    	(default_section_type_flags): Add support for "noinit" section.
    	(default_elf_select_section): Add support for "noinit" attribute.
    
    gcc/c-family/ChangeLog:
    
    2019-07-04  Christophe Lyon  <christophe.lyon@linaro.org>
    
    	* c-attribs.c (c_common_attribute_table): Add "noinit" entry. Add
    	exclusion with "section" attribute.
    	(attr_noinit_exclusions): New table.
    	(handle_noinit_attribute): New function.
    
    gcc/config/ChangeLog:
    
    2019-07-04  Christophe Lyon  <christophe.lyon@linaro.org>
    
    	* msp430/msp430.c (msp430_attribute_table): Remove "noinit" entry.
    
    gcc/testsuite/ChangeLog:
    
    2019-07-04  Christophe Lyon  <christophe.lyon@linaro.org>
    
    	* lib/target-supports.exp (check_effective_target_noinit): New
    	proc.
            * gcc.c-torture/execute/noinit-attribute.c: New test.
    
    Change-Id: Id8e0baa728d187e05541c7520bd5726ccf803c4f

diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
index 48819e7..77ed56d 100644
--- a/gcc/c-family/c-attribs.c
+++ b/gcc/c-family/c-attribs.c
@@ -92,6 +92,7 @@ static tree handle_section_attribute (tree *, tree, tree, int, bool *);
 static tree handle_aligned_attribute (tree *, tree, tree, int, bool *);
 static tree handle_warn_if_not_aligned_attribute (tree *, tree, tree,
 						  int, bool *);
+static tree handle_noinit_attribute (tree *, tree, tree, int, bool *);
 static tree handle_weak_attribute (tree *, tree, tree, int, bool *) ;
 static tree handle_noplt_attribute (tree *, tree, tree, int, bool *) ;
 static tree handle_alias_ifunc_attribute (bool, tree *, tree, tree, bool *);
@@ -235,6 +236,13 @@ static const struct attribute_spec::exclusions attr_const_pure_exclusions[] =
   ATTR_EXCL (NULL, false, false, false)
 };
 
+static const struct attribute_spec::exclusions attr_noinit_exclusions[] =
+{
+  ATTR_EXCL ("noinit", true, true, true),
+  ATTR_EXCL ("section", true, true, true),
+  ATTR_EXCL (NULL, false, false, false),
+};
+
 /* Table of machine-independent attributes common to all C-like languages.
 
    Current list of processed common attributes: nonnull.  */
@@ -307,7 +315,7 @@ const struct attribute_spec c_common_attribute_table[] =
   { "mode",                   1, 1, false,  true, false, false,
 			      handle_mode_attribute, NULL },
   { "section",                1, 1, true,  false, false, false,
-			      handle_section_attribute, NULL },
+			      handle_section_attribute, attr_noinit_exclusions },
   { "aligned",                0, 1, false, false, false, false,
 			      handle_aligned_attribute,
 	                      attr_aligned_exclusions },
@@ -458,6 +466,8 @@ const struct attribute_spec c_common_attribute_table[] =
 			      handle_nocf_check_attribute, NULL },
   { "copy",                   1, 1, false, false, false, false,
 			      handle_copy_attribute, NULL },
+  { "noinit",		      0, 0, true,  false, false, false,
+			      handle_noinit_attribute, attr_noinit_exclusions },
   { NULL,                     0, 0, false, false, false, false, NULL, NULL }
 };
 
@@ -2224,6 +2234,54 @@ handle_weak_attribute (tree *node, tree name,
   return NULL_TREE;
 }
 
+/* Handle a "noinit" attribute; arguments as in struct
+   attribute_spec.handler.  Check whether the attribute is allowed
+   here and add the attribute to the variable decl tree or otherwise
+   issue a diagnostic.  This function checks NODE is of the expected
+   type and issues diagnostics otherwise using NAME.  If it is not of
+   the expected type *NO_ADD_ATTRS will be set to true.  */
+
+static tree
+handle_noinit_attribute (tree * node,
+		  tree   name,
+		  tree   args,
+		  int    flags ATTRIBUTE_UNUSED,
+		  bool *no_add_attrs)
+{
+  const char *message = NULL;
+
+  gcc_assert (DECL_P (*node));
+  gcc_assert (args == NULL);
+
+  if (TREE_CODE (*node) != VAR_DECL)
+    message = G_("%qE attribute only applies to variables");
+
+  /* Check that it's possible for the variable to have a section.  */
+  else if ((TREE_STATIC (*node) || DECL_EXTERNAL (*node) || in_lto_p)
+	   && DECL_SECTION_NAME (*node))
+    message = G_("%qE attribute cannot be applied to variables "
+		 "with specific sections");
+
+  if (!targetm.have_switchable_bss_sections)
+    message = G_("%qE attribute is specific to ELF targets");
+
+  if (message)
+    {
+      warning (OPT_Wattributes, message, name);
+      *no_add_attrs = true;
+    }
+  else
+  /* If this var is thought to be common, then change this.  Common
+     variables are assigned to sections before the backend has a
+     chance to process them.  Do this only if the attribute is
+     valid.  */
+    if (DECL_COMMON (*node))
+      DECL_COMMON (*node) = 0;
+
+  return NULL_TREE;
+}
+
+
 /* Handle a "noplt" attribute; arguments as in
    struct attribute_spec.handler.  */
 
diff --git a/gcc/config/msp430/msp430.c b/gcc/config/msp430/msp430.c
index 365e9eb..264e852 100644
--- a/gcc/config/msp430/msp430.c
+++ b/gcc/config/msp430/msp430.c
@@ -2108,8 +2108,6 @@ const struct attribute_spec msp430_attribute_table[] =
   { ATTR_EITHER,      0, 0, true,  false, false, false, msp430_section_attr,
     NULL },
 
-  { ATTR_NOINIT,      0, 0, true,  false, false, false, msp430_data_attr,
-    NULL },
   { ATTR_PERSIST,     0, 0, true,  false, false, false, msp430_data_attr,
     NULL },
 
@@ -2338,6 +2336,8 @@ msp430_select_section (tree decl, int reloc, unsigned HOST_WIDE_INT align)
     {
       if (TREE_CODE (decl) == FUNCTION_DECL)
 	return text_section;
+      /* FIXME: ATTR_NOINIT is handled generically in
+	 default_elf_select_section.  */
       else if (has_attr (ATTR_NOINIT, decl))
 	return noinit_section;
       else if (has_attr (ATTR_PERSIST, decl))
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index f2619e1..f1af1dc 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -7129,6 +7129,14 @@ The @code{visibility} attribute is described in
 The @code{weak} attribute is described in
 @ref{Common Function Attributes}.
 
+@item noinit
+@cindex @code{noinit} variable attribute
+Any data with the @code{noinit} attribute will not be initialized by
+the C runtime startup code, or the program loader.  Not initializing
+data in this way can reduce program startup times.  Specific to ELF
+targets, this attribute relies on the linker to place such data in the
+right location.
+
 @end table
 
 @node ARC Variable Attributes
diff --git a/gcc/doc/sourcebuild.texi b/gcc/doc/sourcebuild.texi
index 6a66515..d67608a 100644
--- a/gcc/doc/sourcebuild.texi
+++ b/gcc/doc/sourcebuild.texi
@@ -2302,6 +2302,9 @@ Target uses natural alignment (aligned to type size) for types of
 Target uses natural alignment (aligned to type size) for types of
 64 bits or less.
 
+@item noinit
+Target supports the @code{noinit} variable attribute.
+
 @item nonpic
 Target does not generate PIC by default.
 
diff --git a/gcc/testsuite/gcc.c-torture/execute/noinit-attribute.c b/gcc/testsuite/gcc.c-torture/execute/noinit-attribute.c
new file mode 100644
index 0000000..ffcf8c6
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/noinit-attribute.c
@@ -0,0 +1,59 @@
+/* { dg-do run } */
+/* { dg-require-effective-target noinit */
+/* { dg-options "-O2" } */
+
+/* This test checks that noinit data is handled correctly.  */
+
+extern void _start (void) __attribute__ ((noreturn));
+extern void abort (void) __attribute__ ((noreturn));
+extern void exit (int) __attribute__ ((noreturn));
+
+int var_common;
+int var_zero = 0;
+int var_one = 1;
+int __attribute__((noinit)) var_noinit;
+int var_init = 2;
+
+int __attribute__((noinit)) func(); /* { dg-warning "attribute only applies to variables" } */
+int __attribute__((section ("mysection"), noinit)) var_section1; /* { dg-warning "because it conflicts with attribute" } */
+int __attribute__((noinit, section ("mysection"))) var_section2; /* { dg-warning "because it conflicts with attribute" } */
+
+
+int
+main (void)
+{
+  /* Make sure that the C startup code has correctly initialized the ordinary variables.  */
+  if (var_common != 0)
+    abort ();
+
+  /* Initialized variables are not re-initialized during startup, so
+     check their original values only during the first run of this
+     test.  */
+  if (var_init == 2)
+    if (var_zero != 0 || var_one != 1)
+      abort ();
+
+  switch (var_init)
+    {
+    case 2:
+      /* First time through - change all the values.  */
+      var_common = var_zero = var_one = var_noinit = var_init = 3;
+      break;
+
+    case 3:
+      /* Second time through - make sure that d has not been reset.  */
+      if (var_noinit != 3)
+	abort ();
+      exit (0);
+
+    default:
+      /* Any other value for var_init is an error.  */
+      abort ();
+    }
+
+  /* Simulate a processor reset by calling the C startup code.  */
+  _start ();
+
+  /* Should never reach here.  */
+  abort ();
+}
diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
index 815e837..ae05c0a 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/gcc/testsuite/lib/target-supports.exp
@@ -364,6 +364,18 @@ proc check_weak_override_available { } {
     return [check_weak_available]
 }
 
+# The noinit attribute is only supported by some targets.
+# This proc returns 1 if it's supported, 0 if it's not.
+
+proc check_effective_target_noinit { } {
+    if { [istarget arm*-*-eabi]
+	 || [istarget msp430-*-*] } {
+	return 1
+    }
+
+    return 0
+}
+
 ###############################
 # proc check_visibility_available { what_kind }
 ###############################
diff --git a/gcc/varasm.c b/gcc/varasm.c
index 626a4c9..6ddab0ce 100644
--- a/gcc/varasm.c
+++ b/gcc/varasm.c
@@ -6428,6 +6428,9 @@ default_section_type_flags (tree decl, const char *name, int reloc)
       || strncmp (name, ".gnu.linkonce.tb.", 17) == 0)
     flags |= SECTION_TLS | SECTION_BSS;
 
+  if (strcmp (name, ".noinit") == 0)
+    flags |= SECTION_WRITE | SECTION_BSS | 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
@@ -6748,11 +6751,14 @@ decl_readonly_section (const_tree decl, int reloc)
 
 /* Select a section based on the above categorization.  */
 
+static section *noinit_section = NULL;
+
 section *
 default_elf_select_section (tree decl, int reloc,
 			    unsigned HOST_WIDE_INT align)
 {
   const char *sname;
+
   switch (categorize_decl_for_section (decl, reloc))
     {
     case SECCAT_TEXT:
@@ -6790,6 +6796,13 @@ default_elf_select_section (tree decl, int reloc,
       sname = ".tdata";
       break;
     case SECCAT_BSS:
+      if (DECL_P (decl)
+	  && lookup_attribute ("noinit", DECL_ATTRIBUTES (decl)) != NULL_TREE)
+	{
+	  sname = ".noinit";
+	  break;
+	}
+
       if (bss_section)
 	return bss_section;
       sname = ".bss";