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

[Tamar Christina <Tamar dot Christina at arm dot com>]

Hi Christoph,

The noinit testcase is currently failing on x86_64.

Is the test supposed to be running there?

Thanks,
Tamar

-----Original Message-----
From: gcc-patches-owner@gcc.gnu.org <gcc-patches-owner@gcc.gnu.org> On Behalf Of Christophe Lyon
Sent: Wednesday, August 14, 2019 2:18 PM
To: Christophe Lyon <christophe.lyon@linaro.org>; Martin Sebor <msebor@gmail.com>; gcc Patches <gcc-patches@gcc.gnu.org>; Richard Earnshaw <Richard.Earnshaw@arm.com>; nickc@redhat.com; Jozef Lawrynowicz <jozef.l@mittosystems.com>; Richard Sandiford <Richard.Sandiford@arm.com>
Subject: Re: [PATCH] Add generic support for "noinit" attribute

On Wed, 14 Aug 2019 at 14:14, Richard Sandiford <richard.sandiford@arm.com> wrote:
>
> Sorry for the slow response, I'd missed that there was an updated patch...
>
> Christophe Lyon <christophe.lyon@linaro.org> writes:
> >     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.
>
> Second line should be indented by tabs rather than spaces.
>
> > @@ -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");
>
> Maybe make this an else if too?  Or make the VAR_DECL an else if if 
> you think the ELF one should win.  Either way, it seems odd to have 
> the mixture between else if and not.
>
Right, I changed this into an else if.

> > +  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.  */
>
> Comment should be indented two spaces more.
>
> > +    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/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.
>
> Maybe:
>
>    This attribute is specific to ELF targets and relies on the linker to
>    place such data in the right location.
>
Thanks, I thought I had chosen a nice turn of phrase :-)


> > 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;
> > +
>
> No longer needed.
Indeed.

>
> OK with those changes, thanks.
Thanks, committed as r274482.

Christophe

> Richard
>
> >  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";