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

[Christophe Lyon <christophe dot lyon at linaro dot org>]

On Wed, 3 Jul 2019 at 11:51, Richard Earnshaw <Richard.Earnshaw@arm.com> wrote:
>
>
>
> On 02/07/2019 15:49, Christophe Lyon wrote:
> > On Tue, 2 Jul 2019 at 12:30, Richard Earnshaw <Richard.Earnshaw@arm.com> wrote:
> >>
> >>
> >>
> >> On 02/07/2019 11:13, Richard Earnshaw wrote:
> >>>
> >>>
> >>> On 02/07/2019 09:39, Richard Earnshaw wrote:
> >>>>
> >>>>
> >>>> On 01/07/2019 16:58, Kyrill Tkachov wrote:
> >>>>> Hi Christophe,
> >>>>>
> >>>>> On 6/13/19 4:13 PM, Christophe Lyon wrote:
> >>>>>> Hi,
> >>>>>>
> >>>>>> Similar to what already exists for TI msp430 or in TI compilers for
> >>>>>> arm, this patch adds support for "noinit" attribute for arm. It's very
> >>>>>> similar to the corresponding code in GCC for msp430.
> >>>>>>
> >>>>>> It is useful 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.
> >>>>>>
> >>>>>> OK?
> >>>>>>
> >>>>>
> >>>>> This is mostly ok by me, with a few code comments inline.
> >>>>>
> >>>>> I wonder whether this is something we could implement for all targets
> >>>>> in the midend, but this would require linker script support for the
> >>>>> target to be effective...
> >>>>
> >>>> Can't this be done using named sections?  If the sections were of the
> >>>> form .bss.<foo> then it would be easy to make linker scripts do
> >>>> something sane by default and users could filter them out to special
> >>>> noinit sections if desired.
> >>>>
> >>>
> >>> To answer my own question, it would appear to be yes.  You can write today:
> >>>
> >>> int xxx __attribute__ ((section (".bss.noinit")));
> >>>
> >>> int main ()
> >>> {
> >>>     return xxx;
> >>> }
> >>>
> >>> And the compiler will generate
> >>>       .section    .bss.noinit,"aw",@nobits
> >>>       .align 4
> >>>       .type    xxx, @object
> >>>       .size    xxx, 4
> >>> xxx:
> >>>       .zero    4
> >>>
> >>> So at this point, all you need is a linker script to filter .bss.noinit
> >>> into your special part of the final image.
> >>>
> >>> This will most likely work today on any GCC target that supports named
> >>> sections, which is pretty much all of them these days.
> >>>
> >>
> >> Alternatively, we already have:
> >>
> >> https://gcc.gnu.org/ml/gcc-patches/2014-09/msg01302.html
> >>
> >> R.
> >>
> >
> > Hi Richard,
> >
> > Indeed this can already be achieved with the "section" attribute as you propose.
> >
> > The motivation for this patch came from user requests: this feature is
> > already available in some proprietary ARM toolchains (TI, IAR, ...)
> > and it's more convenient for the end-user than having to update linker
> > scripts in addition to adding an attribute to the variable.
> >
>
> ? Your patch has an update to the linker scripts...

Right, but that becomes a feature of the toolchain, rather than having
to edit/create linker scripts for every application.

> > I guess it's a balance between user-friendliness/laziness and GCC
> > developers ability to educate users :-)
>
> Well in that case, this should be done generically, not in just the arm
> backend, or any other backend for that matter.
>

I thought it would be less controversial to mimic msp430, it seems I
was wrong  :)

I'm going to have a look at making this generic, then.

Christophe

> R.
>
> >
> > Christophe
> >
> >
> >>> R.
> >>>
> >>>> R.
> >>>>
> >>>>>
> >>>>> Thanks,
> >>>>>
> >>>>> Kyrill
> >>>>>
> >>>>>> Thanks,
> >>>>>>
> >>>>>> Christophe
> >>>>>
> >>>>> diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> >>>>> index e3e71ea..332c41b 100644
> >>>>> --- a/gcc/config/arm/arm.c
> >>>>> +++ b/gcc/config/arm/arm.c
> >>>>> @@ -151,6 +151,7 @@ static tree arm_handle_notshared_attribute (tree
> >>>>> *, tree, tree, int, bool *);
> >>>>>    #endif
> >>>>>    static tree arm_handle_cmse_nonsecure_entry (tree *, tree, tree,
> >>>>> int, bool *);
> >>>>>    static tree arm_handle_cmse_nonsecure_call (tree *, tree, tree,
> >>>>> int, bool *);
> >>>>> +static tree arm_data_attr (tree *, tree, tree, int, bool *);
> >>>>>    static void arm_output_function_epilogue (FILE *);
> >>>>>    static void arm_output_function_prologue (FILE *);
> >>>>>    static int arm_comp_type_attributes (const_tree, const_tree);
> >>>>> @@ -375,7 +376,8 @@ static const struct attribute_spec
> >>>>> arm_attribute_table[] =
> >>>>>        arm_handle_cmse_nonsecure_entry, NULL },
> >>>>>      { "cmse_nonsecure_call", 0, 0, true, false, false, true,
> >>>>>        arm_handle_cmse_nonsecure_call, NULL },
> >>>>> -  { NULL, 0, 0, false, false, false, false, NULL, NULL }
> >>>>> +  { "noinit", 0, 0, true, false, false, false, arm_data_attr, NULL },
> >>>>> +  { NULL, 0, 0, false, false, false, false, NULL, NULL },
> >>>>>    };
> >>>>>
> >>>>>    /* Initialize the GCC target structure.  */
> >>>>> @@ -808,6 +810,10 @@ static const struct attribute_spec
> >>>>> arm_attribute_table[] =
> >>>>>
> >>>>>    #undef TARGET_CONSTANT_ALIGNMENT
> >>>>>    #define TARGET_CONSTANT_ALIGNMENT arm_constant_alignment
> >>>>> +
> >>>>> +#undef  TARGET_ASM_SELECT_SECTION
> >>>>> +#define TARGET_ASM_SELECT_SECTION arm_select_section
> >>>>> +
> >>>>>
> >>>>>    /* Obstack for minipool constant handling.  */
> >>>>>    static struct obstack minipool_obstack;
> >>>>> @@ -7150,6 +7156,47 @@ arm_handle_cmse_nonsecure_call (tree *node,
> >>>>> tree name,
> >>>>>      return NULL_TREE;
> >>>>>    }
> >>>>>
> >>>>> +/* Called when the noinit attribute is used. 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
> >>>>> +arm_data_attr (tree * node,
> >>>>> +          tree   name,
> >>>>> +          tree   args ATTRIBUTE_UNUSED,
> >>>>> +          int    flags ATTRIBUTE_UNUSED,
> >>>>> +          bool * no_add_attrs ATTRIBUTE_UNUSED)
> >>>>> +{
> >>>>>
> >>>>> no_add_attrs is set in this function, so should not be ATTRIBUTE_UNUSED?
> >>>>> Arguably args is also checked against NULL, so it's technically not
> >>>>> unused either.
> >>>>>
> >>>>> +  const char * message = NULL;
> >>>>> +
> >>>>> +  gcc_assert (DECL_P (* node));
> >>>>>
> >>>>> The house style doesn't have a space after '*'. Same elsewhere in the
> >>>>> patch.
> >>>>>
> >>>>> +  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;
> >>>>> +}
> >>>>> +
> >>>>>    /* Return 0 if the attributes for two types are incompatible, 1 if
> >>>>> they
> >>>>>       are compatible, and 2 if they are nearly compatible (which causes a
> >>>>>       warning to be generated).  */
> >>>>> @@ -27890,6 +27937,8 @@ arm_asm_emit_except_personality (rtx
> >>>>> personality)
> >>>>>
> >>>>>    /* Implement TARGET_ASM_INITIALIZE_SECTIONS.  */
> >>>>>
> >>>>> +static section *noinit_section;
> >>>>> +
> >>>>>    static void
> >>>>>    arm_asm_init_sections (void)
> >>>>>    {
> >>>>> @@ -27902,6 +27951,19 @@ arm_asm_init_sections (void)
> >>>>>      if (target_pure_code)
> >>>>>        text_section->unnamed.data = "\t.section
> >>>>> .text,\"0x20000006\",%progbits";
> >>>>>    #endif
> >>>>> +
> >>>>> +  noinit_section = get_unnamed_section (0, output_section_asm_op,
> >>>>> ".section .noinit,\"aw\"");
> >>>>> +}
> >>>>> +
> >>>>> +static section *
> >>>>> +arm_select_section (tree decl, int reloc, unsigned HOST_WIDE_INT align)
> >>>>> +{
> >>>>>
> >>>>> Please add a function comment.
> >>>>>
> >>>>> +  gcc_assert (decl != NULL_TREE);
> >>>>> +
> >>>>> +  if (DECL_P (decl) && lookup_attribute ("noinit", DECL_ATTRIBUTES
> >>>>> (decl)) != NULL_TREE)
> >>>>> +    return noinit_section;
> >>>>> +  else
> >>>>> +    return default_elf_select_section (decl, reloc, align);
> >>>>>    }
> >>>>>
> >>>>>    /* Output unwind directives for the start/end of a function.  */
> >>>>> @@ -31520,6 +31582,9 @@ arm_elf_section_type_flags (tree decl, const
> >>>>> char *name, int reloc)
> >>>>>      if (decl && TREE_CODE (decl) == FUNCTION_DECL && target_pure_code)
> >>>>>        flags |= SECTION_ARM_PURECODE;
> >>>>>
> >>>>> +  if (strcmp (name, ".noinit") == 0)
> >>>>> +    flags = SECTION_WRITE | SECTION_BSS | SECTION_NOTYPE;
> >>>>> +
> >>>>>
> >>>>>
> >>>>> You're overwriting the flags here. Are you sure you don't want "flags
> >>>>> |= ..." here?
> >>>>>
> >>>>>
> >>>>>    return flags;
> >>>>>    }
> >>>>>
> >>>>> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> >>>>> index 2520835..d544527 100644
> >>>>> --- a/gcc/doc/extend.texi
> >>>>> +++ b/gcc/doc/extend.texi
> >>>>> @@ -6684,6 +6684,7 @@ attributes.
> >>>>>    @menu
> >>>>>    * Common Variable Attributes::
> >>>>>    * ARC Variable Attributes::
> >>>>> +* ARM Variable Attributes::
> >>>>>    * AVR Variable Attributes::
> >>>>>    * Blackfin Variable Attributes::
> >>>>>    * H8/300 Variable Attributes::
> >>>>> @@ -7131,6 +7132,18 @@ given via attribute argument.
> >>>>>
> >>>>>    @end table
> >>>>>
> >>>>> +@node ARM Variable Attributes
> >>>>> +@subsection ARM Variable Attributes
> >>>>> +
> >>>>> +@table @code
> >>>>> +@item noinit
> >>>>> +@cindex @code{noinit} variable attribute, ARM
> >>>>> +Any data with the @code{noinit} attribute will not be initialised by
> >>>>> +the C runtime startup code, or the program loader.  Not initialising
> >>>>> +data in this way can reduce program startup times.
> >>>>> +
> >>>>> +@end table
> >>>>> +
> >>>>>    @node AVR Variable Attributes
> >>>>>    @subsection AVR Variable Attributes
> >>>>>
> >>>>> diff --git a/gcc/testsuite/gcc.target/arm/data-attributes.c
> >>>>> b/gcc/testsuite/gcc.target/arm/data-attributes.c
> >>>>> new file mode 100644
> >>>>> index 0000000..323c8e0
> >>>>> --- /dev/null
> >>>>> +++ b/gcc/testsuite/gcc.target/arm/data-attributes.c
> >>>>> @@ -0,0 +1,51 @@
> >>>>> +/* { dg-do run { target { ! *-*-linux* } } } */
> >>>>> +/* { 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
> >>>>> +main (void)
> >>>>> +{
> >>>>> +  /* Make sure that the C startup code has correctly initialised the
> >>>>> ordinary variables.  */
> >>>>> +  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 ();
> >>>>> +}
> >>>>>