Thanks for doing this in a generic way.
Christophe Lyon <christophe.l...@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.
Since this is specific to ELF targets, I think we should also
warn if !targetm.have_switchable_bss_sections.
> @@ -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.
> 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.
> 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.
> + 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.)
> 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;
Did you consider supporting .noinit.*, e.g. for -fdata-sections?
Thanks,
Richard
