Re: [PATCH v5] LoongArch: add movable attribute

[Xi Ruoyao <xry111@xry111.site>]

Is it OK for trunk or I need to change something?

By the way, I'm seeking a possibility to include this into 12.2.  Then
we leaves only 12.1 without this attribute, and we can just say
"building the kernel needs GCC 12.2 or later".

On Mon, 2022-08-01 at 18:07 +0800, Xi Ruoyao wrote:
> Changes v4 -> v5: Fix changelog.  No code change.
> 
> Changes v3 -> v4:
> 
>  * Use "movable" as the attribute name as Huacai says it's already
> used
>    in downstream GCC fork.
>  * Remove an inaccurate line from the doc. (Initially I tried to
>    implement a "model(...)" like IA64 or M32R. Then I changed my mind
>    but forgot to remove the line copied from M32R doc.)
> 
> -- >8 --
> 
> A linker script and/or a section attribute may locate a local object
> in
> some way unexpected by the code model, leading to a link failure. 
> This
> happens when the Linux kernel loads a module with "local" per-CPU
> variables.
> 
> Add an attribute to explicitly mark an variable with the address
> unlimited by the code model so we would be able to work around such
> problems.
> 
> gcc/ChangeLog:
> 
>         * config/loongarch/loongarch.cc (loongarch_attribute_table):
>         New attribute table.
>         (TARGET_ATTRIBUTE_TABLE): Define the target hook.
>         (loongarch_handle_addr_global_attribute): New static function.
>         (loongarch_classify_symbol): Return SYMBOL_GOT_DISP for
>         SYMBOL_REF_DECL with addr_global attribute.
>         (loongarch_use_anchors_for_symbol_p): New static function.
>         (TARGET_USE_ANCHORS_FOR_SYMBOL_P): Define the target hook.
>         * doc/extend.texi (Variable Attributes): Document new
>         LoongArch specific attribute.
> 
> gcc/testsuite/ChangeLog:
> 
>         * gcc.target/loongarch/addr-global.c: New test.
> ---
>  gcc/config/loongarch/loongarch.cc             | 63
> +++++++++++++++++++
>  gcc/doc/extend.texi                           | 16 +++++
>  .../gcc.target/loongarch/attr-movable.c       | 29 +++++++++
>  3 files changed, 108 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.target/loongarch/attr-movable.c
> 
> diff --git a/gcc/config/loongarch/loongarch.cc
> b/gcc/config/loongarch/loongarch.cc
> index 79687340dfd..6b6026700a6 100644
> --- a/gcc/config/loongarch/loongarch.cc
> +++ b/gcc/config/loongarch/loongarch.cc
> @@ -1643,6 +1643,15 @@ loongarch_classify_symbol (const_rtx x)
>        && !loongarch_symbol_binds_local_p (x))
>      return SYMBOL_GOT_DISP;
>  
> +  if (SYMBOL_REF_P (x))
> +    {
> +      tree decl = SYMBOL_REF_DECL (x);
> +      /* A movable symbol may be moved away from the +/- 2GiB range
> around
> +        the PC, so we have to use GOT.  */
> +      if (decl && lookup_attribute ("movable", DECL_ATTRIBUTES
> (decl)))
> +       return SYMBOL_GOT_DISP;
> +    }
> +
>    return SYMBOL_PCREL;
>  }
>  
> @@ -6068,6 +6077,54 @@ loongarch_starting_frame_offset (void)
>    return crtl->outgoing_args_size;
>  }
>  
> +static tree
> +loongarch_handle_movable_attribute (tree *node, tree name, tree, int,
> +                                   bool *no_add_attrs)
> +{
> +  tree decl = *node;
> +  if (TREE_CODE (decl) == VAR_DECL)
> +    {
> +      if (DECL_CONTEXT (decl)
> +         && TREE_CODE (DECL_CONTEXT (decl)) == FUNCTION_DECL
> +         && !TREE_STATIC (decl))
> +       {
> +         error_at (DECL_SOURCE_LOCATION (decl),
> +                   "%qE attribute cannot be specified for local "
> +                   "variables", name);
> +         *no_add_attrs = true;
> +       }
> +    }
> +  else
> +    {
> +      warning (OPT_Wattributes, "%qE attribute ignored", name);
> +      *no_add_attrs = true;
> +    }
> +  return NULL_TREE;
> +}
> +
> +static const struct attribute_spec loongarch_attribute_table[] =
> +{
> +  /* { name, min_len, max_len, decl_req, type_req, fn_type_req,
> +       affects_type_identity, handler, exclude } */
> +  { "movable", 0, 0, true, false, false, false,
> +    loongarch_handle_movable_attribute, NULL },
> +  /* The last attribute spec is set to be NULL.  */
> +  {}
> +};
> +
> +bool
> +loongarch_use_anchors_for_symbol_p (const_rtx symbol)
> +{
> +  tree decl = SYMBOL_REF_DECL (symbol);
> +
> +  /* A movable attribute indicates the linker may move the symbol
> away,
> +     so the use of anchor may cause relocation overflow.  */
> +  if (decl && lookup_attribute ("movable", DECL_ATTRIBUTES (decl)))
> +    return false;
> +
> +  return default_use_anchors_for_symbol_p (symbol);
> +}
> +
>  /* Initialize the GCC target structure.  */
>  #undef TARGET_ASM_ALIGNED_HI_OP
>  #define TARGET_ASM_ALIGNED_HI_OP "\t.half\t"
> @@ -6256,6 +6313,12 @@ loongarch_starting_frame_offset (void)
>  #undef  TARGET_HAVE_SPECULATION_SAFE_VALUE
>  #define TARGET_HAVE_SPECULATION_SAFE_VALUE
> speculation_safe_value_not_needed
>  
> +#undef  TARGET_ATTRIBUTE_TABLE
> +#define TARGET_ATTRIBUTE_TABLE loongarch_attribute_table
> +
> +#undef  TARGET_USE_ANCHORS_FOR_SYMBOL_P
> +#define TARGET_USE_ANCHORS_FOR_SYMBOL_P
> loongarch_use_anchors_for_symbol_p
> +
>  struct gcc_target targetm = TARGET_INITIALIZER;
>  
>  #include "gt-loongarch.h"
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index 7fe7f8817cd..322d8c05a04 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -7314,6 +7314,7 @@ attributes.
>  * Blackfin Variable Attributes::
>  * H8/300 Variable Attributes::
>  * IA-64 Variable Attributes::
> +* LoongArch Variable Attributes::
>  * M32R/D Variable Attributes::
>  * MeP Variable Attributes::
>  * Microsoft Windows Variable Attributes::
> @@ -8098,6 +8099,21 @@ defined by shared libraries.
>  
>  @end table
>  
> +@node LoongArch Variable Attributes
> +@subsection LoongArch Variable Attributes
> +
> +One attribute is currently defined for the LoongArch.
> +
> +@table @code
> +@item movable
> +@cindex @code{movable} variable attribute, LoongArch
> +Use this attribute on the LoongArch to mark an object possible to be
> moved
> +by the linker, so its address is unlimited by the local data section
> range
> +specified by the code model even if the object is defined locally. 
> This
> +attribute is mostly useful if a @code{section} attribute and/or a
> linker
> +script will move the object somewhere unexpected by the code model.
> +@end table
> +
>  @node M32R/D Variable Attributes
>  @subsection M32R/D Variable Attributes
>  
> diff --git a/gcc/testsuite/gcc.target/loongarch/attr-movable.c
> b/gcc/testsuite/gcc.target/loongarch/attr-movable.c
> new file mode 100644
> index 00000000000..85b1dd4c59a
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/loongarch/attr-movable.c
> @@ -0,0 +1,29 @@
> +/* { dg-do compile } */
> +/* { dg-options "-mexplicit-relocs -mcmodel=normal -O2" } */
> +/* { dg-final { scan-assembler-not "%pc" } } */
> +/* { dg-final { scan-assembler-times "%got_pc_hi20" 3 } } */
> +
> +/* movable attribute should mark x and y possibly outside of the
> local
> +   data range defined by the code model, so GOT should be used
> instead of
> +   PC-relative.  */
> +
> +int x __attribute__((movable));
> +int y __attribute__((movable));
> +
> +int
> +test(void)
> +{
> +  return x + y;
> +}
> +
> +/* The following will be used for kernel per-cpu storage
> implemention. */
> +
> +register char *per_cpu_base __asm__("r21");
> +static int counter __attribute__((section(".data..percpu"),
> movable));
> +
> +void
> +inc_counter(void)
> +{
> +  int *ptr = (int *)(per_cpu_base + (long)&counter);
> +  (*ptr)++;
> +}

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University