Re: [PATCH v3] LoongArch: add addr_global attribute

[Chenghua Xu <xuchenghua@loongson.cn>]

在 2022/7/30 上午1:17, Xi Ruoyao via Gcc-patches 写道:
> Change v2 to v3:
> - Disable section anchor for addr_global symbols.
> - Use -O2 in test to make sure section anchor is disabled.
>
> --
>
> Background:
> https://lore.kernel.org/loongarch/d7670b60-2782-4642-995b-7baa01779a66@loongson.cn/T/#e1d47e2fe185f2e2be8fdc0784f0db2f644119379
>
> Question:  Do you have a better name than "addr_global"?
>
> Alternatives:
>
> 1. Just use "-mno-explicit-relocs -mla-local-with-abs" for kernel
> modules.  It's stupid IMO.
> 2. Implement a "-maddress-local-with-got" option for GCC and use it for
> kernel modules.  It seems too overkill: we might create many unnecessary
> GOT entries.
> 3. For all variables with a section attribute, consider it global.  It
> may make sense, but I just checked x86_64 and riscv and they don't do
> this.
> 4. Implement -mcmodel=extreme and use it for kernel modules.  To me
> "extreme" seems really too extreme.
> 5. More hacks in kernel. (Convert relocations against .data..percpu with
> objtool?  But objtool is not even implemented for LoongArch yet.)
>
> Note: I'll be mostly AFK in the following week.  My attempt to finish
> the kernel support for new relocs before going AFK now failed miserably
> :(.
>
> -- >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             | 61 +++++++++++++++++++
>   gcc/doc/extend.texi                           | 17 ++++++
>   .../gcc.target/loongarch/addr-global.c        | 28 +++++++++
>   3 files changed, 106 insertions(+)
>   create mode 100644 gcc/testsuite/gcc.target/loongarch/addr-global.c
>
> diff --git a/gcc/config/loongarch/loongarch.cc b/gcc/config/loongarch/loongarch.cc
> index 79687340dfd..db6f84d4e66 100644
> --- a/gcc/config/loongarch/loongarch.cc
> +++ b/gcc/config/loongarch/loongarch.cc
> @@ -1643,6 +1643,13 @@ 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);
> +      if (decl && lookup_attribute ("addr_global", DECL_ATTRIBUTES (decl)))
> +	return SYMBOL_GOT_DISP;
> +    }
> +
>     return SYMBOL_PCREL;
>   }
>   
> @@ -6068,6 +6075,54 @@ loongarch_starting_frame_offset (void)
>     return crtl->outgoing_args_size;
>   }
>   
> +static tree
> +loongarch_handle_addr_global_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 } */
> +  { "addr_global", 0, 0, true, false, false, false,
> +    loongarch_handle_addr_global_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);
> +
> +  /* addr_global indicates we don't know how the linker will locate the symbol,
> +     so the use of anchor may cause relocation overflow.  */
> +  if (decl && lookup_attribute ("addr_global", 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 +6311,12 @@ 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..4a1cd7ab5e4 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 +8099,22 @@ 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 addr_global
> +@cindex @code{addr_global} variable attribute, LoongArch
> +@cindex variable addressability on the LoongArch
Can't understand this, Copied and forget to changing?
> +Use this attribute on the LoongArch to mark an object with address
> +unlimited limited 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 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/addr-global.c b/gcc/testsuite/gcc.target/loongarch/addr-global.c
> new file mode 100644
> index 00000000000..46a895c84bb
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/loongarch/addr-global.c
> @@ -0,0 +1,28 @@
> +/* addr_global 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.  */
> +
> +/* { 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 } } */
> +
> +int x __attribute__((addr_global));
> +int y __attribute__((addr_global));
> +
> +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"), addr_global));
> +void
> +inc_counter(void)
> +{
> +  int *ptr = (int *)(per_cpu_base + (long)&counter);
> +  (*ptr)++;
> +}