Re: [PATCH v5] LoongArch: add movable attribute

[WANG Xuerui <i@xen0n.name>]

On 2022/8/3 09:36, Xi Ruoyao wrote:
> 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.

I don't think mindlessly caring for vendor forks is always correct. In 
fact I find the name "movable" too generic, and something like 
"force_got_access" could be better.

I don't currently have time to test this, unfortunately, due to day job. 
Might be able to give it a whirl one or two week later though...

>>   * 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)++;
>> +}