[PATCH] LoongArch: add addr_global attribute

[Xi Ruoyao <xry111@xry111.site>]

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.
	* 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             | 43 +++++++++++++++++++
 gcc/doc/extend.texi                           | 17 ++++++++
 .../gcc.target/loongarch/addr-global.c        | 28 ++++++++++++
 3 files changed, 88 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..5f4e6114fc9 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 +6075,39 @@ 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 }
+};
+
 /* Initialize the GCC target structure.  */
 #undef TARGET_ASM_ALIGNED_HI_OP
 #define TARGET_ASM_ALIGNED_HI_OP "\t.half\t"
@@ -6256,6 +6296,9 @@ 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
+
 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
+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..d48fa8103a0
--- /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" } */
+/* { dg-final { scan-assembler-not "%pc" } } */
+/* { dg-final { scan-assembler-times "%got_pc_hi20" 3 } } */
+
+static 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++;
+}
-- 
2.37.1