bpf: support `naked' function attributes in BPF targets

The kernel selftests and other BPF programs make extensive use of the
`naked' function attribute with bodies written using basic inline
assembly.  This patch adds support for the attribute to
bpf-unkonwn-none, makes it to inhibit warnings due to lack of explicit
`return' statement, and updates documentation and testsuite
accordingly.

Tested in x86_64-linux-gnu host and bpf-unknown-none target.

gcc/ChangeLog

	PR target/111046
	* config/bpf/bpf.cc (bpf_attribute_table): Add entry for the
	`naked' function attribute.
	(bpf_warn_func_return): New function.
	(TARGET_WARN_FUNC_RETURN): Define.
	(bpf_expand_prologue): Add preventive comment.
	(bpf_expand_epilogue): Likewise.
	* doc/extend.texi (BPF Function Attributes): Document the `naked'
	function attribute.

gcc/testsuite/ChangeLog

	* gcc.target/bpf/naked-1.c: New test.

3 files changed, 48 insertions, 0 deletions

diff --git a/gcc/config/bpf/bpf.cc b/gcc/config/bpf/bpf.cc
index 1d0abd7..437bd65 100644
--- a/gcc/config/bpf/bpf.cc
+++ b/gcc/config/bpf/bpf.cc
@@ -154,6 +154,10 @@ static const struct attribute_spec bpf_attribute_table[] =
  { "preserve_access_index", 0, -1, false, true, false, true,
    bpf_handle_preserve_access_index_attribute, NULL },
 
+ /* Support for `naked' function attribute.  */
+ { "naked", 0, 1, false, false, false, false,
+   bpf_handle_fndecl_attribute, NULL },
+
  /* The last attribute spec is set to be NULL.  */
  { NULL,	0,  0, false, false, false, false, NULL, NULL }
 };
@@ -335,6 +339,21 @@ bpf_function_value_regno_p (const unsigned int regno)
 #undef TARGET_FUNCTION_VALUE_REGNO_P
 #define TARGET_FUNCTION_VALUE_REGNO_P bpf_function_value_regno_p
 
+
+/* Determine whether to warn about lack of return statement in a
+   function.  */
+
+static bool
+bpf_warn_func_return (tree decl)
+{
+  /* Naked functions are implemented entirely in assembly, including
+     the return instructions.  */
+  return lookup_attribute ("naked", DECL_ATTRIBUTES (decl)) == NULL_TREE;
+}
+
+#undef TARGET_WARN_FUNC_RETURN
+#define TARGET_WARN_FUNC_RETURN bpf_warn_func_return
+
 /* Compute the size of the function's stack frame, including the local
    area and the register-save area.  */
 
@@ -388,6 +407,9 @@ bpf_expand_prologue (void)
      dynamically.  This should have been checked already and an error
      emitted.  */
   gcc_assert (!cfun->calls_alloca);
+
+  /* If we ever need to have a proper prologue here, please mind the
+     `naked' function attribute.  */
 }
 
 /* Expand to the instructions in a function epilogue.  This function
@@ -399,6 +421,9 @@ bpf_expand_epilogue (void)
   /* See note in bpf_expand_prologue for an explanation on why we are
      not restoring callee-saved registers in BPF.  */
 
+  /* If we ever need to do anything else than just generating a return
+     instruction here, please mind the `naked' function attribute.  */
+
   emit_jump_insn (gen_exit ());
 }
 
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index b363386..f657032 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -5172,6 +5172,17 @@ attribute.  Example:
 int bpf_probe_read (void *dst, int size, const void *unsafe_ptr)
   __attribute__ ((kernel_helper (4)));
 @end smallexample
+
+@cindex @code{naked} function attribute, BPF
+@item naked
+This attribute allows the compiler to construct the requisite function
+declaration, while allowing the body of the function to be assembly
+code.  The specified function will not have prologue/epilogue
+sequences generated by the compiler.  Only basic @code{asm} statements
+can safely be included in naked functions (@pxref{Basic Asm}).  While
+using extended @code{asm} or a mixture of basic @code{asm} and C code
+may appear to work, they cannot be depended upon to work reliably and
+are not supported.
 @end table
 
 @node C-SKY Function Attributes
diff --git a/gcc/testsuite/gcc.target/bpf/naked-1.c b/gcc/testsuite/gcc.target/bpf/naked-1.c
new file mode 100644
index 0000000..cbbc4c5
--- /dev/null
+++ b/gcc/testsuite/gcc.target/bpf/naked-1.c
@@ -0,0 +1,12 @@
+/* Verify that __attribute__((naked)) is accepted and
+   produces a naked function.  Also, the compiler must not
+   warn for the lack of return statement.  */
+/* { dg-do compile } */
+/* { dg-options "-O0 -Wreturn-type" } */
+
+int __attribute__((naked)) foo()
+{
+  __asm__ volatile ("@ naked");
+}
+/* { dg-final { scan-assembler "\t@ naked" } } */
+/* { dg-final { scan-assembler "\texit\n" } } */