[rs6000] ELFv2 ABI 2/8: Remove function descriptors

Hello,

this patch adds the first major feature of the ELFv2 ABI: removal
of function descriptors.

In the current ABI, it is the responsibility of a caller to set
up the TOC pointer needed by the callee by loading the correct
value into r2.  This typically happens in one of these ways:

- For a direct function call within the same module, caller and
  callee share the same TOC, so r2 already contains the correct
  value.

- For a direct function call to a function in another module,
  the linker will interpose a PLT stub which reloads r2 to be
  appropriate for the callee, with help from ld.so.

- For an indirect function call, the "function pointer" points
  to function descriptor that holds the target function address
  as well as the appropriate TOC value (and a static chain
  value for nested functions).  The caller must load those
  values up before transfering control to the callee.

With the new ABI, it is the responsibility of the callee to
set up its own TOC pointer.  This has a number of advantages,
in particular for a callee that doesn't actually need a TOC,
and makes the function pointer call sequence more effective
by avoiding pipeline hazards.

However, it remains true that with the PowerPC ISA, it is
difficult to actually establish addressability to the TOC
without any outside help.  To avoid introducing new performance
regressions, the new ABI instead provides a dual entry point
scheme.

Any function that needs a TOC may provide two entry points:

- The first ("global") entry point is intended to called via
  indirect calls.  It expects r12 to be set up to point to
  the entry point address itself.  (Since in order to perform
  an indirect call, the address must be loaded into some GPR
  anyway, this does not impose a substantial impact on the
  caller ...)  Code at the global entry point is expected
  to load up r2 with the appropriate TOC value for this
  function (and it may use the r12 value to compute that),
  and then fall through to the local entry point.

- The second ("local") entry point expects r2 to be set up
  to the current TOC, much like an entry point in the current
  ABI does.  However, it must not expect r12 to hold any
  particular value.  This means that a local direct function
  call within the same module can be done via a simple "bl"
  instruction just as today.  Note that the linker will
  automatically direct a "bl func" to the *local* entry
  point associated with the symbol "func"; not the global one.

If local and global entry points coincide, the function may
expect no particular value in either r12 or r2; this can be
used by functions that do not need a TOC.

There exists just one single ELF symbol for both local and
global entry points; its symbol value refers to the global
entry point, and flag bits in ELF st_other encode the
offset from there to the local entry point.  The compiler
emits a .localentry directive to announce this location
to the assembler, which will encode it as appropriate.

This patch implements all aspects needed for this scheme:

- Emit ".abiversion 2" assembler directive to help the assembler
  and linker understand the ABI implemented in this file.
- Remove all handling of function descriptors / .opd section /
  dot symbols for the ELFv2 ABI.
- Output a global entry point prologue and local entry point
  marker for functions that use the TOC.
- Implement the ELFv2 function pointer call sequence.
- Use trampolines to handle nested functions, just like on
  32-bit (including marking the stack as executable).
- No longer scan for the old ABI indirect call sequence in
  linux-unwind.h.
- Update assembler code to the new ABI:
   - gcc/config/rs6000/ppc-asm.h (used by libgcc and elsewhere)
   - lititm/config/powerpc/sjlj.S
   - gcc/testsuite/gcc.target/powerpc/ppc64-abi-dfp-1.c

In addition, some related changes are necessary:

- Move the code generating the call to _mcount for -mprofile-kernel
  from output_function_profiler to rs6000_output_function_prologue,
  since this call must happen at the local entry point, not the
  global entry point.
- Disable (now useless) tests for -mpointers-to-nested-functions.
- Fix the libstc++ extract_symvers.in script to ignore markers
  emitted by readelf --symbols that indicate local entry points.
- Fix the "gotest" script to no longer expect function symbols
  to reside in the data segment.  (I understand this last bit
  needs to go into the Go repository first ...)

Bye,
Ulrich

gcc/ChangeLog:

2013-11-11  Ulrich Weigand  <Ulrich.Weigand@de.ibm.com>

	* config/rs6000/rs6000.c (machine_function): New member
	r2_setup_needed.
	(rs6000_emit_prologue): Set r2_setup_needed if necessary.
	(rs6000_output_mi_thunk): Set r2_setup_needed.
	(rs6000_output_function_prologue): Output global entry point
	prologue and local entry point marker if needed for ABI_ELFv2.
	Output -mprofile-kernel code here.
	(output_function_profiler): Do not output -mprofile-kernel
	code here; moved to rs6000_output_function_prologue.
	(rs6000_file_start): Output ".abiversion 2" for ABI_ELFv2.

	(rs6000_emit_move): Do not handle dot symbols for ABI_ELFv2.
	(rs6000_output_function_entry): Likewise.
	(rs6000_assemble_integer): Likewise.
	(rs6000_elf_encode_section_info): Likewise.
	(rs6000_elf_declare_function_name): Do not create dot symbols
	or .opd section for ABI_ELFv2.

	(rs6000_trampoline_size): Update for ABI_ELFv2 trampolines.
	(rs6000_trampoline_init): Likewise.
	(rs6000_elf_file_end): Call file_end_indicate_exec_stack
	for ABI_ELFv2.

	(rs6000_call_aix): Handle ELFv2 indirect calls.  Do not check
	for function descriptors in ABI_ELFv2.

	* config/rs6000/rs6000.md ("*call_indirect_aix<mode>"): Support
	on ABI_AIX only, not ABI_ELFv2.
	("*call_value_indirect_aix<mode>"): Likewise.
	("*call_indirect_elfv2<mode>"): New pattern.
	("*call_value_indirect_elfv2<mode>"): Likewise.

	* config/rs6000/predicates.md ("symbol_ref_operand"): Do not
	check for function descriptors in ABI_ELFv2.
	("current_file_function_operand"): Likewise.

	* config/rs6000/ppc-asm.h [__powerpc64__ && _CALL_ELF == 2]:
	(toc): Undefine.
	(FUNC_NAME): Define ELFv2 variant.
	(JUMP_TARGET): Likewise.
	(FUNC_START): Likewise.
	(HIDDEN_FUNC): Likewise.
	(FUNC_END): Likeiwse.

libitm/ChangeLog:

2013-11-11  Ulrich Weigand  <Ulrich.Weigand@de.ibm.com>

	* config/powerpc/sjlj.S [__powerpc64__ && _CALL_ELF == 2]:
	(FUNC): Define ELFv2 variant.
	(END): Likewise.
	(HIDDEN): Likewise.
	(CALL): Likewise.
	(BASE): Likewise.
	(LR_SAVE): Likewise.

libgcc/ChangeLog:

2013-11-11  Ulrich Weigand  <Ulrich.Weigand@de.ibm.com>
	    Alan Modra  <amodra@gmail.com>

	* config/rs6000/tramp.S [__powerpc64__ && _CALL_ELF == 2]:
	(trampoline_initial): Provide ELFv2 variant.
	(__trampoline_setup): Likewise.

	* config/rs6000/linux-unwind.h (frob_update_context): Do not
	check for AIX indirect function call sequence if _CALL_ELF == 2.

gcc/testsuite/ChangeLog:

2013-11-11  Ulrich Weigand  <Ulrich.Weigand@de.ibm.com>

	* gcc.target/powerpc/ppc64-abi-dfp-1.c (FUNC_START): New macro.
	(WRAPPER): Use it.
	* gcc.target/powerpc/no-r11-1.c: Skip on powerpc_elfv2.
	* gcc.target/powerpc/no-r11-2.c: Skip on powerpc_elfv2.
	* gcc.target/powerpc/no-r11-3.c: Skip on powerpc_elfv2.

libstdc++/ChangeLog:

2013-11-11  Ulrich Weigand  <Ulrich.Weigand@de.ibm.com>

	* scripts/extract_symvers.in: Ignore <localentry: > fields
	in readelf --symbols output.

[rs6000] ELFv2 ABI 2/8: Remove function descriptors

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