Recognize virtual tail call frames.
	* Makefile.in (SFILES): Add dwarf2-frame-tailcall.c.
	(HFILES_NO_SRCDIR): Add dwarf2-frame-tailcall.h.
	(COMMON_OBS): Add dwarf2-frame-tailcall.o.
	* dwarf2-frame-tailcall.c: New file.
	* dwarf2-frame-tailcall.h: New file.
	* dwarf2-frame.c: Include dwarf2-frame-tailcall.h.
	(execute_cfa_program): New function comment.  Return INSN_PTR.  Reset
	REGS.PREV only after CIE execution.
	(struct dwarf2_frame_cache): New field tailcall_cache.
	(dwarf2_frame_cache): New variables entry_pc, entry_cfa_sp_offset,
	entry_cfa_sp_offset_p and instr.  Execute FDE instructions in two
	parts, try to find entry_cfa_sp_offset.  Call
	dwarf2_tailcall_sniffer_first.
	(dwarf2_frame_prev_register): Call dwarf2_tailcall_prev_register_first
	when appropriate.
	(dwarf2_frame_dealloc_cache): New function.
	(dwarf2_frame_sniffer): Preinitialize cache by dwarf2_frame_cache.
	(dwarf2_frame_unwind): Install dwarf2_frame_dealloc_cache.
	(dwarf2_signal_frame_unwind): Do not install dwarf2_frame_dealloc_cache.
	(dwarf2_append_unwinders): Add dwarf2_tailcall_frame_unwind.
	(dwarf2_frame_cfa): Support also dwarf2_tailcall_frame_unwind.
	* dwarf2loc.c (func_addr_to_tail_call_list)
	(tailcall_dump, call_sitep, VEC (call_sitep), chain_candidate)
	(call_site_find_chain_1, call_site_find_chain): New.
	* dwarf2loc.h (struct call_site_chain): New.
	(call_site_find_chain): New declaration.
	* frame.c (get_frame_address_in_block): Support also TAILCALL_FRAME.
	* frame.h (enum frame_type): New entry TAILCALL_FRAME.
	* python/py-frame.c (gdbpy_initialize_frames): Add TAILCALL_FRAME.
	* stack.c (frame_info): Support also TAILCALL_FRAME.

gdb/doc/
	Recognize virtual tail call frames.
	* gdb.texinfo (Optimized Code): Add reference to Tail Call Frames.
	(Tail Call Frames): New node.
	(Frames In Python): Add gdb.TAILCALL_FRAME.

gdb/testsuite/
	Recognize virtual tail call frames.
	* gdb.arch/amd64-entry-value.cc (c, a, b, amb_z, amb_y, amb_x, amb)
	(amb_b, amb_a): New.
	(main): Call a and b.
	* gdb.arch/amd64-entry-value.exp (tailcall: breakhere, tailcall: bt)
	(tailcall: p i, tailcall: p j, set $sp0=$sp, up, p $sp0 == $sp, frame 3)
	(p $sp0 + sizeof (void *) == $sp, ambiguous: breakhere, ambiguous: bt):
	New tests.