[avr] Add -mno-call-main to tweak running main()

    AVR: target/118806 - Add -mno-call-main to tweak running main().
    
    On devices with very limited resources, it may be desirable to run
    main in a more efficient way than provided by the startup code
    
       XCALL main
       XJMP  exit
    
    from section .init9.  In AVR-LibC v2.3, that code has been moved to
    libmcu.a, hence symbol __call_main can be satisfied so that the
    respective code is no more pulled in from that library.
    Instead, main can be run by putting it in section .init9.
    
    The patch adds attributes noreturn and section(".init9"), and
    sets __call_main=0 when it encounters main().
    
    gcc/
            PR target/118806
            * config/avr/avr.opt (-mcall-main): New option and...
            (avropt_call_main): ...variable.
            * config/avr/avr.cc (avr_no_call_main_p): New variable.
            (avr_insert_attributes) [-mno-call-main, main]: Add attributes
            noreturn and section(".init9") to main.  Set avr_no_call_main_p.
            (avr_file_end) [avr_no_call_main_p]: Define symbol __call_main.
            * doc/invoke.texi (AVR Options) <-mno-call-main>: Document.
            <-mnodevicelib>: Extend explanation.

diff --git a/gcc/config/avr/avr.cc b/gcc/config/avr/avr.cc
index 9bfb3785ddb..c31d06d6d20 100644
--- a/gcc/config/avr/avr.cc
+++ b/gcc/config/avr/avr.cc
@@ -230,6 +230,9 @@  bool avr_need_clear_bss_p = false;
 bool avr_need_copy_data_p = false;
 bool avr_has_rodata_p = false;
 
+/* To track if we satisfy __call_main from AVR-LibC.  */
+bool avr_no_call_main_p = false;
+
 /* Counts how often pass avr-fuse-add has been executed.  Is is kept in
    sync with cfun->machine->n_avr_fuse_add_executed and serves as an
    insn condition for shift insn splitters.  */
@@ -11712,6 +11715,32 @@  avr_insert_attributes (tree node, tree *attributes)
 			       NULL, *attributes);
     }
 
+#if defined WITH_AVRLIBC
+  if (avropt_call_main == 0
+      && TREE_CODE (node) == FUNCTION_DECL
+      && MAIN_NAME_P (DECL_NAME (node)))
+    {
+      if (lookup_attribute ("section", *attributes))
+	{
+	  warning (OPT_Wattributes, "%<section%> attribute on main function"
+		   " inhibits %<-mno-call-main%>");
+	}
+      else
+	{
+	  if (!lookup_attribute ("noreturn", *attributes))
+	    *attributes = tree_cons (get_identifier ("noreturn"),
+				     NULL_TREE, *attributes);
+	  // Put main into section .init9 so that it is executed even
+	  // though it's not called.
+	  tree init9 = build_string (1 + strlen (".init9"), ".init9");
+	  tree arg = build_tree_list (NULL_TREE, init9);
+	  *attributes = tree_cons (get_identifier ("section"),
+				   arg, *attributes);
+	  avr_no_call_main_p = true;
+	}
+    } // -mno-call-main
+#endif // AVR-LibC
+
   avr_handle_isr_attribute (node, attributes, "signal");
   avr_handle_isr_attribute (node, attributes, "interrupt");
 
@@ -12311,6 +12340,15 @@  avr_file_end (void)
 
   if (avr_need_clear_bss_p)
     fputs (".global __do_clear_bss\n", asm_out_file);
+
+  /* Don't let __call_main call main() and exit().
+     Defining this symbol will keep the code from being pulled
+     in from lib<mcu>.a as requested by AVR-LibC's gcrt1.S.
+     We invoke main() by other means: putting it in .init9.  */
+
+  if (avr_no_call_main_p)
+    fputs (".global __call_main\n"
+	   "__call_main = 0\n", asm_out_file);
 }
 
 
diff --git a/gcc/config/avr/avr.opt b/gcc/config/avr/avr.opt
index 0184b40f4e5..af46898ba78 100644
--- a/gcc/config/avr/avr.opt
+++ b/gcc/config/avr/avr.opt
@@ -42,6 +42,10 @@  mn-flash=
 Target RejectNegative Joined Var(avropt_n_flash) UInteger Init(-1)
 This option is used internally. Set the number of 64 KiB flash segments.
 
+mcall-main
+Target Var(avropt_call_main) UInteger Init(1)
+Call main and exit.
+
 mskip-bug
 Target Mask(SKIP_BUG)
 This option is used internally. Indicate presence of a processor erratum.  Do not skip 32-bit instructions.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 0aef2abf05b..986b1a5fb8b 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -908,7 +908,7 @@  Objective-C and Objective-C++ Dialects}.
 @gccoptlist{-mmcu=@var{mcu}  -mabsdata  -maccumulate-args  -mcvt
 -mbranch-cost=@var{cost}  -mfuse-add=@var{level}  -mfuse-move=@var{level}
 -mcall-prologues  -mgas-isr-prologues  -mint8  -mflmap
--mdouble=@var{bits}  -mlong-double=@var{bits}
+-mdouble=@var{bits}  -mlong-double=@var{bits}  -mno-call-main
 -mn_flash=@var{size}  -mfract-convert-truncate  -mno-interrupts
 -mmain-is-OS_task  -mrelax  -mrmw  -mstrict-X  -mtiny-stack
 -mrodata-in-ram  -msplit-bit-shift  -msplit-ldst
@@ -24457,6 +24457,18 @@  Do not save registers in @code{main}.  The effect is the same like
 attaching attribute @ref{AVR Function Attributes,,@code{OS_task}}
 to @code{main}. It is activated per default if optimization is on.
 
+@opindex mno-call-main
+@opindex mcall-main
+@item -mno-call-main
+Don't call @code{main} and @code{exit}.
+Instead, put @code{main} in section
+@w{@uref{https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init,@code{.init9}}}.
+
+This option can be used for devices with very limited resources in order
+to save a few bytes of code and stack space.  It will work as expected since
+@w{@uref{https://github.com/avrdudes/avr-libc/issues/1012,AVR-LibC v2.3}}.
+With older versions, there will be no performance gain.
+
 @opindex mno-interrupts
 @item -mno-interrupts
 Generated code is not compatible with hardware interrupts.
@@ -24535,7 +24547,19 @@  Allow to use truncation instead of rounding towards zero for fractional fixed-po
 
 @opindex nodevicelib
 @item -nodevicelib
-Don't link against AVR-LibC's device specific library @code{lib<mcu>.a}.
+Don't link against AVR-LibC's device specific library @code{lib@var{mcu}.a}.
+
+Notice that since AVR-LibC v2.3, that library contains code that is
+essential for the correct functioning of a program.  In particular,
+it contains parts of the startup code like:
+@w{@uref{https://github.com/avrdudes/avr-libc/issues/1011,@code{__init_sp}}}
+to initialize the stack pointer with symbol @code{__stack},
+@w{@uref{https://github.com/avrdudes/avr-libc/issues/1010,@code{__init_cvt}}}
+to set up the hardware to use a compact vector table with @option{-mcvt},
+@w{@uref{https://github.com/avrdudes/avr-libc/issues/1012,@code{__call_main}}}
+to call @code{main} and @code{exit}, and
+@w{@uref{https://github.com/avrdudes/avr-libc/issues/931,@code{__do_flmap_init}}}
+to set up FLMAP according to symbol @code{__flmap}.
 
 @opindex nodevicespecs
 @item -nodevicespecs