AVR: Support .rodata in Flash for AVR64* and AVR128* Devices.

These devices see a 32 KiB block of their program memory (flash) in
the RAM address space.  This can be used to support .rodata in flash
provided Binutils support PR31124 (Add new emulations which locate
.rodata in flash).  This patch does the following:

* configure checks availability of Binutils PR31124.

* Add new command line options -mrodata-in-ram and -mflmap.
While -flmap is for internal usage (communicate hardware properties
from device-specs to the compiler proper), -mrodata-in-ram is a user
space option that allows to return to the current rodata-in-ram layout.

* Adjust gen-avr-mmcu-specs.cc so that device-specs are generated
that sanity check options, and that translate -m[no-]rodata-in-ram
to its emulation.

* Objects in .rodata don't drag __do_copy_data.

* Document new options and built-in macros.

	PR target/112944

gcc/
	* configure.ac [target=avr]: Check availability of emulations
	avrxmega2_flmap and avrxmega4_flmap, resulting in new config vars
	HAVE_LD_AVR_AVRXMEGA2_FLMAP and HAVE_LD_AVR_AVRXMEGA4_FLMAP.
	* configure: Regenerate.
	* config.in: Regenerate.
	* doc/invoke.texi (AVR Options): Document -mflmap, -mrodata-in-ram,
	__AVR_HAVE_FLMAP__, __AVR_RODATA_IN_RAM__.

	* config/avr/avr.opt (-mflmap, -mrodata-in-ram): New options.
	* config/avr/avr-arch.h (enum avr_device_specific_features):
	Add AVR_ISA_FLMAP.
	* config/avr/avr-mcus.def (AVR_MCU) [avr64*, avr128*]: Set isa flag
	AVR_ISA_FLMAP.
	* config/avr/avr.cc (avr_arch_index, avr_has_rodata_p): New vars.
	(avr_set_core_architecture): Set avr_arch_index.
	(have_avrxmega2_flmap, have_avrxmega4_flmap)
	(have_avrxmega3_rodata_in_flash): Set new static const bool according
	to configure results.
	(avr_rodata_in_flash_p): New function using them.
	(avr_asm_init_sections): Let readonly_data_section->unnamed.callback
	track avr_need_copy_data_p only if not avr_rodata_in_flash_p().
	(avr_asm_named_section): Track avr_has_rodata_p.
	(avr_file_end): Emit __do_copy_data also when avr_has_rodata_p
	and not avr_rodata_in_flash_p ().
	* config/avr/specs.h (CC1_SPEC): Add %(cc1_rodata_in_ram).
	(LINK_SPEC): Add %(link_rodata_in_ram).
	(LINK_ARCH_SPEC): Remove.
	* config/avr/gen-avr-mmcu-specs.cc (have_avrxmega3_rodata_in_flash)
	(have_avrxmega2_flmap, have_avrxmega4_flmap): Set new static
	const bool according to configure results.
	(diagnose_mrodata_in_ram): New function.
	(print_mcu): Generate specs with the following changes:
	<*cc1_misc, *asm_misc, *link_misc>: New specs so that we don't
	need to extend avr/specs.h each time we add a new bell or whistle.
	<*cc1_rodata_in_ram, *link_rodata_in_ram>: New specs to diagnose
	-m[no-]rodata-in-ram.
	<*cpp_rodata_in_ram>: New. Does -D__AVR_RODATA_IN_RAM__=0/1.
	<*cpp_mcu>: Add -D__AVR_AVR_FLMAP__ if it applies.
	<*cpp>: Add %(cpp_rodata_in_ram).
	<*link_arch>: Use emulation avrxmega2_flmap, avrxmega4_flmap as
	requested.
	<*self_spec>: Add -mflmap or %<mflmap as needed.

gcc/testsuite/
	* gcc.target/avr/torture/pr112944-flmap-0.c: New test.
	* gcc.target/avr/torture/pr112944-flmap-1.c: New test.

1 files changed, 38 insertions, 2 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index b7a2013..1773f0d 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -883,11 +883,11 @@ Objective-C and Objective-C++ Dialects}.
 @emph{AVR Options}
 @gccoptlist{-mmcu=@var{mcu}  -mabsdata  -maccumulate-args
 -mbranch-cost=@var{cost}
--mcall-prologues  -mgas-isr-prologues  -mint8
+-mcall-prologues  -mgas-isr-prologues  -mint8 -mflmap
 -mdouble=@var{bits} -mlong-double=@var{bits}
 -mn_flash=@var{size}  -mno-interrupts
 -mmain-is-OS_task  -mrelax  -mrmw  -mstrict-X  -mtiny-stack
--mfract-convert-truncate
+-mrodata-in-ram -mfract-convert-truncate
 -mshort-calls  -nodevicelib  -nodevicespecs
 -Waddr-space-convert  -Wmisspelled-isr}
 
@@ -23719,6 +23719,20 @@ differ from instructions in the assembler code.
 Relaxing must be turned on if linker stubs are needed, see the
 section on @code{EIND} and linker stubs below.
 
+@opindex mrodata-in-ram
+@item -mrodata-in-ram
+@itemx -mno-rodata-in-ram
+Locate the @code{.rodata} sections for read-only data in RAM resp.@:
+in program memory.
+For most devices, there is no choice and this option acts rather
+like an assertion.
+
+Since v14 and for the AVR64* and AVR128* devices, @code{.rodata}
+is located in flash memory per default, provided the required GNU Binutils
+support (@w{@uref{https://sourceware.org/PR31124,PR31124}}) is available.
+In that case, @option{-mrodata-in-ram} can be used to return to the old
+layout with @code{.rodata} in RAM.
+
 @opindex mstrict-X
 @item -mstrict-X
 Use address register @code{X} in a way proposed by the hardware.  This means
@@ -24112,6 +24126,23 @@ description file, and is currently available for
 there is no need to use address spaces like @code{__flash} or
 features like attribute @code{progmem} and @code{pgm_read_*}.
 
+@item __AVR_HAVE_FLMAP__
+This macro is defined provided the following conditions are met:
+@itemize @bullet
+@item The device has the @code{NVMCTRL_CTRLB.FLMAP} bitfield.
+This applies to the AVR64* and AVR128* devices.
+@item It's not known at assembler-time which emulation will be used.
+@end itemize
+This implies the compiler was configured with GNU Binutils that implement
+@w{@uref{https://sourceware.org/PR31124,PR31124}}.
+
+@item __AVR_RODATA_IN_RAM__
+This macro is undefined when the code is compiled for a core architecture.
+
+When the code is compiled for a device, the macro is defined to@tie{}1
+when the @code{.rodata} sections for read-only data is located in RAM;
+and defined to@tie{}0, otherwise.
+
 @item __WITH_AVRLIBC__
 The compiler is configured to be used together with AVR-Libc.
 See the @option{--with-avrlibc} configure option.
@@ -24165,6 +24196,11 @@ files.
 Assume that the flash memory has a size of @var{num} times 64@tie{}KiB.
 This determines which @code{__flash@var{N}} address spaces are available.
 
+@opindex mflmap
+@item -mflmap
+The device has the @code{FLMAP} bit field located in special function
+register @code{NVMCTRL_CTRLB}.
+
 @opindex mrmw
 @item -mrmw
 Assume that the device supports the Read-Modify-Write