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[AVR][PATCH] Add linker relaxation support

[AVR][PATCH] Add linker relaxation support / Fix 64 bit bug in gas rev. 5

Björn Haase bjoern.m.haase@web.de
Mon Nov 7 21:02:00 GMT 2005 
Hello,

ok, Nick, I indeed have forgotten to attach the patch. Here it is now. Here I 
also have added the appropriate *(.data.*) and *(.bss.*) lines for making the 
-fdata-sections switch useable.

Bjoern.


2005-10-26  Bjoern Haase  <bjoern.m.haase@web.de>

	* include/elf/avr.h: 

	R_AVR_MS8_LDI,R_AVR_MS8_LDI_NEG: Add.
	EF_AVR_LINKRELAX_PREPARED: Add.

	* bfd/elf32-avr.c:

	avr_reloc_map: 
	insert BFD_RELOC_AVR_MS8_LDI and R_AVR_MS8_LDI
	bfd_elf_avr_final_write_processing: 
	set EF_AVR_LINKRELAX_PREPARED in e_flags field.
        elf32_avr_relax_section: add.
        elf32_avr_relax_delete_bytes: add.
        elf32_avr_get_relocated_section_contents: add.
	avr_pc_wrap_around: add.
	avr_relative_distance_considering_wrap_around: add.
	avr_final_link_relocate: Handle negative int8t_t immediate for R_AVR_LDI
        
        * gas/config/tc-avr.c: 

	avr_mod_hash_value: Add.
	md_apply_fix, exp_mod: 
	Use BFD_RELOC_HH8_LDI and BFD_RELOC_MS8_LDI for hlo8() and hhi8()
	md_begin: 
	set linkrelax variable to 1, use avr_mod_hash_value instead of int 
	avr_ldi_expression: use avr_mod_hash_value instead of (int)
	tc_gen_reloc:
	handle substractions of symbols, if possible do fixups, abort otherwise
	
	* gas/config/tc-avr.h:
	TC_LINKRELAX_FIXUP, TC_VALIDATE_FIX, tc_fix_adjustable: add.
	
	* bfd/bfd-in2.h: Add BFD_RELOC_AVR_MS8_LDI and BFD_RELOC_AVR_LDI_NEG
	* bfd/libbfd.h: Ditto.
	* bfd/reloc.c: Ditto.

	* ld/scripttempl/avr.sc:
	add *(.jumptables) *(.lowtext), add KEEP() , add *(.data.*) and *(.bss.*)


P.S.: Again the same description of the patch as known from rev. 4. Only 
addition: .data.* and .bss.* sections are now added.

GAS:

In order to make relaxation work, we need to make gas preserve more
relocs than it used to do. The issue is that all the relative instruction
offsets no longer must be calculated at assembly time but at linke time.
For this purpose the patch disables the fixups for all of the program-space
relative relocs by setting linkrelax to the value 1.
I also have made use of <inttypes.h> in order to fix a bug that shows up
for 64 bit machines for present mainline.

Because there was no way to find out from the object file itself if it is
suitable for relaxing or not, It is necessary to avoid that ld attempts to
run relaxing optimization on old object files. In order to guarantee this,
I have introduced a new flag bit in the e_flag portion of the elf header. 
Bits 0-3 have the same meaning (new define symbol EF_AVR_LINKRELAX_PREPARED).

In order to be able to leave the fixup for the higher two bytes of 32 bit 
values
to the linker, I have changed the present workaround. Before my change gas
internally made use of negated BFD enum values for these higher bytes.
I replaced this by adding proper new relocs to bfd and the elf files
(BFD_RELOC_AVR_MS8_LDI and R_AVR_MS8_LDI as well as the negated variants)
for the most significant byte of 32 bit values.


BFD:

I first had to implement a couple of changes in bfd for supporting the new 
relocs for the most significant bytes.

Three functions for implementing linker relaxation support are now new.
Before the relaxation machine starts, it is checked that the bfd
has the EF_AVR_LINKRELAX_PREPARED flag set in the elf header so that we could
safely continue to link old object files.

In the ".jumptables" and ".vectors" sections only a reduced relaxation
machine runs: Jumps and Calls are replaced by the shorter variants
without changing the relative addresses.

In elf32-avr.c I then have introduced a new global variable determinating
the wrap-around value of the avr program memory space. This value is
presently initialized with a value signalizing that no wraparound could occur,
but if it is filled one day with the correct value,
the relaxation machine will make use of the wrap-around shortcuts for jumps.
Since this is interesting only for targets with 16k and 32k of program memory,
I thought about adding a target-specific options for ld 
(-muse-16k-wraparound-jumps) or (-muse-32k-wraparound-jumps).

LD:

In order to fix the --gc-sections bug I have added the required KEEP() 
statements in the linker script.
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