On resolving the MIPS gas branch reloc issue

[Alexandre Oliva]

On Feb 26, 2003, Thiemo Seufer <ica2_ts@csv.ica.uni-stuttgart.de> wrote:

> The Embedded PIC ABI did something similiar earlier but with a GNU
> extension relocation. To keep compatible to it, we need now one new
> BFD_RELOC_MIPSEMB_16_PCREL_S2 bfd reloc, because BFD has no way to
> find out the ABI early enough.

Have you tested this heavily with n32 and/or n64?  I ran into all
sorts of trouble because of weirdnesses such as the code next to the
following comments in gas/config/tc-mips.c:

	      /* In this case, the bfd_install_relocation routine will
		 incorrectly add the symbol value back in.  We just want
		 the addend to appear in the object file.

		 howto->pcrel_offset is added for R_MIPS_PC16, which is
		 generated for code like

		 	globl g1 .text
			.text
			.space 20
		 g1:
		 x:
		 	bal g1
	       */


	      /* Make sure the addend is still non-zero.  If it became zero
		 after the last operation, set it to a spurious value and
		 subtract the same value from the object file's contents.  */


      /* This code was generated using trial and error and so is
	 fragile and not trustworthy.  If you change it, you should
	 rerun the elf-rel, elf-rel2, and empic testcases and ensure
	 they still pass.  */

	  /* BFD's REL handling, for MIPS, is _very_ weird.
	     This gives the right results, but it can't possibly
	     be the way things are supposed to work.  */


      /* If 'value' is zero, the remaining reloc code won't actually
	 do the store, so it must be done here.  This is probably
	 a bug somewhere.  */


All of these chunks assume REL relocations, and since n32 and n64 use
RELA, there's no way to generate appropriate relocations for certain
border values that trigger special conditions in the generic
relocation handling code.  I tried very hard to fix them, but failed.
I wish the fix was this simple :-(

Perhaps we should just go with branches to symbols in the same
sections for now?  Do you still have patch that you send me the other
day to that effect handy?  Perhaps you could propose it here as well?

-- 
Alexandre Oliva   Enjoy Guarana', see http://www.ic.unicamp.br/~oliva/
Red Hat GCC Developer                 aoliva@{redhat.com, gcc.gnu.org}
CS PhD student at IC-Unicamp        oliva@{lsd.ic.unicamp.br, gnu.org}
Free Software Evangelist                Professional serial bug killer