MIPS: Changing the PC stored from a "and link" instruction

["Brandon H. Dwiel" <bhdwiel at ncsu dot edu>]

Hello,

I am a student working on a project involving generating a MIPS processor.
We have decided to NOT implement logic to handle branch delay slots and instead
work with generating a compiler that will emit code without these delay slots.
The compiler tool chain versions are:

binutils:      2.15
gcc:           3.4.5
glibc:         2.3.6
linux-headers: 2.6.12

So far, I have been mostly successful in removing these delay slots. (nops still
exist in some situations).
However, the offsets are still with respect to the branch delay slot.
This is not a problem in most cases because we just add 4 to the offset in the pipeline
instead of adding the PC of the delay-slot instruction to the offset. There is one
case where this solution does not work. Here is the segment of code:

  400184: 04110001  bal 40018c<__start+0xc>

  400188: 00000000  nop

  40018c: 3c1c0fc1  lui gp,0xfc1

  400190: 279cbc64  addiu gp,gp,-17308

  400194: 039fe021  addu  gp,gp,ra


The bal instruction here is expected to push the PC of the instruction after
the delay slot (0x40018c). This value is then later added to the $gp register
in the last instruction of the sequence. Because of this behavior, putting
the PC of the instruction after the branch results in incorrect execution.

Looking at another segment of code from the same program:

  400204: 0320f809  jalr  t9

  400208: 8fbc0010  lw  gp,16(sp)

  40020c: 8fbf0018  lw  ra,24(sp)


Notice here that if I push the PC of the instruction after the delay slot (0x40020c),
then I will skip the load that writes to the $gp. I realize that in this case it is possible
that 0x40020c is expected to be pushed, but I have not noticed any
repercussions of simply pushing 0x400208 instead.

I would like to make the changes necessary so that the compiler expects the PC of the
instruction directly after the branch to be put in the $ra register.

I cannot locate where it is specified that PC+8 of an "and link" instruction is to
be put in the $ra so that I may change it.

-Brandon