The GNU Debugger and me

At the end of last week I tried to profile the performance of matmult and parfib, but discovered it was broken. Specifically, the allocation count for some cost centers is getting corrupted. For example:

COST CENTRE            MODULE    no. entries  %time %alloc   %time %alloc
MAIN        MAIN     1           0   0.0   20.3     0.0  100.0
 main       Main     172 30590380562441576   0.0   19.0     0.0   19.0
 CAF        Main     166 30590376291721096   0.0   20.2     0.0   20.2
  main      Main     173           0   0.0   0.0    0.0    0.0
   dude     Main     174 30619204112395264   0.0    0.0     0.0    0.0

Sadly, for some reason it's also getting corrupted with the via-c build, so my usual approach of swapping out suspect -fasm closure code for known good -fvia-c code isn't going to fly.

That puts me squarely in GDB territory, and the nastiness of breakpoints and single stepping through instructions. I figure GDB is like the SWAT team of software engineering. It's fun to recount its exploits to your pals over beer, but you hope you're never in a situation where you actually need to call on its services.

Anyway, some time later I've got a Cmm line:

        I32[CCCS] = CAF_ccs;
        I64[I32[R1 + 4] + 16] = I64[I32[R1 + 4] + 16] + 1 :: W64;

Which produces:

        
        sethi   %hi(Main_CAFs_cc_ccs),%g1
        or      %g1,%lo(Main_CAFs_cc_ccs),%g1 -- ok, load CAF_ccs into %g1
        sethi   %hi(CCCS),%g2
        or      %g2,%lo(CCCS),%g2    -- ok, load addr of CCCS into %g2
        st      %g1,[%g2]            -- ok, I32[CCCS] = CAF_ccs
        add     %g1,1,%g1            -- hmmm: produce a misaligned addr into %g1
        addx    %g2,%g0,%g2          -- hmmm: a nop. %g0 is always zero
        ld      [%l1+4],%g3          -- ok, load addr of the cost centre record into %g3
        add     %g3,16,%g3           -- ok, calculate the addr of the allocation count
                                     --     for that cost centre 

        st      %g2,[%g3]            -- badness! write addr into the allocation count field
        st      %g1,[%g3+4]          --          ditto

Finding being 80% of fixing, hopefully this'll be working again tomorrow.