Disassembly Oddities.

dchokola · 2014-01-11 18:36

I have been trying to debug some LMM code with a single .cog module, so I compiled both with -O0 and -g, then did an objdump. I'm new to Propeller assembly (In fact, I'm using C to *avoid* using assembly.) so I'm not sure if I just don't understand what I'm seeing or if there is some other problem.

Throughout the dump, particularly in the .cog module, there are lines like this:

0000015c <__LMM_JMP>:
 15c:   1122bc08                        rdlong  44 <pc>, 44 <pc>

00000000 <r0>:
   0:   0000bca0                        mov     40 <sp>, 40 <sp>
00000004 <r1>:
   4:   0000bca0                        mov     0 <r0>, 0 <r0>
00000008 <r2>:
   8:   0000bc0a                        rdlong  4 <r1>, 4 <r1> wz

Wouldn't these instructions be useless since the sources and destinations are identical?

dchokola · 2014-01-11 18:51

I should mention that this is propeller-elf-gcc (propellergcc-alpha_v1_9_0_2277) 4.6.1 (Latest from propgcc Mercurial as of today.)

David Betz · 2014-01-11 19:17

dchokola wrote: »

I should mention that this is propeller-elf-gcc (propellergcc-alpha_v1_9_0_2277) 4.6.1 (Latest from propgcc Mercurial as of today.)

No, they're not useless. They are essentially: pc = *pc;

dchokola · 2014-01-11 19:29

David Betz wrote: »

No, they're not useless. They are essentially: pc = *pc;

Thank you! I really over-thought this one.

ersmith · 2014-01-13 04:56

Actually the problem is with the disassembler -- it's only able to handle one relocation at a time. I can't remember why this is, but I remember looking at it and it wasn't trivial to fix. So disassembly of relocatable object files tends to be kind of useless; the instructions come out right, but you can't rely on the operands.

Disassembly of final executable binaries is OK, though, because those no longer have relocations.

Disassembly Oddities.

Comments