Re: [Lightning] jit_movr broken on MIPS

[Paul Cercueil]

Hi Paulo,

>    Please post the disassembly log (you may post it off list to me to
> avoid a 6M+ mail, and I would reply with only what should matter).
> Please also have in the log a call to jit_print, and edit the log to tell
> where it was optimized out. If the jit_print call already removed the
> jit_movr please annotate in the log where it was done.
>    This should make it easier to understand what condition led
> to the instruction being optimized out.

I uploaded three snippets of log:

The reference, generated by a different JIT, that shows hashes of the 2 
MiB of RAM, of the (emulated) 32 MIPS GPR and 32 CP0 registers before 
and after the execution of the code:
http://crapouillou.net/~paul/reference.txt

The machine code generated by my lightning-based dynarec, with latest 
GIT of Lightning. The issue I'm talking about can be seen line 47, where 
a call to jit_retval_i() doesn't generate any code.
http://crapouillou.net/~paul/broken.txt

Finally, the machine code generated by the *exact* same binary, not 
recompiled in between, with the GIT version of Lightning patched to use 
jit_orr(dst, 0, src) instead of jit_movr(dst, src) within the function 
jit_retval_i():
http://crapouillou.net/~paul/fixed.txt

There, the bug doesn't appear anymore. The return value of the function 
call is correctly fetched into s0. The hashes corresponding to the final 
state match perfectly the ones of the reference JIT.

>    Note that, unless you found a bug, it may optimize out a store
> to a non callee save register under certain conditions, like before
> a jit_ret* (non callee save register liveness reach epilog) or a
> jit_jmpr, or a jit_jmpi patched to an absolute address; it treats
> jit_jmpr and jit_jmpi to absolute address as a function call, as
> otherwise it would need to consider all non callee save registers
> live on a jump to an undefined target, and that would defeat the
> register allocator.

I would have understood that; But in my case the optimized-out register 
is s0, which is a callee-saved register.

Regards,

Paul