Re: [Lightning] jit_qdivr_u trashes JIT_R0 on x86_64

[Paulo César Pereira de Andrade]

Em sáb, 31 de ago de 2019 às 10:37, Paul Cercueil <address@hidden> escreveu:
>
> Hi Paulo,

  Hi Paul,

  I just pushed a jit_live wrapper to check/lightning.c so that it makes
it easier to test patterns using jit_live().

> I added jit_live() calls to my known live registers after each call to
> an external function, and a lot of bugs suddently disappeared :)

  I believe the function your code calls is to avoid inlining identical code,
if that is the case, you could use a simple block of code, to avoid the
prolog/epilog cost. If it is a C function, the compiler should generate
an optimized version, or you could write it in assembly.

  For example, using the new 'live' wrapper (btw, in this specific
example, using 'live' is not required, because it understands %r0 is
live due to using it as a printf argument):
"""
#define USE_JMPR    1
.data    16
fmt:
.c    "%d %d %d\n"
.code
    jmpi main
helper:
    movi %v1 5
    movi %v2 7
#if USE_JMPR
    jmpr %v0
#else
    jmpi label
#endif
main:
    prolog
    movi %v1 2
    movi %v2 3
    addr %r0 %v1 %v2
#if USE_JMPR
    movi %v0 label
#endif
    jmpi helper
label:
#if USE_JMPR
    live %r0
#endif
    qdivr %v1 %v2 %v2 %v1
    prepare
        pushargi fmt
        ellipsis
        pushargr %r0
        pushargr %v1
        pushargr %v2
    finishi @printf
    ret
    epilog
"""

  Suppose 'helper' above is a common code, then, your code can
just jump to the common code, and from the common code, jump
back; you might also make it inside a jit function, because if the
common code needs to spill/reload a temporary, it must have
a stack frame, in that case, just create an block only reachable
from a jump to the common code.
  If the address to jump back is known, the helper code can jump
back to it, allowing lightning to follow the jump and understand the
live registers, otherwise, can use jmpr, passing the 'jump back'
address in a register.
  jmpr allows flexibility in that kind of construct, as well as different
approaches for dispatch tables.
  Another alternative is to use jit_tramp and jit_frame. See check/tramp.tst
or check/ctramp.c for an example, implementing a Fibonacci number
calculator and tail call optimized code.

> So, case closed. Thank you for your help!
>
> -Paul

Thanks,
Paulo