LuaJIT - loop conditional code generation

[Jani Piitulainen <jani.piitulainen+ll@...>]

Is there a way to stop simple conditionals in a loop from jumping out of a compiled trace? 

"bit.band" with one argument is used here to tell LuaJIT result is a 32-bit integer, it's compiled to nothing. It can sometimes be useful reducing unnecessary double <-> int conversions (like cvtsi2sd & cvtsd2si).

  for i=1, 1000000, 1 do

    if band(i, 7)==0 then

      c = band(c + 1)

    end

    if band(i, 7)==3 then

      c = band(c - 1)

    end

    a = band(a + c)

  end

Using LuaJIT git head x64. The generated code is very similar for x86.

ebx = i, ebp = a, eax = c

->LOOP:

394cffd0  mov r15d, ebx

394cffd3  and r15d, +0x07

394cffd7  jz 0x394c0028 ->6 -- this jumps out of the JITted trace, right? why not jnz to "cmp r15d, +0x03" here + add eax, 1?

394cffdd  cmp r15d, +0x03

394cffe1  jz 0x394c002c ->7

394cffe7  add ebp, eax

394cffe9  add ebx, +0x01

394cffec  cmp ebx, 0x000f4240

394cfff2  jle 0x394cffd0        ->LOOP

Full code:

local bit = require("bit");

local band=bit.band;

function test()

  local a=band(0)

  local c=band(1)

  for i=1, 1000000, 1 do

    if band(i, 7)==0 then

      c = band(c + 1)

    end

    if band(i, 7)==3 then

      c = band(c - 1)

    end

    -- changing conditions into following form produces effectively identical x64 code

    -- c = band(c + (band(i, 7)==0 and 1 or 0))

    -- c = band(c - (band(i, 7)==3 and 1 or 0))

    a = band(a + c)

  end

  return a,c

end

for j=1, 10, 1 do

  print(test())

end

Thanks,

Jani Piitulainen