[Caml-list] Re: ARM code generator problem

[Jeffrey Scofield <jeffsco@psellos.com>]

OCamlers, Benedikt:

>> The result is that a value in d7 is sometimes destroyed by a use of s14
>> as a scratch register.  In my code it was a call to float_of_int that
>> destroyed a float value being kept in d7.
> 
> If you look at destroyed_at_oper in asmcomp/arm/proc.ml, you'll see that
> d7 (s14+s15) is marked as destroyed for those operations where it is
> used as scratch register.

I was able to reproduce this behavior with the stock OCaml 4.00.0 compiler,
so I really do think there's a problem.

I whittled my code down to just a few lines.  Here it is:

    let rate_pos scounts : float =
        let m_MIN = -999.0 
        in let max1s = Array.make 14 m_MIN
        in let max2s = Array.make_matrix 14 14 m_MIN
        in let try_build (k1: int) (m: float) : unit =
            let denom = 12
            in let try1b (sawk1, xct) k =
                let () =
                    if max2s.(k1).(k) > m then
                        let adjm = if m <= m_MIN then 0.0 else m
                        in let numer =
                            if k = k1 then 48
                            else if sawk1 then 36
                            else 24
                        in let f = float_of_int numer /. float_of_int denom
                        in let () =
                            if max1s.(k1) <= m_MIN then max1s.(k1) <- 0.0
                        in
                            max1s.(k1) <-
                                max1s.(k1) +. (max2s.(k1).(k) -. adjm) *. f
                in
                    if k = k1 then
                        (true, xct)
                    else
                        (sawk1, xct + scounts.(k))
            in
                ignore (List.fold_left try1b (false, 0) [])
        in let () = Array.iteri try_build max1s
        in
            0.0

(This is a heavily hacked up piece of an evaluation function for a card
game app.)

Here is my OCaml command line (running on Linux/ARM inside Qemu, as you
suggested--it works!):

$ ocamlopt -ffpu vfpv3 -c -S rate.ml

I'm using vfpv3 because that's what I use for my iOS port.  The system
type is linux_eabihf, which is what you need to get vfpv3 support.

The section that seems to misbehave is these three lines:

    in let f = float_of_int numer /. float_of_int denom
    in let () =
        if max1s.(k1) <= m_MIN then max1s.(k1) <- 0.0

Here is the assembly code with added annotations:

        ldr     r12, [r2, #16]    @ r12 <- m_MIN block
        mov     r0, r7, asr #1    
        ldr     r7, [r2, #20]
        movs    r6, #0xc          @ r6 <- denom
        fmsr    s14, r6           
        fsitod  d10, s14          @ d10 <- float_of_int denom
        ldr     r6, [r7, #-4]
        fldd    d7, [r12, #0]     @ d7 <- m_MIN
        ldr     r12, [r2, #28]
        fmsr    s14, r0           @ *** d7 is destroyed here ***
        fsitod  d9, s14           @ d9 <- float_of_int numer
        cmp     r12, r6, lsr #10
        bcs     .L111
        add     r6, r7, r12, lsl #2
        fldd    d6, [r6, #-4]     @ d6 <- max1s.(k1)
        fdivd   d8, d9, d10
        fcmpd   d6, d7            @ *** This comparison fails ***
        fmstat
        bhi     .L104

I built the OCaml 4.00.0 compiler from sources inside Qemu.  The
line for configure was just this:

    $ ./configure --host armv5tejl-unknown-linux-gnueabihf

After that, I just built as usual.

If you agree that this is a problem, I can create a Mantis
bug report for it (if you like).

Best regards,

Jeffrey