[Caml-list] Re: ARM code generator problem

[Benedikt Meurer <benedikt.meurer@googlemail.com>]

On Aug 13, 2012, at 21:21 , Jeffrey Scofield wrote:

> OCamlers, Benedikt:

Hey Jeffrey,

>>> 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).

Jep, that's a bug indeed. Somewhow ocamlopt seems to believe that the Ifloatofint instruction preserves d7 although it is marked as destroyed for this operation.

> Best regards,
> Jeffrey

greets,
Benedikt