Original bug ID: 5731
Reporter: jeffsco
Assigned to: meurer
Status: closed (set by @xavierleroy on 2015-12-11T18:19:43Z)
Resolution: fixed
Priority: normal
Severity: major
Platform: ARM
OS: Debian
OS Version: armel 2.6.32-5
Version: 4.00.0
Fixed in version: 4.00.1+dev
Category: back end (clambda to assembly)
Monitored by: cookedm

Bug description

Code generation error. A value is loaded into d7. A call to float_of_int generates code that loads s14, which overwrites the value in d7 (low order mantissa). The value in d7 is then used in a later computation, but the value has been destroyed.

If I make small changes to the code, the load of d7 happens after the float_of_int computation, and things work OK. Possibly some constraints on instruction orderings aren't being applied properly.

Steps to reproduce

Need a version of ocamlopt with support for armv7 with hardware floating point (VFPv3).

Then compile the following code:

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

My compile line looks like this:

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

I'm attaching the full assembly code output. The section with the error contains the code for the following lines:

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

The erroneous code looks like this (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

Additional information

I ran my tests on a stock OCaml 4.00.0 compiler in an emulated Linux ARMEL environment under QEMU on OS X. I don't think this affects the results; I see exactly the same instruction sequence in my experimental cross-compiler builds (compiling for iOS under OS X).

Here is my configure line:

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

(Need to use gnueabihf to get VFPv3 support.)

Then build ocamlopt as usual.

File attachments

• rate.s