Re: [Caml-list] ARM code generator problem

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

On Aug 10, 2012, at 23:41 , Jeffrey Scofield wrote:

> Greetings,

Hey Jeffrey,

> While working on porting OCaml 4.00.0 to iOS, I ran across
> what looks like a problem in the ARM code generation.
> 
> If you look at asmcomp/arm/emit.mlp you see lots of places where
> s14 is used as a scratch register.  The one that showed up in my
> code is the code sequence for float_of_int:
> 
>    | Lop(Ifloatofint) ->
>        `       fmsr    s14, {emit_reg i.arg.(0)}\n`;
>        `       fsitod  {emit_reg i.res.(0)}, s14\n`; 2
> 
> Note that the emitted code always uses s14 (unconditionally).  This
> suggests that s14 should be set aside as a scratch register.
> 
> However, s14 is also an alias for the low order part of d7.  If you look
> at asmcomp/arm/proc.ml you'll see that d7 is used as a general purpose
> register.
> 
> 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.

If possible, it would probably also make sense to merge some of the iOS related code into the upstream ARM backend, in case you are interested.

> Regards,
> Jeffrey

greets,
Benedikt