Re: RFC: [MIPS] Add an option to disable ldc1/sdc1

[Ralf Baechle <ralf at linux-mips dot org>]

On Fri, Feb 15, 2013 at 01:23:14AM +0000, Maciej W. Rozycki wrote:

> > What about 64-bit targets?  We can sometimes access doubles using GPRs,
> > so on 64-bit targets we could end up using LD and SD to access a double
> > even when this option disables LDC1 and SDC1.  I think we'd need to
> > patch the move patterns as well.
> 
>  As far as Linux is concerned -- which from the context of this 
> consideration I infer is the case -- there is no issue, because unless an 
> app has explicitly requested this feature to be disabled, with the use of 
> the sysmips(2) FIXADE API (which of course hardly any does), all address 
> error exceptions triggered by an unaligned memory access made with a CPU 
> instruction are trapped by the kernel and the access emulated.

The sysmips(MIPS_FIXADE, ...) API is crafted after other, older MIPS
UNIX variants which as far as I was able to find out all only support
fixing up of integer accesses.

>  However no such accesses are emulated when made with a coprocessor 
> instruction, whether the FPU or CP2 or the legacy CP1 and CP3 units as 
> supported by earlier processors.  There was once a proposal to emulate FPU 
> instructions as well, posted on the linux-mips mailing list here:
> 
> http://www.linux-mips.org/cgi-bin/mesg.cgi?a=linux-mips&i=20120615234641.6938B58FE7C%40mail.viric.name
> 
> -- but the MIPS/Linux maintainer rejected the idea.  I have cc-ed Ralf on 
> this reply in case he wanted to add something.
> 
>  Of course an option to GCC to disable any such instructions may have some 
> value to some people -- for bare-iron targets if nothing else -- but I 
> fear this is going to end up with a lot of hassle with 64-bit ABIs or even 
> just 64-bit FPU (-mabi=32 -mfp64) as individual halves of 64-bit registers 
> are not addressable in the MIPS architecture (e.g. there's no LWHC1 
> instruction), so you'll need to use scratch registers.
> 
>  Which is why I think any resources put into this effort would better be 
> used to clean up such broken code and, perhaps more importantly, to 
> educate people to write their programs properly in the first place.  
> Writing portable code is really not a big deal, you just need to remember 
> all the world is not x86 and apply a few simple rules.

And I'd like to remind that emulation of missaligned accesses in software
is on the order of 1000 times slower than a properly aligned access.  As I
understand right now the issue only affects webkit and code based on it so
fixing it in webkit should be the way to go rather than introducing a
compatibility hack.

Even on x86 (or MIPS variants such as the Octeon core) which fixup
misaligned accesses in hardware there is still a performance penalty for
misalignment so proper alignment always matters.

Unless maybe when packing structures for space reasons.

  Ralf