Re: [PATCH, GCC/ARM] Remove ARMv8-M code for D17-D31

[Thomas Preudhomme <thomas dot preudhomme at foss dot arm dot com>]

Hi Richard,

On 28/06/17 16:56, Richard Earnshaw (lists) wrote:
> On 20/06/17 16:01, Thomas Preudhomme wrote:
> > Hi,
> >
> > Function cmse_nonsecure_entry_clear_before_return has code to deal with
> > high VFP register (D16-D31) while ARMv8-M Baseline and Mainline both do
> > not support more than 16 double VFP registers (D0-D15). This makes this
> > security-sensitive code harder to read for not much benefit since
> > libcall for cmse_nonsecure_call functions do not deal with those high
> > VFP registers anyway.
> >
> > This commit gets rid of this code for simplicity and fixes 2 issues in
> > the same function:
> >
> > - stop the first loop when reaching maxregno to avoid dealing with VFP
> >    registers if targetting Thumb-1 or using -mfloat-abi=soft
> > - include maxregno in that loop
>
> This is silently baking in dangerous assumptions about GCC's internal
> numbering of the registers.  That's not a good idea from a long-term
> portability perspective.
>
> At the very least you need to assert that all the interesting registers
> are numbered in the range 0..63; but ideally the code should just handle
> pretty much any assignment of internal register numbers.

Well there is already this:

gcc_assert ((unsigned) maxregno <= sizeof (to_clear_mask) * __CHAR_BIT__);

> Did you consider using sbitmaps rather than doing all the multi-word
> stuff by steam?

No but am happy to. I'll respin the patch.

Best regards,

Thomas