Re: [PATCH v2 3/3] crypto: arm64/aegis128 - implement plain NEON version

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Mon, 12 Aug 2019 at 20:31, Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Mon, Aug 12, 2019 at 10:22 AM Ard Biesheuvel
> <ard.biesheuvel@linaro.org> wrote:
> >
> > On Mon, 12 Aug 2019 at 19:50, Nick Desaulniers <ndesaulniers@google.com> wrote:
> > >
> > > On Sun, Aug 11, 2019 at 3:59 PM Ard Biesheuvel
> > > <ard.biesheuvel@linaro.org> wrote:
> > > > diff --git a/crypto/Makefile b/crypto/Makefile
> > > > index 99a9fa9087d1..0d2cdd523fd9 100644
> > > > --- a/crypto/Makefile
> > > > +++ b/crypto/Makefile
> > > > @@ -98,7 +98,14 @@ CFLAGS_aegis128-neon-inner.o += -mfpu=crypto-neon-fp-armv8
> > > >  aegis128-$(CONFIG_CRYPTO_AEGIS128_SIMD) += aegis128-neon.o aegis128-neon-inner.o
> > > >  endif
> > > >  ifeq ($(ARCH),arm64)
> > > > -CFLAGS_aegis128-neon-inner.o += -ffreestanding -mcpu=generic+crypto
> > > > +aegis128-cflags-y := -ffreestanding -mcpu=generic+crypto
> > > > +aegis128-cflags-$(CONFIG_CC_IS_GCC) += -ffixed-q16 -ffixed-q17 -ffixed-q18 \
> > > > +                                      -ffixed-q19 -ffixed-q20 -ffixed-q21 \
> > > > +                                      -ffixed-q22 -ffixed-q23 -ffixed-q24 \
> > > > +                                      -ffixed-q25 -ffixed-q26 -ffixed-q27 \
> > > > +                                      -ffixed-q28 -ffixed-q29 -ffixed-q30 \
> > > > +                                      -ffixed-q31
> > >
> > > I've filed https://bugs.llvm.org/show_bug.cgi?id=42974 for a feature
> > > request for this in Clang.
> > >
> >
> > Good. But even GCC has issues here. Most notably, something like
> >
> > register uint8x16_t foo asm ("v16");
> >
> > should permit a register that is excluded from general allocation to
> > be used explicitly, but this throws a warning on GCC and an error with
> > Clang.
>
> Consider filing bugs against GCC's issue tracker so that they're aware
> of the issue if you think there's more that can be improved on their
> end (for bugs in Clang, I'm always happy to help submit bug reports).
> What is the warning?
>

Cheers.

With Clang, I get

$ clang -target aarch64-linux-gnu -S -o - -O2 neon.c
neon.c:4:1: error: bad type for named register variable
register uint8x16_t foo asm ("v0");
^
1 error generated.

but given that -fixed-vNN is not implemented either, this is not
unexpected. But the latter is much more useful if the former is
supported as well.

I can't actually get the GCC warning to reproduce, so it might have
been operator error :-) But if I define the sbox like this

register uint8x16x4_t sbox0 asm ("v16");
register uint8x16x4_t sbox1 asm ("v20");
register uint8x16x4_t sbox2 asm ("v24");
register uint8x16x4_t sbox3 asm ("v28");

and use it in the sbox substitution, GCC emits lots of code like

     2bc:       4eb01e08        mov     v8.16b, v16.16b
     2c0:       4eb11e29        mov     v9.16b, v17.16b
     2c4:       4eb21e4a        mov     v10.16b, v18.16b
     2c8:       4eb31e6b        mov     v11.16b, v19.16b
     ..
     2d4:       4e0e610e        tbl     v14.16b, {v8.16b-v11.16b}, v14.16b
     2d8:       4e0d610d        tbl     v13.16b, {v8.16b-v11.16b}, v13.16b

i.e., it moves the register contents around rather than use it in the
tbl/tbx instructions directly, and the resulting code is absolutely
dreadful.

I will bring this up with the GCC developers, but it might be more
useful to grab someone internally rather than go via the bugzilla.





> for -ffixed-q* and `asm ("v16")`, on aarch64, what are the q registers
> and v registers?  I assume they're related to NEON, but I'd only even
> worked with w* and x* GPRs.  I *think* the explicit register syntax
> works for GPRs in Clang; maybe the v* and q* registers being broken is
> just oversight and can be fixed.
>

Yes. NEON/FP registers are referenced in different ways based on context:

v0.16b, v0.8h, v0.4s, v0.2d for SIMD bytes, halfwords, single words double words

q0/d0/s0 etc for scalar 128/64/32 bit FP


> > > With those 2 recommendations:
> > > Acked-by: Nick Desaulniers <ndesaulniers@google.com>
> > > in regards to compiling w/ Clang.  Someone else should review the
> > > implementation of this crypto routine.
> --
> Thanks,
> ~Nick Desaulniers