Re: [PATCH v3 24/29] crypto: lib/curve25519 - work around Clang stack spilling issue

On Mon, 14 Oct 2019 at 16:14, Jason A. Donenfeld [off-list ref] wrote:

Hi Ard,

On Mon, Oct 7, 2019 at 6:46 PM Ard Biesheuvel [off-list ref] wrote:

quoted

Arnd reports that the 32-bit generic library code for Curve25119 ends
up using an excessive amount of stack space when built with Clang:

  lib/crypto/curve25519-fiat32.c:756:6: error: stack frame size
      of 1384 bytes in function 'curve25519_generic'
      [-Werror,-Wframe-larger-than=]

Let's give some hints to the compiler regarding which routines should
not be inlined, to prevent it from running out of registers and spilling
to the stack. The resulting code performs identically under both GCC
and Clang, and makes the warning go away.

Are you *sure* about that? Couldn't we fix clang instead? I'd rather
fixes go there instead of gimping this. The reason is that I noticed
before that this code, performance-wise, was very inlining sensitive.
Can you benchmark this on ARM32-noneon and on MIPS32? If there's a
performance difference there, then maybe you can defer this part of
the series until after the rest lands, and then we'll discuss at
length various strategies? Alternatively, if you benchmark those and
it also makes no difference, then it indeed makes no difference.

I tested this using a 32-bit ARM VM running under an 64-bit KVM
hypervisor, doing 100 iterations of the selftest.