Re: [PATCH] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Sep 17, 2026 at 11:26:46AM +0200, Jason A. Donenfeld wrote:
> I don't suspect this is the right change. On x86_64, this changes to
> code from:
> 
>     rep stosq
> 
> into:
> 
>     loc_2D9:
>     mov     dword ptr [rbx+rax*4+0Ch], 0
>     add     rax, 1
>     cmp     rax, 0Ch
>     jbe     short loc_2D9
> 
> Which is a lot less compact. It seems like the actual solution is for
> gcc&clang to emit this inline memset mnemonic when the platform has a
> good one, and otherwise not. But disabling optimizations for all
> platforms, because it's broken on one, seems bad.

Yes, that is certainly fair criticism. That said, this optimization
happens in the middle end as far as I can tell, so I am not sure how
much target specific knowledge is available at that point.
Additionally...

> In this case, the compiler is being smart: it identifies a loop and
> rightly turns it into memset. But if this isn't a compilation
> environment that has an outline function, it should do something else.

As I mentioned in the commit message, compilers require all environments
(hosted or not) to provide memset(), so the "doing something else" is
nothing :)

  GCC requires the freestanding environment provide memcpy, memmove,
  memset and memcmp.

I guess another option is to just include a basic memset() like the one
in lib/string.c so that it is only used if the compiler makes this sort
of transformation, while leaving all other architectures alone.

-- 
Cheers,
Nathan