Re: [PATCH v3 1/5] AArch64: Improve A64FX memset

[Szabolcs Nagy via Libc-alpha <libc-alpha@sourceware.org>]

The 08/02/2021 14:38, Wilco Dijkstra via Libc-alpha wrote:
> > We discussed how should be defined BTI_C macro before, at that time conclusion
> > was "NOP" rather than empty unless HAVE_AARCH64_BTI.
> > Now the above code defines BTI_C as empty unconditionally.
> > A64FX doesn't support BTI, so this code is OK.
> > But I'm just interested in the reason why it is changed.
> 
> We changed to NOP in the generic code, so that works for all string functions.
> In this specific case removing the initial NOP as well allows all performance critical
> code for <= 512 bytes to be perfectly aligned to 16-byte fetch blocks.

yes, this makes sense:

originally BTI_C was always hint 34, but since that can be
slow it was changed for !HAVE_AARCH64_BTI. We don't want the
layout of asm code to change based on toolchain configuration
so BTI_C is defined as a place holder nop then.

but in a64fx specific code bti is never needed so we also
don't need the place holder nop, BTI_C can be unconditionally
empty.