Re: Support for memcpy with equal source and destination

[Alexander Monakov <amonakov@ispras.ru>]

On Mon, 27 Nov 2023, Ralf Jung wrote:

> > Please note that GCC does not use memcpy for "sufficiently small" structure
> > copies at -O2, as it's faster to emit the necessary loads+stores inline.
> > 
> > The threshold for "sufficiently small" varies with target and compiler
> > version; for instance, it is "above 64 bytes" for 32-bit arm and "above 8192
> > bytes" for x86_64 with current trunk (it also depends on default
> > -march/-mtune, etc.).
> 
> Wow, 8 kilobytes?!?

It inlines 'rep movsq' for sizes between 32 and 8192 for generic tuning, but
it varies substantially for different CPU families. For example, with
-mtune=znver[234] the thresholds are:

* memcpy is used above 64 bytes, or if size is unknown
* 'rep movsq' above 16 bytes (up to 64 bytes)

> > So on x86 at least adding such a branch in memcpy is not a practical choice.
> 
> Sorry, how does that follow from GCC not using memcpy for small copies?

If you add a branch into memcpy, every single invocation of memcpy will pay a
tiny cost for that branch, even though it matters only for copying huge
structs, which is vanishingly rare compared to all uses of memcpy.

Alexander