Re: [edk2-devel] [PATCH v2 3/3] MdeModulePkg: Use CopyMem instead of GUID assignment

["Leif Lindholm" <leif@nuviainc.com>]

On Fri, Mar 13, 2020 at 04:08:12 +0000, Chang, Abner (HPS SW/FW Technologist) wrote:
> > -----Original Message-----
> > From: Leif Lindholm [mailto:leif@nuviainc.com]
> > Sent: Friday, March 13, 2020 5:20 AM
> > To: devel@edk2.groups.io; lersek@redhat.com
> > Cc: Chang, Abner (HPS SW/FW Technologist) <abner.chang@hpe.com>;
> > Schaefer, Daniel (DualStudy) <daniel.schaefer@hpe.com>; Chen, Gilbert
> > <gilbert.chen@hpe.com>; afish@apple.com; michael.d.kinney@intel.com;
> > pete@akeo.ie; Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > Subject: Re: [edk2-devel] [PATCH v2 3/3] MdeModulePkg: Use CopyMem
> > instead of GUID assignment
> 
> The current NULL instance of CompilerIntrinsicsLib is applied on
> every modules, this means it's not flexible for overwriting memcpy
> (for example) with the faster algorithm (such as SSEx instructions)
> for the specific module in the same DSC, right? That says we can't
> assign a special version of memcpy to just one particular module.

All true.
But ... are you planning to contribute lots of code that performs
large (certainly larger than GUIDs in order for effects to be
noticeable) struct assignments on critical paths?

Even if that is the case, as commented on the other fork of this
thread: if we add the -O3 jiggle, GCC will automatically insert what
it considers to be the optimal implementation for the specified target
when encountering the naïve implementation.

> > On Thu, Mar 12, 2020 at 20:42:52 +0100, Laszlo Ersek wrote:
> > > On 03/12/20 15:44, Leif Lindholm wrote:
> > > > And what would you propose we do the next time the RISC-V toolchain
> > > > generates a memcpy call based on some other completely valid change
> > > > to core code?
> > >
> > > We could choose to enable the intrinsics library for RISC-V at that point.
>
> and I would like to see the flexibility of overwriting memory
> library functions for particular modules. There is no special
> algorithm of memory manipulation so far in RISC-V spec, however, the
> working group of Vector extension does propose the new instruction
> sets.

Use of CompilerIntrinsicsLib has no effect on explicit CopyMem & co
operations, only on memcpy/memset generated by the compiler. Which
will generate build failures if not handled.

/
    Leif