[PATCH v2 1/2] eal: provide leading and trailing zero bit count abstraction

[Thomas Monjalon]

06/01/2023 19:58, Tyler Retzlaff:
> On Fri, Jan 06, 2023 at 02:40:59PM +0100, Thomas Monjalon wrote:
> > 06/01/2023 13:41, Morten Brørup:
> > > > From: Bruce Richardson [mailto:bruce.richardson at intel.com]
> > > > Sent: Friday, 6 January 2023 12.48
> > > > 
> > > > On Thu, Jan 05, 2023 at 04:32:40PM -0800, Stephen Hemminger wrote:
> > > > > On Thu, 5 Jan 2023 09:21:18 -0800
> > > > > Tyler Retzlaff <roretzla at linux.microsoft.com> wrote:
> > > > >
> > > > > > On Thu, Jan 05, 2023 at 10:01:31AM +0100, Thomas Monjalon wrote:
> > > > > > > 05/01/2023 08:09, Morten Brørup:
> > > > > > > > > From: Tyler Retzlaff [mailto:roretzla at linux.microsoft.com]
> > > > > > > > > +/**
> > > > > > > > > + * @warning
> > > > > > > > > + * @b EXPERIMENTAL: this API may change, or be removed,
> > > > without prior
> > > > > > > > > notice
> > > > > > > > > + *
> > > > > > > > > + * Get the count of leading 0-bits in v.
> > > > > > > > > + *
> > > > > > > > > + * @param v
> > > > > > > > > + *   The value.
> > > > > > > > > + * @return
> > > > > > > > > + *   The count of leading zero bits.
> > > > > > > > > + */
> > > > > > > > > +__rte_experimental
> > > > > > > > > +static inline unsigned int
> > > > > > > > > +rte_clzl(unsigned long v)
> > > > > > > >
> > > > > > > > Don't use l (long) and ll (long long) for names (and types),
> > > > use explicit bit lengths, 32 and 64.
> > > > > > > >
> > > > > > > > E.g.: rte_clz32(uint32_t v)
> > > > > > >
> > > > > > > I agree on using numbers.
> > > > > > >
> > > > > >
> > > > > > love the idea, fewer functions too.
> > > > > >
> > > > > > though it is a shame we cannot adopt C11 standard because we could
> > > > just
> > > > > > do away with the bit suffixes entirely.
> > > > >
> > > > > We could but the project needs to support older RHEL releases
> > > > > which have older tool sets. Though probably this is moot point given
> > > > > how much meson seems to change.
> > > > 
> > > > True, though meson tends to be a bit easier to update than GCC on a
> > > > system
> > > > - no "pip3 install --upgrade gcc", sadly :-)
> > > > 
> > > > If we can't go all the way to C11 support, how about at least going to
> > > > C99
> > > > support? As far as I know DPDK has never updated its minimum C-standard
> > > > version, and it might be a good idea to start the process of doing so,
> > > > even
> > > > if it is a baby step.
> > 
> > I am in favor of this baby step: define -std=c99 porject-wise
> > and see what are the effects during the year.
> > 
> > 
> > > The DPDK Getting Started Guide [1] says:
> > > 
> > > "Required Tools and Libraries:
> > > [...]
> > > a supported C compiler such as gcc (version 4.9+)"
> > > 
> > > GCC version 4.9 supports C11 [2]:
> > > "GCC 4.9 Changes: "ISO C11 support is now at a similar level of completeness to ISO C99 support [...]"
> > > 
> > > So why are we not going to support C11?
> > 
> > We should make a plan to switch to C11 during next year.
> > 
> > 
> > > Probably because of RHEL 7, which only provides GCC 4.8 [3].
> > > 
> > > RHEL 7 was released for GA on June 10, 2014 [4]. If someone has a server with a nine year old distro still used in production, it is probably because it is running some legacy application, which is difficult to get up and running on a newer distro. Partial conclusion: RHEL 7 is probably still widely used in production.
> > > 
> > > However, I have a hard time understanding why anyone would build and/or deploy a brand new DPDK application (based on DPDK 23.03) on such a server. Can someone please justify this?
> > > 
> > > Are we really going to postpone C11 support in DPDK until June 30, 2026, when RHEL 7 ends its Extended Life Cycle Support [4]?
> > 
> > RHEL does its own choices to support old software for long.
> > Upstream development should move forward.
> > 
> > 
> > > If so, then the GCC version mentioned in the DPDK Getting Started Guide should be corrected accordingly.
> > 
> > No let's keep GCC 4.9 as a minimum for now.
> > If needed we could upgrade it later.
> 
> but i think the point Morten is making is that RHEL 7 gcc is 4.8 and
> therefore we implicitly no longer support it if we document requiring
> gcc 4.9.

Yes I got it.
I think everything in DPDK works on RHEL 7 today,
but I believe RHEL 7 is not a strong requirement anymore for the mainline.
Asking for confirmation here.

> so i think the way to do this is through clarification at the next
> release / ltsc release. starting with dpdk 23.xx will require
> compiler conforming to standard X and optional features / annexes from
> standard X. anyone building applications targeting that version of dpdk
> release will need a conforming implementation.
> 
> from there we just need to take care not to backport any code to
> stable branches that depend on standard features that exceed the
> requirements documented for that release.

We could even start testing C99 requirement in 23.03 I think.