From: Tyler Retzlaff <roretzla@linux.microsoft.com>
To: Bruce Richardson <bruce.richardson@intel.com>
Cc: "Morten Brørup" <mb@smartsharesystems.com>,
dev@dpdk.org, david.marchand@redhat.com, thomas@monjalon.net
Subject: Re: [PATCH v2 1/2] eal: provide leading and trailing zero bit count abstraction
Date: Wed, 5 Apr 2023 10:25:15 -0700 [thread overview]
Message-ID: <20230405172515.GA14891@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> (raw)
In-Reply-To: <ZC2ZDse5eCj2WNHU@bricha3-MOBL.ger.corp.intel.com>
On Wed, Apr 05, 2023 at 04:51:42PM +0100, Bruce Richardson wrote:
> On Wed, Apr 05, 2023 at 08:22:16AM -0700, Tyler Retzlaff wrote:
> > On Wed, Apr 05, 2023 at 09:44:54AM +0100, Bruce Richardson wrote:
> > > On Tue, Apr 04, 2023 at 02:23:22PM -0700, Tyler Retzlaff wrote:
> > > > On Thu, Jan 05, 2023 at 08:09:19AM +0100, Morten Brørup wrote:
> > > > > > From: Tyler Retzlaff [mailto:roretzla@linux.microsoft.com]
> > > > > > Sent: Thursday, 24 November 2022 00.43
> > > > > >
> > > > > > Provide an abstraction for leading and trailing zero bit counting
> > > > > > functions to hide compiler specific intrinsics and builtins.
> > > > > >
> > > > > > Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
> > > > > > ---
> > > > > > lib/eal/include/meson.build | 1 +
> > > > > > lib/eal/include/rte_bitcount.h | 265
> > > > > > +++++++++++++++++++++++++++++++++++++++++
> > > > > > 2 files changed, 266 insertions(+)
> > > > > > create mode 100644 lib/eal/include/rte_bitcount.h
> > > > > >
> > > > > > diff --git a/lib/eal/include/meson.build b/lib/eal/include/meson.build
> > > > > > index cfcd40a..8ff1d65 100644
> > > > > > --- a/lib/eal/include/meson.build
> > > > > > +++ b/lib/eal/include/meson.build
> > > > > > @@ -5,6 +5,7 @@ includes += include_directories('.')
> > > > > >
> > > > > > headers += files(
> > > > > > 'rte_alarm.h',
> > > > > > + 'rte_bitcount.h',
> > > > > > 'rte_bitmap.h',
> > > > > > 'rte_bitops.h',
> > > > > > 'rte_branch_prediction.h',
> > > > > > diff --git a/lib/eal/include/rte_bitcount.h
> > > > > > b/lib/eal/include/rte_bitcount.h
> > > > > > new file mode 100644
> > > > > > index 0000000..587de52
> > > > > > --- /dev/null
> > > > > > +++ b/lib/eal/include/rte_bitcount.h
> > > > > > @@ -0,0 +1,265 @@
> > > > > > +/* SPDX-License-Identifier: BSD-3-Clause
> > > > > > + * Copyright (C) 2022 Microsoft Corporation
> > > > > > + */
> > > > > > +
> > > > > > +#ifndef _RTE_BITCOUNT_H_
> > > > > > +#define _RTE_BITCOUNT_H_
> > > > > > +
> > > > > > +#include <rte_compat.h>
> > > > > > +
> > > > > > +#ifdef __cplusplus
> > > > > > +extern "C" {
> > > > > > +#endif
> > > > > > +
> > > > > > +#ifdef RTE_TOOLCHAIN_MSVC
> > > > > > +
> > > > > > +/**
> > > > > > + * @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_clz(unsigned int v)
> > > > > > +{
> > > > > > + unsigned long rv;
> > > > > > +
> > > > > > + (void)_BitScanReverse(&rv, v);
> > > > > > +
> > > > > > + return (unsigned int)rv;
> > > > > > +}
> > > > > > +
> > > > > > +/**
> > > > > > + * @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)
> > > >
> > > > so i just noticed this, but sometimes these functions receive size_t so
> > > > naming them specifically 32/64 bit becomes problematic because are going
> > > > to end up with promotion on sizeof(size_t) == sizeof(long) == 4
> > > > platforms.
> > > >
> > > > i.e.
> > > > size_t s = ...;
> > > > x = rte_clz64(s); // assume 64-bit today
> > > >
> > > > this code is now broken because on 32-bit platform s will get promoted
> > > > and the extra 32 zero-bits will be returned in the result breaking
> > > > calculations.
> > > >
> > > > any thoughts? should we go back to l, ll?
> > > >
> > >
> > > Yes, promotion will happen, but I still think that the 32 and 64 versions
> > > are far clearer here in all cases. Anyone looking at the code will
> > > recognise that the result will be the leading zero count of a 64-bit number
> > > irrespective of the type actually passed in. It's less confusing now IMHO.
> >
> > here's an example in the code that would result in a bad calculation or
> > at least i believe so at a glance. switching to rte_clz32() would break
> > on 64-bit since it would truncate.
> >
> > lib/eal/common/malloc_elem.c
> >
> > -log2 = sizeof(size) * 8 - __builtin_clzl(size);
> > +log2 = sizeof(size) * 8 - rte_clz64(size);
> >
> > if i'm right you'd have to conditionally compile at the site.
> >
> > #ifdef 64-bit
> > rte_clz64()
> > #else
> > rte_clz32()
> > #endif
> >
>
> Why can clz64 not be used in both 32 and 64 bit cases? You know the result
> will always include zeros in high bits of a 64-bit value, and the result
> will still fit inside even an 8-bit variable?
of course. doh.
sorry for the noise!
and thank you as always.
>
> /Bruce
next prev parent reply other threads:[~2023-04-05 17:25 UTC|newest]
Thread overview: 81+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-23 22:14 [PATCH 0/2] eal: provide leading and trailing zero bit count Tyler Retzlaff
2022-11-23 22:14 ` [PATCH 1/2] eal: provide leading and trailing zero bit count abstraction Tyler Retzlaff
2023-04-03 21:46 ` Mattias Rönnblom
2022-11-23 22:14 ` [PATCH 2/2] test/bitcount: add bitcount tests Tyler Retzlaff
2022-11-23 23:43 ` [PATCH v2 0/2] eal: provide leading and trailing zero bit count Tyler Retzlaff
2022-11-23 23:43 ` [PATCH v2 1/2] eal: provide leading and trailing zero bit count abstraction Tyler Retzlaff
2022-11-24 10:17 ` Morten Brørup
2022-11-28 17:13 ` Tyler Retzlaff
2022-11-28 17:22 ` Morten Brørup
2022-11-28 17:27 ` Tyler Retzlaff
2023-01-05 9:04 ` Thomas Monjalon
2023-01-05 17:23 ` Tyler Retzlaff
2023-01-05 17:27 ` Tyler Retzlaff
2023-01-05 20:57 ` Tyler Retzlaff
2023-01-05 21:34 ` Morten Brørup
2023-01-05 22:06 ` Tyler Retzlaff
2023-01-05 23:10 ` Morten Brørup
2023-01-06 1:04 ` Tyler Retzlaff
2023-01-06 10:09 ` Thomas Monjalon
2023-01-06 10:00 ` Thomas Monjalon
2023-01-05 7:09 ` Morten Brørup
2023-01-05 9:01 ` Thomas Monjalon
2023-01-05 17:21 ` Tyler Retzlaff
2023-01-06 0:32 ` Stephen Hemminger
2023-01-06 11:48 ` Bruce Richardson
2023-01-06 12:41 ` Morten Brørup
2023-01-06 13:40 ` Thomas Monjalon
2023-01-06 18:58 ` Tyler Retzlaff
2023-01-06 20:51 ` Thomas Monjalon
2023-01-10 9:18 ` Ferruh Yigit
2023-01-06 18:47 ` Tyler Retzlaff
2023-01-09 8:50 ` Bruce Richardson
2023-04-04 21:23 ` Tyler Retzlaff
2023-04-05 8:44 ` Bruce Richardson
2023-04-05 15:22 ` Tyler Retzlaff
2023-04-05 15:51 ` Bruce Richardson
2023-04-05 17:25 ` Tyler Retzlaff [this message]
2022-11-23 23:43 ` [PATCH v2 2/2] test/bitcount: add bitcount tests Tyler Retzlaff
2023-01-04 23:46 ` [PATCH v2 0/2] eal: provide leading and trailing zero bit count Tyler Retzlaff
2023-01-09 17:36 ` [PATCH v4 0/2] eal: provide leading and trailing zero bit count abstraction Tyler Retzlaff
2023-01-09 17:36 ` [PATCH v4 1/2] eal: move bit operation functions from common to bitops header Tyler Retzlaff
2023-01-10 13:56 ` Ferruh Yigit
2023-01-09 17:36 ` [PATCH v4 2/2] eal: provide leading and trailing zero bit count abstraction Tyler Retzlaff
2023-01-10 13:55 ` Ferruh Yigit
2023-01-10 17:34 ` Tyler Retzlaff
2023-01-10 18:37 ` Tyler Retzlaff
2023-01-10 13:56 ` [PATCH v4 0/2] " Ferruh Yigit
2023-01-06 22:01 ` [PATCH v3 0/3] eal: provide leading and trailing zero bit count Tyler Retzlaff
2023-01-06 22:01 ` [PATCH v3 1/3] eal: move bit functions from common to bitops header Tyler Retzlaff
2023-01-06 22:01 ` [PATCH v3 2/3] eal: provide leading and trailing zero bit count abstraction Tyler Retzlaff
2023-01-06 22:01 ` [PATCH v3 3/3] test/bitcount: add bitcount tests Tyler Retzlaff
2023-01-07 8:15 ` Thomas Monjalon
2023-01-09 16:57 ` Tyler Retzlaff
2023-01-09 17:26 ` Tyler Retzlaff
2023-01-07 13:40 ` Morten Brørup
2023-01-09 8:51 ` [PATCH v3 0/3] eal: provide leading and trailing zero bit count Bruce Richardson
2023-01-10 19:38 ` [PATCH v5 0/2] eal: provide leading and trailing zero bit count abstraction Tyler Retzlaff
2023-01-10 19:38 ` [PATCH v5 1/2] eal: move bit operation common to bitops header Tyler Retzlaff
2023-01-10 19:38 ` [PATCH v5 2/2] eal: provide leading and trailing zero bit count abstraction Tyler Retzlaff
2023-01-10 19:46 ` [PATCH v6 0/2] " Tyler Retzlaff
2023-01-10 19:46 ` [PATCH v6 1/2] eal: move bit operation common to bitops header Tyler Retzlaff
2023-01-10 19:46 ` [PATCH v6 2/2] eal: provide leading and trailing zero bit count abstraction Tyler Retzlaff
2023-01-20 22:14 ` [PATCH v6 0/2] " Tyler Retzlaff
2023-02-02 9:14 ` David Marchand
2023-02-02 10:56 ` David Marchand
2023-02-02 15:57 ` Tyler Retzlaff
2023-02-03 9:14 ` David Marchand
2023-02-02 15:56 ` Tyler Retzlaff
2023-02-03 9:21 ` David Marchand
2023-04-01 0:45 ` [PATCH v7 0/4] eal: provide abstracted bit counting functions Tyler Retzlaff
2023-04-01 0:45 ` [PATCH v7 1/4] eal: move bit count functions to bitops header Tyler Retzlaff
2023-04-01 0:45 ` [PATCH v7 2/4] eal: provide abstracted bit count functions Tyler Retzlaff
2023-04-01 0:45 ` [PATCH v7 3/4] pipeline: add include of bitops Tyler Retzlaff
2023-04-01 0:45 ` [PATCH v7 4/4] maintainers: add bitcount test under EAL API and common code Tyler Retzlaff
2023-04-01 7:08 ` [PATCH v7 0/4] eal: provide abstracted bit counting functions Morten Brørup
2023-04-04 0:11 ` [PATCH v8 0/3] " Tyler Retzlaff
2023-04-04 0:11 ` [PATCH v8 1/3] eal: move bit count functions to bitops header Tyler Retzlaff
2023-04-04 0:11 ` [PATCH v8 2/3] eal: provide abstracted bit count functions Tyler Retzlaff
2023-04-04 0:11 ` [PATCH v8 3/3] maintainers: add bitcount test under EAL API and common code Tyler Retzlaff
2023-04-04 8:27 ` [PATCH v8 0/3] eal: provide abstracted bit counting functions Bruce Richardson
2023-08-25 8:41 ` David Marchand
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