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From: Bruce Richardson <bruce.richardson@intel.com>
To: Tyler Retzlaff <roretzla@linux.microsoft.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 16:51:42 +0100	[thread overview]
Message-ID: <ZC2ZDse5eCj2WNHU@bricha3-MOBL.ger.corp.intel.com> (raw)
In-Reply-To: <20230405152216.GA28418@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>

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?

/Bruce

  reply	other threads:[~2023-04-05 15:51 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 [this message]
2023-04-05 17:25               ` Tyler Retzlaff
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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