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

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

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?

ty