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:roret...@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 <roret...@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 and that seems very undesirable. another solution is to defer this change until post 23.07 release (where C11 can be used) and we could then just provide a single generic. with C11 i can provide a single macro that doesn't need 8/16/32/64 suffix. size_t v; n = rte_clz(v); // sizeof(v) doesn't matter. > > /Bruce
