Re: [PATCH v4 2/2] eal: provide leading and trailing zero bit count abstraction

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

On Tue, Jan 10, 2023 at 01:55:59PM +0000, Ferruh Yigit wrote:
> On 1/9/2023 5:36 PM, Tyler Retzlaff wrote:
> > From: Tyler Retzlaff <roretzla@microsoft.com>
> > 
> > Provide an abstraction for leading and trailing zero bit counting
> > functions to hide compiler specific intrinsics and builtins.
> > 
> > Include basic unit test of following functions added.
> > 
> >     rte_clz32
> >     rte_clz64
> >     rte_ctz32
> >     rte_ctz64
> > 
> > Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
> > ---
> >  app/test/meson.build         |   2 +
> >  app/test/test_bitcount.c     |  71 ++++++++++++++++++
> >  lib/eal/include/rte_bitops.h | 168 +++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 241 insertions(+)
> >  create mode 100644 app/test/test_bitcount.c
> > 
> > diff --git a/app/test/meson.build b/app/test/meson.build
> > index f34d19e..d1277bc 100644
> > --- a/app/test/meson.build
> > +++ b/app/test/meson.build
> > @@ -13,6 +13,7 @@ test_sources = files(
> >          'test_alarm.c',
> >          'test_atomic.c',
> >          'test_barrier.c',
> > +        'test_bitcount.c',
> >          'test_bitops.c',
> >          'test_bitmap.c',
> >          'test_bpf.c',
> > @@ -160,6 +161,7 @@ test_deps += ['bus_pci', 'bus_vdev']
> >  fast_tests = [
> >          ['acl_autotest', true, true],
> >          ['atomic_autotest', false, true],
> > +        ['bitcount_autotest', true, true],
> >          ['bitmap_autotest', true, true],
> >          ['bpf_autotest', true, true],
> >          ['bpf_convert_autotest', true, true],
> > diff --git a/app/test/test_bitcount.c b/app/test/test_bitcount.c
> > new file mode 100644
> > index 0000000..36eb05c
> > --- /dev/null
> > +++ b/app/test/test_bitcount.c
> > @@ -0,0 +1,71 @@
> > +/* SPDX-License-Identifier: BSD-3-Clause
> > + * Copyright (C) 2022 Microsoft Corporation
> > + */
> > +
> > +#include <string.h>
> > +
> > +#include <rte_bitops.h>
> > +#include <rte_debug.h>
> > +
> > +#include "test.h"
> > +
> > +RTE_LOG_REGISTER(bitcount_logtype_test, test.bitcount, INFO);
> > +
> > +static int
> > +test_clz32(void)
> > +{
> > +	uint32_t v = 1;
> > +	RTE_TEST_ASSERT(rte_clz32(v) == sizeof(v) * CHAR_BIT - 1,
> > +	    "Unexpected count.");
> > +
> > +	return 0;
> > +}
> > +
> > +static int
> > +test_clz64(void)
> > +{
> > +	uint64_t v = 1;
> > +	RTE_TEST_ASSERT(rte_clz64(v) == sizeof(v) * CHAR_BIT - 1,
> > +	    "Unexpected count.");
> > +
> > +	return 0;
> > +}
> > +
> > +static int
> > +test_ctz32(void)
> > +{
> > +	uint32_t v = 2;
> > +	RTE_TEST_ASSERT(rte_ctz32(v) == 1, "Unexpected count.");
> > +
> > +	return 0;
> > +}
> > +
> > +static int
> > +test_ctz64(void)
> > +{
> > +	uint64_t v = 2;
> > +	RTE_TEST_ASSERT(rte_ctz64(v) == 1, "Unexpected count.");
> > +
> > +	return 0;
> > +}
> > +
> > +static struct unit_test_suite bitcount_test_suite = {
> > +	.suite_name = "bitcount autotest",
> > +	.setup = NULL,
> > +	.teardown = NULL,
> > +	.unit_test_cases = {
> > +		TEST_CASE(test_clz32),
> > +		TEST_CASE(test_clz64),
> > +		TEST_CASE(test_ctz32),
> > +		TEST_CASE(test_ctz64),
> > +		TEST_CASES_END()
> > +	}
> > +};
> > +
> > +static int
> > +test_bitcount(void)
> > +{
> > +	return unit_test_suite_runner(&bitcount_test_suite);
> > +}
> > +
> > +REGISTER_TEST_COMMAND(bitcount_autotest, test_bitcount);
> > diff --git a/lib/eal/include/rte_bitops.h b/lib/eal/include/rte_bitops.h
> > index 531479e..387d7aa 100644
> > --- a/lib/eal/include/rte_bitops.h
> > +++ b/lib/eal/include/rte_bitops.h
> > @@ -1,5 +1,7 @@
> >  /* SPDX-License-Identifier: BSD-3-Clause
> >   * Copyright(c) 2020 Arm Limited
> > + * Copyright(c) 2010-2019 Intel Corporation
> > + * Copyright(c) 2023 Microsoft Corporation
> >   */
> >  
> >  #ifndef _RTE_BITOPS_H_
> > @@ -275,6 +277,172 @@
> >  	return val & mask;
> >  }
> >  
> > +#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_clz32(uint32_t v)
> > +{
> > +	unsigned long rv;
> > +
> > +	(void)_BitScanReverse(&rv, v);
> > +
> > +	return (unsigned int)rv;
> > +}
> 
> 
> OK, this looks wrong to me,
> 
> 
> '_BitScanReverse' is [1]:
> "Search the mask data from most significant bit (MSB) to least
> significant bit (LSB) for a set bit (1)."
> 
> As far as I can see index starts from zero and from lsb to msb.
> 
> So _BitScanReverse() returns following index values:
> 0x2 => 1
> 0xffffffff => 31
> 
> If above is correct, above function doesn't return number of leading
> zeros, but it should return "31 - (unsigned int)rv".
> 
> Please check godbolt experiment for above:
> https://godbolt.org/z/znYn54f57
> 
> 
> As far as I can see unit test is correct, so above should fail with
> windows compiler, and I assume you run unit test with windows compiler,
> so probably I am missing something but please help me understand.

the test is wrong, i will fix both the test and the implementation.

thank you for the careful review.