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From: "Stefan Kanthak" <stefan.kanthak@nexgo.de>
To: "Pavan Nikhilesh Bhagavatula" <pbhagavatula@marvell.com>,
<dev@dpdk.org>
Cc: <hannes@stressinduktion.org>
References: <0D8EBD99DC5246E394511624D3B09B10@W340>
<CY4PR1801MB1863A73B62C8E3F8475A7D0BDE590@CY4PR1801MB1863.namprd18.prod.outlook.com>
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Date: Fri, 5 Apr 2019 21:17:53 +0200
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Subject: Re: [dpdk-dev] [BUG] Maintainer
forlib/librte_eal/common/rte_reciprocal.c?
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"Pavan Nikhilesh Bhagavatula" <pbhagavatula@marvell.com> wrote Thursday, March 28, 2019 6:35 AM:
> Hi Stefan,
>
> Thanks for the heads up, I am planning to use a modified version of
> https://github.com/hcs0/Hackers-Delight/blob/master/divluh.c.txt
> for simplicity as we don't require the remainder.
Why don't you refer to the ORIGINAL source
<https://www.hackersdelight.org/hdcodetxt/divluh.c.txt> instead?
BUT: you also shouldn't use these routines, they too show quite bad
implementations of a binary division!
As Don Knuth wrote in volume 2 of TAoCP, hardware implementations
are most often NOT well-suited for implementation in software.
Take a look at the following STRAIGHTFORWARD implementation, both
in ANSI C and x86 assembly (for Microsoft's Visual C compiler):
unsigned long long divide(unsigned long long dividendlow,
unsigned long long dividendhigh,
unsigned long long divisor,
unsigned long long *remainder)
{
#ifndef _M_IX86
unsigned int shift = 64;
do
{
if (dividendhigh < divisor)
{
dividendhigh <<= 1;
dividendhigh |= dividendlow >> 63;
dividendlow <<= 1;
}
else
{
dividendhigh -= divisor;
dividendhigh <<= 1;
dividendhigh |= dividendlow >> 63;
dividendlow <<= 1;
dividendlow |= 1;
}
} while (--shift != 0);
if (*remainder != NULL)
remainder = dividendhigh;
return dividendlow;
#else
__asm
{
mov eax, dword ptr dividendlow
mov edx, dword ptr dividendlow+4
mov ecx, dword ptr dividendhigh
mov ebx, dword ptr dividendhigh+4
mov edi, dword ptr divisor
mov esi, dword ptr divisor+4
mov ebp, 64
NEXT:
cmp ebx, esi
ja SUBTRACT
jb SHIFT
cmp ecx, edi
jb SHIFT
SUBTRACT:
sub ecx, edi
sbb ebx, esi
SHIFT:
cmc
adc eax, eax
adc edx, edx
adc ecx, ecx
adc ebx, ebx
dec ebp
jnz NEXT
or ebp, remainder
jz EXIT
mov [ebp], ecx
mov [ebp+4], ebx
EXIT:
}
#endif
}
> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Stefan Kanthak
> Sent: Tuesday, March 26, 2019 6:41 AM
> To: dev@dpdk.org
> Cc: hannes@stressinduktion.org
> Subject: [dpdk-dev] [BUG] Maintainer for
> lib/librte_eal/common/rte_reciprocal.c?
>
> Hi @ll,
>
> <https://git.dpdk.org/dpdk/plain/MAINTAINERS> does NOT list a maintainer
> for lib/librte_eal/common/rte_reciprocal.c
>
> Whoever it is should take a close look at <https://skanthak.homepage.t-
> online.de/division.html>,
> then fix the most obvious bug plus the many deficiencies of
> divide_128_div_64_to_64().
>
> regards
> Stefan Kanthak
From mboxrd@z Thu Jan 1 00:00:00 1970
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Message-ID: <992998AF3ADA4F28BBF7D7CC257A2034@H270>
From: "Stefan Kanthak" <stefan.kanthak@nexgo.de>
To: "Pavan Nikhilesh Bhagavatula" <pbhagavatula@marvell.com>,
<dev@dpdk.org>
Cc: <hannes@stressinduktion.org>
References: <0D8EBD99DC5246E394511624D3B09B10@W340>
<CY4PR1801MB1863A73B62C8E3F8475A7D0BDE590@CY4PR1801MB1863.namprd18.prod.outlook.com>
In-Reply-To: <CY4PR1801MB1863A73B62C8E3F8475A7D0BDE590@CY4PR1801MB1863.namprd18.prod.outlook.com>
Date: Fri, 5 Apr 2019 21:17:53 +0200
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Subject: Re: [dpdk-dev] [BUG] Maintainer
forlib/librte_eal/common/rte_reciprocal.c?
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"Pavan Nikhilesh Bhagavatula" <pbhagavatula@marvell.com> wrote Thursday, March 28, 2019 6:35 AM:
> Hi Stefan,
>
> Thanks for the heads up, I am planning to use a modified version of
> https://github.com/hcs0/Hackers-Delight/blob/master/divluh.c.txt
> for simplicity as we don't require the remainder.
Why don't you refer to the ORIGINAL source
<https://www.hackersdelight.org/hdcodetxt/divluh.c.txt> instead?
BUT: you also shouldn't use these routines, they too show quite bad
implementations of a binary division!
As Don Knuth wrote in volume 2 of TAoCP, hardware implementations
are most often NOT well-suited for implementation in software.
Take a look at the following STRAIGHTFORWARD implementation, both
in ANSI C and x86 assembly (for Microsoft's Visual C compiler):
unsigned long long divide(unsigned long long dividendlow,
unsigned long long dividendhigh,
unsigned long long divisor,
unsigned long long *remainder)
{
#ifndef _M_IX86
unsigned int shift = 64;
do
{
if (dividendhigh < divisor)
{
dividendhigh <<= 1;
dividendhigh |= dividendlow >> 63;
dividendlow <<= 1;
}
else
{
dividendhigh -= divisor;
dividendhigh <<= 1;
dividendhigh |= dividendlow >> 63;
dividendlow <<= 1;
dividendlow |= 1;
}
} while (--shift != 0);
if (*remainder != NULL)
remainder = dividendhigh;
return dividendlow;
#else
__asm
{
mov eax, dword ptr dividendlow
mov edx, dword ptr dividendlow+4
mov ecx, dword ptr dividendhigh
mov ebx, dword ptr dividendhigh+4
mov edi, dword ptr divisor
mov esi, dword ptr divisor+4
mov ebp, 64
NEXT:
cmp ebx, esi
ja SUBTRACT
jb SHIFT
cmp ecx, edi
jb SHIFT
SUBTRACT:
sub ecx, edi
sbb ebx, esi
SHIFT:
cmc
adc eax, eax
adc edx, edx
adc ecx, ecx
adc ebx, ebx
dec ebp
jnz NEXT
or ebp, remainder
jz EXIT
mov [ebp], ecx
mov [ebp+4], ebx
EXIT:
}
#endif
}
> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Stefan Kanthak
> Sent: Tuesday, March 26, 2019 6:41 AM
> To: dev@dpdk.org
> Cc: hannes@stressinduktion.org
> Subject: [dpdk-dev] [BUG] Maintainer for
> lib/librte_eal/common/rte_reciprocal.c?
>
> Hi @ll,
>
> <https://git.dpdk.org/dpdk/plain/MAINTAINERS> does NOT list a maintainer
> for lib/librte_eal/common/rte_reciprocal.c
>
> Whoever it is should take a close look at <https://skanthak.homepage.t-
> online.de/division.html>,
> then fix the most obvious bug plus the many deficiencies of
> divide_128_div_64_to_64().
>
> regards
> Stefan Kanthak