From: Richard Henderson <richard.henderson@linaro.org>
To: LIU Zhiwei <zhiwei_liu@c-sky.com>,
alistair23@gmail.com, chihmin.chao@sifive.com,
palmer@dabbelt.com
Cc: guoren@linux.alibaba.com, wenmeng_zhang@c-sky.com,
qemu-riscv@nongnu.org, qemu-devel@nongnu.org,
wxy194768@alibaba-inc.com
Subject: Re: [PATCH v6 25/61] target/riscv: vector single-width averaging add and subtract
Date: Fri, 27 Mar 2020 18:22:16 -0700 [thread overview]
Message-ID: <ce13869a-141f-cc4d-8f1e-6a3742197654@linaro.org> (raw)
In-Reply-To: <f50a4a63-c5cf-4322-9663-98fb15dde715@c-sky.com>
On 3/27/20 6:07 PM, LIU Zhiwei wrote:
>
>
> On 2020/3/28 8:32, Richard Henderson wrote:
>> On 3/18/20 8:46 PM, LIU Zhiwei wrote:
>>> +static inline int32_t asub32(CPURISCVState *env, int vxrm, int32_t a, int32_t b)
>>> +{
>>> + int64_t res = (int64_t)a - b;
>>> + uint8_t round = get_round(vxrm, res, 1);
>>> +
>>> + return (res >> 1) + round;
>>> +}
>>> +
>>>
>>> I find a corner case here. As the spec said in Section 13.2
>>>
>>> "There can be no overflow in the result".
>>>
>>> If the a is 0x7fffffff, b is 0x80000000, and the round mode is round to up(rnu),
>>> then the result is (0x7fffffff - 0x80000000 + 1) >> 1, equals 0x80000000,
>>> according the v0.7.1
>> That's why we used int64_t as the intermediate type:
>>
>> 0x000000007fffffff - 0xffffffff80000000 + 1
>> = 0x000000007fffffff + 0x0000000080000000 + 1
>> = 0x00000000ffffffff + 1
>> = 0x0000000100000000
>>
>> Shift that right by 1 and you do indeed get 0x80000000.
>> There's no saturation involved.
>
> The minuend 0x7fffffff is INT32_MAX, and the subtrahend 0x80000000 is INT32_MIN.
>
> The difference between the minuend and the subtrahend should be a positive
> number. But the result here is 0x80000000.
>
> So it is overflow. However, according to the spec, it should not overflow.
Unless I'm missing something, the spec is wrong about "there can be no
overflow", the above being a counter-example.
Do you have hardware to compare against? Perhaps it is in fact "overflow is
ignored", as the 0.8 spec says for vasubu?
I wouldn't add saturation, because the spec says nothing about saturation, and
does mention truncation, at least for vasubu.
r~
next prev parent reply other threads:[~2020-03-28 1:23 UTC|newest]
Thread overview: 120+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-17 15:05 [PATCH v6 00/61] target/riscv: support vector extension v0.7.1 LIU Zhiwei
2020-03-17 15:05 ` [PATCH v6 01/61] target/riscv: add vector extension field in CPURISCVState LIU Zhiwei
2020-03-17 15:05 ` [PATCH v6 02/61] target/riscv: implementation-defined constant parameters LIU Zhiwei
2020-03-17 15:05 ` [PATCH v6 03/61] target/riscv: support vector extension csr LIU Zhiwei
2020-03-17 15:05 ` [PATCH v6 04/61] target/riscv: add vector configure instruction LIU Zhiwei
2020-03-23 6:51 ` Kito Cheng
2020-03-23 7:10 ` LIU Zhiwei
2020-03-17 15:05 ` [PATCH v6 05/61] target/riscv: add an internals.h header LIU Zhiwei
2020-03-18 23:45 ` Alistair Francis
2020-03-27 23:41 ` Richard Henderson
2020-03-17 15:05 ` [PATCH v6 06/61] target/riscv: add vector stride load and store instructions LIU Zhiwei
2020-03-18 23:54 ` Alistair Francis
2020-03-17 15:05 ` [PATCH v6 07/61] target/riscv: add vector index " LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 08/61] target/riscv: add fault-only-first unit stride load LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 09/61] target/riscv: add vector amo operations LIU Zhiwei
2020-03-19 17:01 ` Alistair Francis
2020-03-27 23:44 ` Richard Henderson
2020-03-17 15:06 ` [PATCH v6 10/61] target/riscv: vector single-width integer add and subtract LIU Zhiwei
2020-03-20 18:31 ` Alistair Francis
2020-03-27 23:54 ` Richard Henderson
2020-03-28 14:42 ` LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 11/61] target/riscv: vector widening " LIU Zhiwei
2020-03-19 16:28 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 12/61] target/riscv: vector integer add-with-carry / subtract-with-borrow instructions LIU Zhiwei
2020-03-19 17:29 ` Alistair Francis
2020-03-28 0:00 ` Richard Henderson
2020-03-17 15:06 ` [PATCH v6 13/61] target/riscv: vector bitwise logical instructions LIU Zhiwei
2020-03-20 18:34 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 14/61] target/riscv: vector single-width bit shift instructions LIU Zhiwei
2020-03-19 20:10 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 15/61] target/riscv: vector narrowing integer right " LIU Zhiwei
2020-03-20 18:43 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 16/61] target/riscv: vector integer comparison instructions LIU Zhiwei
2020-03-25 17:32 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 17/61] target/riscv: vector integer min/max instructions LIU Zhiwei
2020-03-20 18:49 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions LIU Zhiwei
2020-03-25 17:36 ` Alistair Francis
2020-03-28 0:06 ` Richard Henderson
2020-03-28 15:17 ` LIU Zhiwei
2020-03-28 15:47 ` Richard Henderson
2020-03-28 16:13 ` LIU Zhiwei
2020-03-29 4:00 ` LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 19/61] target/riscv: vector integer divide instructions LIU Zhiwei
2020-03-20 18:51 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 20/61] target/riscv: vector widening integer multiply instructions LIU Zhiwei
2020-03-25 17:25 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 21/61] target/riscv: vector single-width integer multiply-add instructions LIU Zhiwei
2020-03-25 17:27 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 22/61] target/riscv: vector widening " LIU Zhiwei
2020-03-25 17:42 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 23/61] target/riscv: vector integer merge and move instructions LIU Zhiwei
2020-03-26 17:57 ` Alistair Francis
2020-03-28 0:18 ` Richard Henderson
2020-03-17 15:06 ` [PATCH v6 24/61] target/riscv: vector single-width saturating add and subtract LIU Zhiwei
2020-03-28 0:20 ` Richard Henderson
2020-03-17 15:06 ` [PATCH v6 25/61] target/riscv: vector single-width averaging " LIU Zhiwei
2020-03-19 3:46 ` LIU Zhiwei
2020-03-28 0:32 ` Richard Henderson
2020-03-28 1:07 ` LIU Zhiwei
2020-03-28 1:22 ` Richard Henderson [this message]
2020-03-28 15:37 ` LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 26/61] target/riscv: vector single-width fractional multiply with rounding and saturation LIU Zhiwei
2020-03-28 1:08 ` Richard Henderson
2020-03-28 15:41 ` LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 27/61] target/riscv: vector widening saturating scaled multiply-add LIU Zhiwei
2020-03-28 1:23 ` Richard Henderson
2020-03-17 15:06 ` [PATCH v6 28/61] target/riscv: vector single-width scaling shift instructions LIU Zhiwei
2020-03-28 1:24 ` Richard Henderson
2020-03-17 15:06 ` [PATCH v6 29/61] target/riscv: vector narrowing fixed-point clip instructions LIU Zhiwei
2020-03-28 1:50 ` Richard Henderson
2020-03-17 15:06 ` [PATCH v6 30/61] target/riscv: vector single-width floating-point add/subtract instructions LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 31/61] target/riscv: vector widening " LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 32/61] target/riscv: vector single-width floating-point multiply/divide instructions LIU Zhiwei
2020-03-25 17:46 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 33/61] target/riscv: vector widening floating-point multiply LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 34/61] target/riscv: vector single-width floating-point fused multiply-add instructions LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 35/61] target/riscv: vector widening " LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 36/61] target/riscv: vector floating-point square-root instruction LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 37/61] target/riscv: vector floating-point min/max instructions LIU Zhiwei
2020-03-25 17:47 ` Alistair Francis
2020-03-17 15:06 ` [PATCH v6 38/61] target/riscv: vector floating-point sign-injection instructions LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 39/61] target/riscv: vector floating-point compare instructions LIU Zhiwei
2020-03-28 2:01 ` Richard Henderson
2020-03-28 15:44 ` LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 40/61] target/riscv: vector floating-point classify instructions LIU Zhiwei
2020-03-28 2:06 ` Richard Henderson
2020-03-17 15:06 ` [PATCH v6 41/61] target/riscv: vector floating-point merge instructions LIU Zhiwei
2020-03-28 3:23 ` Richard Henderson
2020-03-28 15:47 ` LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 42/61] target/riscv: vector floating-point/integer type-convert instructions LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 43/61] target/riscv: widening " LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 44/61] target/riscv: narrowing " LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 45/61] target/riscv: vector single-width integer reduction instructions LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 46/61] target/riscv: vector wideing " LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 47/61] target/riscv: vector single-width floating-point " LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 48/61] target/riscv: vector widening " LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 49/61] target/riscv: vector mask-register logical instructions LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 50/61] target/riscv: vector mask population count vmpopc LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 51/61] target/riscv: vmfirst find-first-set mask bit LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 52/61] target/riscv: set-X-first " LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 53/61] target/riscv: vector iota instruction LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 54/61] target/riscv: vector element index instruction LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 55/61] target/riscv: integer extract instruction LIU Zhiwei
2020-03-28 3:36 ` Richard Henderson
2020-03-28 16:23 ` LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 56/61] target/riscv: integer scalar move instruction LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 57/61] target/riscv: floating-point scalar move instructions LIU Zhiwei
2020-03-28 3:44 ` Richard Henderson
2020-03-28 16:31 ` LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 58/61] target/riscv: vector slide instructions LIU Zhiwei
2020-03-28 3:50 ` Richard Henderson
2020-03-28 13:40 ` LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 59/61] target/riscv: vector register gather instruction LIU Zhiwei
2020-03-28 3:57 ` Richard Henderson
2020-03-17 15:06 ` [PATCH v6 60/61] target/riscv: vector compress instruction LIU Zhiwei
2020-03-17 15:06 ` [PATCH v6 61/61] target/riscv: configure and turn on vector extension from command line LIU Zhiwei
2020-03-25 17:49 ` Alistair Francis
2020-03-28 4:00 ` Richard Henderson
2020-03-17 20:47 ` [PATCH v6 00/61] target/riscv: support vector extension v0.7.1 no-reply
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ce13869a-141f-cc4d-8f1e-6a3742197654@linaro.org \
--to=richard.henderson@linaro.org \
--cc=alistair23@gmail.com \
--cc=chihmin.chao@sifive.com \
--cc=guoren@linux.alibaba.com \
--cc=palmer@dabbelt.com \
--cc=qemu-devel@nongnu.org \
--cc=qemu-riscv@nongnu.org \
--cc=wenmeng_zhang@c-sky.com \
--cc=wxy194768@alibaba-inc.com \
--cc=zhiwei_liu@c-sky.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).