Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply

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Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply

From:	Richard Henderson
Subject:	Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions
Date:	Sat, 28 Mar 2020 08:47:29 -0700
User-agent:	Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 3/28/20 8:17 AM, LIU Zhiwei wrote:
>> Missed the improvement here.  See tcg_gen_mulsu2_i64.
> Though I have not gotten the principle, the code in tcg_gen_mulsu2_i64 is much
> tidier.

Let A = signed operand,
    B = unsigned operand
    P = unsigned product

If the sign bit A is set, then P is too large.
In that case we subtract 2**64 * B to fix that:

    HI_P -= (A < 0 ? B : 0)

where the conditional is computed as (A >> 63) & B.


r~

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[PATCH v6 15/61] target/riscv: vector narrowing integer right shift instructions, (continued)
- [PATCH v6 15/61] target/riscv: vector narrowing integer right shift instructions, LIU Zhiwei, 2020/03/17
  - Re: [PATCH v6 15/61] target/riscv: vector narrowing integer right shift instructions, Alistair Francis, 2020/03/20
- [PATCH v6 16/61] target/riscv: vector integer comparison instructions, LIU Zhiwei, 2020/03/17
  - Re: [PATCH v6 16/61] target/riscv: vector integer comparison instructions, Alistair Francis, 2020/03/25
- [PATCH v6 17/61] target/riscv: vector integer min/max instructions, LIU Zhiwei, 2020/03/17
  - Re: [PATCH v6 17/61] target/riscv: vector integer min/max instructions, Alistair Francis, 2020/03/20
- [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions, LIU Zhiwei, 2020/03/17
  - Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions, Alistair Francis, 2020/03/25
  - Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions, Richard Henderson, 2020/03/27
    - Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions, LIU Zhiwei, 2020/03/28
    - Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions, Richard Henderson <=
    - Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions, LIU Zhiwei, 2020/03/28
    - Re: [PATCH v6 18/61] target/riscv: vector single-width integer multiply instructions, LIU Zhiwei, 2020/03/29
- [PATCH v6 19/61] target/riscv: vector integer divide instructions, LIU Zhiwei, 2020/03/17
  - Re: [PATCH v6 19/61] target/riscv: vector integer divide instructions, Alistair Francis, 2020/03/20
- [PATCH v6 20/61] target/riscv: vector widening integer multiply instructions, LIU Zhiwei, 2020/03/17
  - Re: [PATCH v6 20/61] target/riscv: vector widening integer multiply instructions, Alistair Francis, 2020/03/25
- [PATCH v6 21/61] target/riscv: vector single-width integer multiply-add instructions, LIU Zhiwei, 2020/03/17
  - Re: [PATCH v6 21/61] target/riscv: vector single-width integer multiply-add instructions, Alistair Francis, 2020/03/25
- [PATCH v6 22/61] target/riscv: vector widening integer multiply-add instructions, LIU Zhiwei, 2020/03/17
  - Re: [PATCH v6 22/61] target/riscv: vector widening integer multiply-add instructions, Alistair Francis, 2020/03/25

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