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Re: [Qemu-riscv] [Qemu-devel] RISC-V: insn32.decode: Confusing encodings

From: Richard Henderson
Subject: Re: [Qemu-riscv] [Qemu-devel] RISC-V: insn32.decode: Confusing encodings
Date: Wed, 7 Aug 2019 11:40:58 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 8/6/19 5:48 AM, Maxim Blinov wrote:
> slli     00.... ......    ..... 001 ..... 0010011 @sh
> srli     00.... ......    ..... 101 ..... 0010011 @sh
> srai     01.... ......    ..... 101 ..... 0010011 @sh
> First question: Why does the %sh10 field exist? There are no 10-bit
> shamt fields anywhere in the spec.
> Second question: For rv32i, "SLLI" is defined as follows in the spec:
> 0000000 shamt[4:0] rs1[4:0] 001 rd[4:0] 0010011  |  SLLI

Bits [9:5] of the field are checked against zero later, with

    if (a->shamt >= TARGET_LONG_BITS) {
        return false;

It was done this way to be compatible between rv32, rv64, and a future rv128.
Which I admit would only need 7 bits not 10, but it didn't seem to matter
either way.

> Consider the case that we have a 32 bit cpu and we wanted to have a
> custom instruction encoded like so:
>       |This bit set
>       v
> 0000001 shamt[4:0] rs1[4:0] 001 rd[4:0] 0010011  |  MY_INSN
> In 64 bit risc-v, we can't have that instruction because that bit is
> used in the shift field for the SLLI instruction.  But it should be
> fine to use in 32-bit risc-v.

Ah, well, for that you would in fact need to adjust the decode files.

I do question why you'd want to define MY_INSN in such a way as to be
incompatible with an rv64 implementation.  Why not place your new bit higher in
the field?

> Why not have two separate insn32.decode and insn64.decode files?

To avoid unnecessary duplication, of course.


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