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Re: [Qemu-riscv] [PATCH v5 00/35] target/riscv: Convert to decodetree

From: Palmer Dabbelt
Subject: Re: [Qemu-riscv] [PATCH v5 00/35] target/riscv: Convert to decodetree
Date: Fri, 25 Jan 2019 15:54:58 -0800 (PST)

On Tue, 22 Jan 2019 13:38:52 PST (-0800), address@hidden wrote:
On 1/22/19 1:28 AM, Bastian Koppelmann wrote:

this patchset converts the RISC-V decoder to decodetree in four major steps:

1) Convert 32-bit instructions to decodetree [Patch 1-16]:
    Many of the gen_* functions are called by the decode functions for 16-bit
    and 32-bit functions. If we move translation code from the gen_*
    functions to the generated trans_* functions of decode-tree, we get a lot of
    duplication. Therefore, we mostly generate calls to the old gen_* function
    which are properly replaced after step 2).

    Each of the trans_ functions are grouped into files corresponding to their
    ISA extension, e.g. addi which is in RV32I is translated in the file

2) Convert 16-bit instructions to decodetree [Patch 17-19]:
    All 16 bit instructions have a direct mapping to a 32 bit instruction. Thus,
    we convert the arguments in the 16 bit trans_ function to the arguments of
    the corresponding 32 bit instruction and call the 32 bit trans_ function.

3) Remove old manual decoding in gen_* function [Patch 20-30]:
    this move all manual translation code into the trans_* instructions of
    decode tree, such that we can remove the old decode_* functions.

4) Simplify RVC by reusing as much as possible from the RVG decoder as suggested
   by Richard. [Patch 31-35]

full tree available at


v4 -> v5:
    - fixed rebase error
    - moved TARGET_LONG_BITS check of shift instructions before rd == 0 check
    - removed extra sign extension of sraiw
    - removed rs2 == 0 special cases in sraw/srlw

All looks good to me now.  Thanks for persevering.

Thanks for reviewing this.

Bastian: what sort of testing have you done here? This is a pretty big change, and while it's been fairly extensively reviewed I'm just generally paranoid.

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