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Re: [PATCH] decodetree: Allow custom var width load functions

From: Richard Henderson
Subject: Re: [PATCH] decodetree: Allow custom var width load functions
Date: Tue, 13 Apr 2021 18:10:11 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1

On 4/13/21 11:16 AM, Luis Pires wrote:
This is useful in situations where you want decodetree
to handle variable width instructions but you want to
provide custom code to load the instructions. Suppressing
the generation of the load function is necessary to avoid
compilation errors due to the load function being unused.

This will be used by the PowerPC decodetree code.

Signed-off-by: Luis Pires<luis.pires@eldorado.org.br>
  scripts/decodetree.py | 18 +++++++++++-------
  1 file changed, 11 insertions(+), 7 deletions(-)

So, in the previous cases where we have very controlled "variable length", we've simply created separate decode files each with fixed length.

E.g. for arm thumb, in thumb_tr_translate_insn:

    insn = arm_lduw_code(env, dc->base.pc_next, dc->sctlr_b);
    is_16bit = thumb_insn_is_16bit(dc, dc->base.pc_next, insn);
    if (is_16bit) {
        disas_thumb_insn(dc, insn);
    } else {
        disas_thumb2_insn(dc, insn);

Similarly, riscv's decode_opc.

I would think that Power would work the same, with 32-bit and 64-bit instructions. I'd like you to talk me through what you want to do here.


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