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Re: Question about TCG backend correctness

From: Alex Bennée
Subject: Re: Question about TCG backend correctness
Date: Tue, 18 Oct 2022 10:22:28 +0100
User-agent: mu4e 1.9.1; emacs 28.2.50

Richard Henderson <richard.henderson@linaro.org> writes:

> On 10/18/22 01:27, LIU Zhiwei wrote:
>> Maybe I can run RISU on qemu-aarch64(x86) and qemu-aarch64(risc-v)
>> to check the RISC-V backend.
> This is a good start for debugging a tcg backend. It's not
> comprehensive, because RISU executes one instruction at a time then
> raises an exception to check the results.  This means that the tcg
> optimizer doesn't have much to work with, which means that the tcg
> backend is not as stressed as it could be.
>>> I've long wanted to have the ability to have TCG unit tests where a
>>> virtual processor could be defined for the purpose of directly
>>> exercising TCG.
>> We already have many ISAs as the front end of TCG. Will the virtual
>> processor here be some
>> different?
> It wouldn't.  This is my argument against creating a new virtual
> processor.

I'm not wedded to the idea - but it could be a super simple processor
with a lot less extra baggage than a full ISA - purely for throwing TCG
ops at rather than decoding any machine code.

> I do think we should be better about creating regression tests for
> bugs fixed, in the form of small focused assembly test cases which get
> run via check-tcg.

Can we detect optimisation failures with guest tests? Have we written
any so far?

Is it worth doing anything to lower the barrier of entry for these small
micro tests or is the current check-tcg framework enough?

> r~

Alex Bennée

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