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Re: Question about TCG backend correctness

From: LIU Zhiwei
Subject: Re: Question about TCG backend correctness
Date: Wed, 19 Oct 2022 19:59:10 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.3.3

On 2022/10/18 13:22, Richard Henderson wrote:
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.

After fixing some bugs, RISU can run  some instructions now. Thanks.

IMHO,  the next 2 requests should be satisfied for TCG test.

1. Add a unit test case for any lower level and small function is easy.

For example, the function in risc-v backend,

static bool tcg_out_mov(TCGContext *s, TCGType type, TCGReg ret, TCGReg arg)
     if (ret == arg) {
         return true;
     switch (type) {
     case TCG_TYPE_I32:
     case TCG_TYPE_I64:
         tcg_out_opc_imm(s, OPC_ADDI, ret, arg, 0);
     return true;

Write a unit test case for it is not easy currently.  I don't know how to fill the TCGContext struct and other parameters.
And there is no test framework where I can reuse.

The others may say that we can run a lot of benchmark to ensure we have a good coverage and don't
need such a low level test case. Some reasonable. But it will lead to a very high test burden and it is very hard to get
a good coverage only through high level test.

2. Add a unit test case for any high level function is easy, such as tcg_gen_code.

The check-tcg test belongs to the high level testing.

Best Regards,

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

It wouldn't.  This is my argument against creating a new virtual processor.

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.


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