Re: Question about TCG backend correctness

[LIU Zhiwei]

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);
         break;
     default:
         g_assert_not_reached();
     }
     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，
Zhiwei

> 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 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.
>       
>       
>       
>       r~
>       
>