Re: [PATCH v2 4/5] target/riscv: Add support for PC-relative translation

[Richard Henderson]

On 3/29/23 18:09, liweiwei wrote:
> > > @@ -51,26 +59,43 @@ static bool trans_jal(DisasContext *ctx, arg_jal *a)
> > >   static bool trans_jalr(DisasContext *ctx, arg_jalr *a)
> > >   {
> > >       TCGLabel *misaligned = NULL;
> > > +    TCGv succ_pc = tcg_temp_new();
> >
> > succ_pc can by null for !CF_PCREL...
> I think this is OK since it's only used for CF_PCREL.

It allocates an unused temp.  Not a bug per se, but an easily fixable mistake.

> > ... or initialized like
> >
> >        } else {
> >            succ_pc = tcg_constant_tl(ctx->pc_succ_insn);
> >        }

If you do this, you can avoid the test/set/seti later.

> > >       if (misaligned) {
> > >           gen_set_label(misaligned);
> > > -        gen_exception_inst_addr_mis(ctx);
> > > +        gen_exception_inst_addr_mis(ctx, target_pc);
> > >       }
> >
> > This is what I expected from patch 3: cpu_pc is unchanged, with the new (incorrect) 
> > address passed to inst_addr_mis for assigning to badaddr.  Bug being fixed here, thus 
> > should really be a separate patch.
>
> It's OK to update cpu_pc before gen_exception_inst_addr_mis() since it will restore the 
> current pc by gen_set_pc_imm() after update cpu_pc into badaddr.

True, but I think it's confusing to set cpu_pc for it's mere use in copying to badaddr, 
and rely on generate_exception to reset cpu_pc to the correct value.

> However, after PC-relative translation is enabled, we cannot use gen_set_pc to directly 
> update cpu_pc in above case, since gen_set_pc() will break the pc_save, and make 
> gen_set_pc_imm() in gen_exception_inst_addr_mis() failed. So we introduce a temp target_pc 
> instead of cpu_pc to compute the destination pc and use it to do misaligned check.

Exactly.

Which is why I think it is better to simply pass gen_exception_inst_addr_mis the value to 
use with badaddr in a normal temp (or constant).  And do this always, not simply in the 
one case where it is absolutely required to not clobber cpu_pc.


r~