Re: [PATCH 21/30] tcg/loongarch: Implement tcg_out_call

[WANG Xuerui]

Hi Richard,

On 9/21/21 00:35, Richard Henderson wrote:
> On 9/20/21 9:31 AM, Richard Henderson wrote:
> > On 9/20/21 1:04 AM, WANG Xuerui wrote:
> > > +    } else if (TCG_TARGET_REG_BITS == 32 || offset == 
> > > (int32_t)offset) {
> > > +        /* long jump: +/- 2GiB */
> > > +        tcg_out_opc_pcaddu12i(s, TCG_REG_TMP0, 0);
> > > +        tcg_out_opc_jirl(s, link, TCG_REG_TMP0, 0);
> > > +        ret = reloc_call(s->code_ptr - 2, arg);
> > > +        tcg_debug_assert(ret == true);
> >
> > Just inline reloc_call here, so that you can provide the correct 
> > offsets to the pcadd and jirl instructions directly.  The assert will 
> > vanish, because you've already done the range check with "offset == 
> > (int32_t)offset".
>
> Actually, don't you want offset == sextract64(offset, 0, 34), and use 
> pcaddu18i? Depending on the memory map of qemu, those extra bits could 
> make the difference in directly reaching the main executable.
Whoa, silly me, I actually didn't realize a single expected use case of 
pcaddu18i until I read this, the low 2 bits are always clear so 18 is 
exactly the amount of shift needed when paired with jirl!

I'll of course rework this to use pcaddu18i+jirl instead.

> r~