Re: [PATCH v2 2/2] LoongArch: When the code model is extreme, the symbol address is obtained through macro instructions regardless of the value of -mexplicit-relocs.

[Xi Ruoyao <xry111@xry111.site>]

On Fri, 2024-01-05 at 17:57 +0800, chenglulu wrote:
> 
> 在 2024/1/5 下午4:37, Xi Ruoyao 写道:
> > On Fri, 2024-01-05 at 11:40 +0800, Lulu Cheng wrote:
> > >   bool
> > >   loongarch_explicit_relocs_p (enum loongarch_symbol_type type)
> > >   {
> > > +  /* Instructions pcalau12i, addi.d, lu32i.d and lu52i.d must be adjancent
> > > +     so that the linker can infer the PC of pcalau12i to apply relocations
> > > +     to lu32i.d and lu52i.d.  Otherwise, the results would be incorrect if
> > > +     these four instructions are not in the same 4KiB page.
> > > +     Therefore, macro instructions are used when cmodel=extreme.  */
> > > +  if (loongarch_symbol_extreme_p (type))
> > > +    return false;
> > I think this is a bit of strange.  With -mexplicit-relocs={auto,always}
> > we should still use explicit relocs, but coding all 4 instructions
> > altogether as
> > 
> > "pcalau12i.d\t%1,%pc64_hi12(%2)\n\taddi.d\t%0,$r0,%pclo12(%2)\n\tlu32i.d\t%0,%pc64_lo20(%2)\n\tlu52i.d\t%0,%0,%pc64_hi12(%2)"
> > 
> > Give me several hours trying to implement this...
> > 
> I think there is no difference between macros and these instructions put 
> together. If implement it in a split form, I think I can try it through 
> TARGET_SCHED_MACRO_FUSION_PAIR_P

There is a difference:

int x;
int t() { return x; }

pcalau12i.d t0, %pc_hi20(x)
addi.d t1, r0, %pc_lo12(x)
lu32i.d t1, %pc64_lo20(x)
lu52i.d t1, t1, %pc64_hi12(x)
ldx.w a0, t0, t1

is slightly better than

pcalau12i.d t0, %pc_hi20(x)
addi.d t1, r0, %pc_lo12(x)
lu32i.d t1, %pc64_lo20(x)
lu52i.d t1, t1, %pc64_hi12(x)
addi.d t0, t0, t1
ld.w a0, t0, 0

And generating macros when -mexplicit-relocs=always can puzzle people
(it says "always" :-\ ).

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University