[llvm] [PowerPC] Peephole address calculation in TOC memops (PR #76488)

[Chen Zheng via llvm-commits]

chenzheng1030 wrote:

> > Something useful related to this came up in @stephenpeckham's presentation. Adding an offset to the large code model relocation expressions is not allowed, so I belive we couldn't transform something like
> > ```
> > addis r3, x[TD]@u(2)
> > addi  r3, x[TD]@l(r3)
> > ld.      r5, 2(r3)
> > ```
> > 
> > 
> >     
> >       
> >     
> > 
> >       
> >     
> > 
> >     
> >   
> > to
> > ```
> > addis r3, x[TD]@u(2)
> > ld  r5, x[TD]@l + 2)(r3)
> > ```
> > 
> > 
> >     
> >       
> >     
> > 
> >       
> >     
> > 
> >     
> >   
> > I though I'd comment here now so we won't forget to test this.
> 
> ```
> int a = 100;
> int main()
> {
>   return a;
> }
> ```
> 
> `1.c -S -mcmodel=large -mtocdata -m64`
> 
> ```
> .main:
> # %bb.0:                                # %entry
>         li 3, 0
>         stw 3, -12(1)
>         addis 3, a[TD]@u(2)
>         la 3, a[TD]@l(3)
>         lwz 3, 0(3)
>         extsw 3, 3
>         blr
> ```
> 
> After manually changing the assembly to:
> 
> ```
> .main:
> # %bb.0:                                # %entry
>         li 3, 0
>         stw 3, -12(1)
>         addis 3, a[TD]@u(2)
>         lwz 3, a[TD]@l(3)
>         extsw 3, 3
>         blr
> ```
> 
> It is still able to get good running result.
> 
> Do you mean other cases @mandlebug ?

Seems related to https://github.com/llvm/llvm-project/commit/b54579fab6571501f9e82a3e413ad36fd34aaa75

```
addis r3, x[TD]@u(2)
addi  r3, x[TD]@l(r3)
ld      r5, offset(r3)
```

If the offset is bigger than 8, then the `x[TD]@u(2)` may resolve to different values with/without the `ld(addi)` combination. But this should already be handled with existing code?

https://github.com/llvm/llvm-project/pull/76488