[llvm] [BPF] Support Jump Table (PR #149715)

[via llvm-commits]

eddyz87 wrote:

> > > R_BPF_64_NODYLD32
> > 
> > 
> > Even I forgot that it is for R_BPF_64_NODYLD32. I is related to dynamic linking and initially for .BTF and .BTF.ext sections. I will take a look again about what it is and what we should do about it.
> > The original commit:
> > commit [6a2ea84](https://github.com/llvm/llvm-project/commit/6a2ea84600ba4bd3b2733bd8f08f5115eb32164b) Author: Yonghong Song [yhs at fb.com](mailto:yhs at fb.com) Date: Tue May 18 08:35:59 2021 -0700
> > ```
> > BPF: Add more relocation kinds
> > ```
> 
> Okay, I checked the above commit. R_BPF_64_NODYLD32 is mostly for linker when lld tries to merge different progs. Unfortunately this is never materialized. So you can treat R_BPF_64_NODYLD32 as a normal R_BPF_64_ABS32 (data) relocation.

I think relocations can be made to go away if table content would be implemented as instruction offsets, computed as difference between label and section start. The piece of code I posted [here](https://github.com/llvm/llvm-project/pull/149715#issuecomment-3097615612) generated the following assembly:

```
        .section        .jumptables,"", at progbits
.L0_0_set_7 = ((LBB0_7-.LBPF.JX.0.0)>>3)-1
.L0_0_set_2 = ((LBB0_2-.LBPF.JX.0.0)>>3)-1
.L0_0_set_8 = ((LBB0_8-.LBPF.JX.0.0)>>3)-1
.L0_0_set_9 = ((LBB0_9-.LBPF.JX.0.0)>>3)-1
.L0_0_set_10 = ((LBB0_10-.LBPF.JX.0.0)>>3)-1
BPF.JT.0.0:
        .long   .L0_0_set_7
        .long   .L0_0_set_2
        .long   .L0_0_set_2
        .long   .L0_0_set_2
        ...
```
And there were no relocations for ".jumptables":

```
Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
  [ 1] .strtab           STRTAB          0000000000000000 000320 000065 00      0   0  1
  [ 2] .text             PROGBITS        0000000000000000 000040 0000f0 00  AX  0   0  8
  [ 3] .rel.text         REL             0000000000000000 0002f0 000030 10   I  6   2  8
  [ 4] .jumptables       PROGBITS        0000000000000000 000130 000130 00      0   0  1
  [ 5] .llvm_addrsig     LLVM_ADDRSIG    0000000000000000 000320 000000 00   E  6   0  1
  [ 6] .symtab           SYMTAB          0000000000000000 000260 000090 18      1   2  8
```

Do we need these relocations to simplify linking by libbpf?

https://github.com/llvm/llvm-project/pull/149715