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

[via llvm-commits]

eddyz87 wrote:

The following example is not handled:

```c++
int bar(int a) {
  __label__ l1, l2;
  void * volatile tgt;
  int ret = 0;
  if (a)
    tgt = &&l1;
  else
    tgt = &&l2;
  goto *tgt;
 l1: ret += 1;
 l2: ret += 2;
 return ret;
}
```

Currently the following code is produced:

```
$ (use-my-llvm ; clang -O2 -S -o - --target=bpf jt-vars.c )
        .file   "jt-vars.c"
        .text
        .globl  bar                             # -- Begin function bar
        .p2align        3
        .type   bar, at function
bar:                                    # @bar
# %bb.0:                                # %entry
        r2 = .Ltmp0 ll
        if w1 == 0 goto LBB0_2
# %bb.1:                                # %entry
        r2 = .Ltmp1 ll
LBB0_2:                                 # %entry
        *(u64 *)(r10 - 8) = r2
        r1 = *(u64 *)(r10 - 8)
        gotox r1
.Ltmp1:                                 # Block address taken
LBB0_3:                                 # %l1
        w0 = 3
        goto LBB0_5
.Ltmp0:                                 # Block address taken
LBB0_4:                                 # %l2
        w0 = 2
LBB0_5:                                 # %.split
        exit
.Lfunc_end0:
        .size   bar, .Lfunc_end0-bar
                                        # -- End function
        .addrsig
```

As discussed previously, `r2 = .Ltmp0 ll` should be converted to an access to a single element jump table. Otherwise kernel won't be able to adjust `.Ltmp0` properly in `verifier.c:do_misc_fixups`.

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