On Wed, Jun 18, 2025 at 9:30 AM Anton Protopopov
<a.s.protopopov@xxxxxxxxx> wrote:
>
> On 25/06/18 09:01AM, Alexei Starovoitov wrote:
> > On Wed, Jun 18, 2025 at 7:43 AM Anton Protopopov
> > <a.s.protopopov@xxxxxxxxx> wrote:
> > >
> > > On 25/06/17 08:24PM, Alexei Starovoitov wrote:
> > > > On Sun, Jun 15, 2025 at 1:55 AM Anton Protopopov
> > > > <a.s.protopopov@xxxxxxxxx> wrote:
> > > > > +SEC("syscall")
> > > > > +int two_towers(struct simple_ctx *ctx)
> > > > > +{
> > > > > + switch (ctx->x) {
> > > > >
> > > >
> > > > Not sure why you went with switch() statements everywhere.
> > > > Please add few tests with explicit indirect goto
> > > > like interpreter does: goto *jumptable[insn->code];
> > >
> > > This requires to patch libbpf a bit more, as some meta-info
> > > accompanying this instruction should be emitted, like LLVM does with
> > > jump_table_sizes. And this probably should be a different section,
> > > such that it doesn't conflict with LLVM/GCC. I thought to add this
> > > later, but will try to add to the next version.
> >
> > Hmm. I'm not sure why llvm should handle explicit indirect goto
> > any different than the one generated from switch.
> > The generated bpf.o should be the same.
>
> For a switch statement LLVM will create a jump table
> and create the {,.rel}.llvm_jump_table_sizes tables.
>
> For a direct goto *, say
>
> static const void *table[] = {
> &&l1, &&l2, &&l3, &&l4, &&l5,
> };
> if (index > ARRAY_SIZE(table))
> return 0;
> goto *table[index];
>
> it will not generate {,.rel}.llvm_jump_table_sizes. I wonder, does
> LLVM emit the size of `table`? (If no, then some assembly needed to
> emit it.) In any case it should be easy to add this case, but still
> it is a bit of coding, thus a bit different case.)
It's controlled by -emit-jump-table-sizes-section flag.
I haven't looked at pending llvm/bpf diff, but it should be possible
to standardize. Emit it for both or for none.
My preference would be for _none_.
Not sure why you made libbpf rely on that section name.
Relocations against text can be in other rodata sections.
Normal behavior for x86 and other backends.
