On Wed, Feb 8, 2023 at 9:26 AM Dave Thaler <dthaler=40microsoft.com@xxxxxxxxxxxxxx> wrote: > > David Vernet wrote: > > > +Reserved instructions > > > +==================== > > > > small nit: Missing a = > > Ack. > > > > +Clang will generate the reserved ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d) > > instruction if ``-O0`` is used. > > > > Are we calling this out here to say that BPF_CALL in clang -O0 builds is not > > supported? That would seem to be the case given that we say that BPF_CALL > > | BPF_X | BPF_JMP in reserved and not permitted in instruction-set.rst. > > Yes, exactly. I could update the language to add something like > "... so BPF_CALL in clang -O0 builds is not supported". That will not be a correct statement. BPF_CALL is a valid insn regardless of optimization flags. BPF_CALLX will be a valid insn when the verifier support is added. Compilers need to make a choice which insn to use on a case by case basis. When compilers have no choice, but to use call by register they will use callx. That what happens with = (void *)1 hack that we use for helpers. It can happen with -O2 just as well.
